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WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY 


W. O. HOTCHKISS, Director and State Geologist 
a sf BULLETIN NO. 64 SCIENTIFIC SERIES NO. 13 
te a 
Th LUCLOGY 
c Crustacea 
Inland Lakes of Wisconsin 
2 
“a 
‘n 
The Plankton 3 e 
I. Its Quantity and Chemical Composition me 
CO 14 
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Fad 
» 
Q 
oe 
Y 
By 
EDWARD A. BIRGE and CHANCEY JUDAY 
: & AG ONAN NSTI 
MVOEDT enn ie oer \ 
WNVERTES +} ATE “284 s18° 
\ Z001 OGY dh. 7 
Crustaces 


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“A f Mi Iau AL w\) A 


el 


MADISON, WISCONSIN 
PUBLISHED BY THE STATE 
1922 


Wisconsin Geological and Natural History Survey 


BOARD OF COMMISSIONERS 


JOHN J. BLAINE, 
Governor of the State. 


EDWARD A. BIRGE, President. 
President of the University of Wisconsin. 


JOHN CALLAHAN, Vice-President. 
State Superintendent of Public Instruction. 


MELVIN A. BRANNON, 
President of the Wisconsin Academy of Sciences, Arts and Letters. 


STAFF OF THE SURVEY, 1922 


ADMINISTRATION: 
William O. Hotchkiss, State Geologist, Director and Superintendent. 
In immediate charge of Geology Division. 
Ernest I’. Bean, Assistant State Geologist. 
Lillian M. Veerhusen, Chief Clerk. 
Amy F. Mueller, Clerk and Stenographer. 
Angeline Doll, Clerk. 


GEOLOGY DIVISION: 
William O. Hotchkiss, In charge. 
Ernest F. Bean, Geologist, Mineral Land Classification. 
Thomas C. Chamberlin, Consulting Geologist, Pleistocene Geology. 
Edward O. Ulrich, Consulting Geologist, Stratigraphy, by cooperation of the 
USS. Gis: 
Henry R. Aldrich, Geologist, Mineral Land Classification. 
Ray Hughes Whitbeck, Geographer. 
Edward Steidtmann, Geologist, Limestones. 
Frederic T. Thwaites, Well Records, Educational Rock Collection. 


NATURAL HISTORY DIVISION: 
Edward A. Birge, In charge. 
Chancey Juday, Lake Survey. 


DIVISION OF SOILS: 
A. R. Whitson, In charge. 
W. J. Geib, Inspector and Editor. 
T. J. Dunnewald, Field Assistant and Analyst. 
IIMD 


TABLE OF CONTENES 


Page 

Pre MBIT COS t rcea katie, ate Replene ais Ge ee ca hie ndsrow tie. g)a\ aut mien bev eles Statepianti ons, & calsta'elsi = Vv 
TERETE GO 6 SRI ple As aed fer teak te tetale han Ae IES See aE) Se ee A vil 
MB ETOLEIT CLIO Ti eee ore ce Meet Ae agree eee Panay Elo ac pollen cedicw co lagere a yim at aca le g's aise Sho 1 
Chapter I. Apparatus, methods and purpose of investigation............. 7 
HPT OTN aa alee CaN oss os cu ahh COMME Teas msimMod cet alt cohen al asad Bletey Meus argos Sze "bora 7 

Hee RETINS) FeNd OLN TUG ices ora) 4) Wicvieve ian «des chore eter aio fateh « lols ei alavece IN cas nee 8 aie As Mea Me eas 9 
Bet ed tec aP A sian scat ey ofc sh omens Med neh a2) ye Galt su eee) ce vesel aaa arena ear ease, aia s 12 
Bie ran Oe le IMC LIMOS oe) Secor a celeb ages be tombe ja water evensanpacca el ey anntend mvarteh seme eke Soa e 16 
EAE OSCH Oe AIUVE SUNS A LUO, vehi or -ucloj storeys Wit fale set'n deelsr'ap ePellpandis-suala ache th oe ele le 17 
Chapter II. The net plankton of Lake Mendota......................04. 20 
ETON Shr a bil Ce HOM sys \cts oc cote iedateyotaretes «!eoni was (ar lol spice aie) 1a Waker ents eR eteiicles2 20 
Cuan or pwaper and: Ot splamlebome ys 07 Yeh) Serer aha slaie ete oie wlame a) ec 23 
Weiecee VCCMOLOG age ares shcie ten Ui aeapacis oaetenoh sa hece uy tateos rans) teacba Gre eae io Re etala Gs & Grae 25 
Ocroquice Matern das Net, law VOM: ai le\atdepainils pease eel ee e Waleie Oe sb eae 25 
Ohrensie nl sres MLS ae ccd st eee tebe ees eal sy sed che Ge waaay Seis t Ng oa ae 31 

Nitro Ceny and seride WEOLCLM sce ces tes ciacic se 6. sek a eiee wg ee es al 

SEH MOWS ORT A Ch). <tr selma Reet Badat bal toh ace tobelot a, o,5, 53/6, sc ee voce beh areae wom a's 40 

Mano aUavess ) inte amiwcda raped feist mds) Asie « an ls aed ge Gleee ates S 42 

RUMOR GED rane tie LG ca hanes Acts ape Sahn s aes Go ena Ba coenTOLG 43 
INGPFOCeRCEne en OR ENACE Macrae: codutat Metall eteisia s.2.a. 9 die: eehee ene eae eee a 44 

NSTI eee a eunal ciate atonery ayaa wher ater? drakgrac ee ature nig Wein erate ce hae, eee Be uh 45 
SouostiwemlsriOle asi Loa coer eats che wae a aet we eek, Gk a ieee ee 52 

ReSUPiS Oke cOMer: MAVEN tL LORS: .65)., ac a/e,0 4 4 aie oe se ba, 6 8 Cele eae 54 

OATS MAS 1 ee toletatchon eres lomtee! bale aa ctaiet Schaef elaig ers ais 6S 6 bls etki Sriete 5d 

Chapter III. The nannoplankton of Lake Mendota....................... 62 
MEG COMO kn GIN LEM MOTO a arena anh wel td Craik cee S Maas Ki pee, bres Moe eee ints. 4) ol hee 64 

Gh INCA nia OMe LOREM ete epeiene deSak sr oot cot aul aia chal cy Sek. Ma capcrore Sik lw ate Maal al af 66 

SO rye QTM AED OUgra so einen Siac at agar talelici/or'ebet afiay'al eta ai 5) oY a's: ear e's halte-sese oom eam ea geie le oy eae 67 
NaLRO eM: AMG CHUGLS |; PLO CEI Sa cp arco: cise roi ies hale Sia.e Gio aoe sstoyeue lee el cleorw d w cebenel lel 70 

JED SIEWSITe, SSD: cUra B21 Gh Me DG PN ad Fo. ee Re a GO gg gv EU a RE rt UR 75 
ge E OS ANUS Wy Peas of cy cater rey nts MRA SS GG eae Ata GJ Skis a Sea 1eiNS was galted nse hablo celle ben 79 
(CPTI s ET OVS gr a Ray Sd UN COR ea gene Re ae St SoA Oo 79 
NLR OCOUOEPEC FOX EEA Ct ste ee aeurd Goths ae gk 6 a ees aw inie clin lerh sdelel eed lietola) over abehepere te 80 
US MMe a ap leod sve Nah ee et aa eau ae Pe A a igi U5 Nose alae fay Oeetcnapomunaloe ata ei aiehg deg 80 
TATU STAUS NG aE ledtey tect ot ee Sener Ne ie (2 « Sl 2.15 east Rilobaha hehebetla avthehenes aohereuss 83 
HUGUE Oy WAOCEIT CN cfd eae RAN ee hha Soe ea itoic 6 Gk Bulb lo aye siiebet elolelaietuba spaie 88 
Chapter IV. The total plankton of Lake Mendota....................... 92 
VUE MEME OMS AIT CUA INLEGW wine tere jsvace dats cox) Sie ie ibe epee ba uw acteaa terest chal hey Waka ehoratonats 92 
Mcuniatlantiwhy and. chemical) Compositionl .. 645 ecco sea Oe delle a 97 
ROME OLOEE OEE MLM PATE Aiken eit Bucher mett se winucds eee telaie chap dal dea ds apena seal ara 98 


uch r OG COGLRG, + LA WON GRAIN RSE 2 eID A MENA mies coos Nae She Orn MR it x 2 102 


iv TABLE OF CONTENTS 


Page 

Chapter ‘V. The plankton’ of “Lake Monona). 0c, face acre ee ee 104 
ING ERM GIO Me cing eam Gedo oa dios Go ua Ioan wooo OM Db eo Ou ob Oooo OY 104 
Organisms, of net plamktone ss ee ice se mee ete cone rata ours sean ceae tense mete 110 
INannoplam kom yy is ete cies mua emes etc avemeienes arene lapel alcetcoual ae saan ma speye tareteue 112 
Organisms of nannoplankton.7. ste os tee = elie tee nie oe darole al ae icee eaenaeee 118 
Totalepplanilet omy 3 sic ae heualeeiers: ovens tele eee ele te weet ohenmitok ae leita ten el arog et eee naam are 119 
Organic aniatter: per unit area® occa sas 6 eo c.0 ss este ebensdone so oh lake enone eee 122 
Chapter VI. The plankton of Lakes Waubesa and Kegonsa.............. 124 
Tide Wa esa oo sc: i ies Gag 7s a cis eile ete etek lrae iolio meets dohemel et eohen cc aeneh aera nreae 124 
Net iplamk tom. siccs a5 5G). Wisstarcie aver sloiaevsnsue es cts Cherehopeetensist ewes wee Tete 124 
Organisms of net planktom .(i.: 05 Spseie’s) se slenctoele dekeh smal tore eh ieee 128 
INannoplamktom fe eas eels eene. acere he iota cisions ie eyes nemercigctic ee et ea nN eeraete 130 
Organisms of nannoplanktom 52% c<.ascccume etoile oo Cee eee 135 

Total; plank tomy 07s ees eis esa ate bile chesoun ciniea tone wee ee te eae sie el ee eae 137 

Lake) Re gonsa, 256 stiles eee as ace oc wl a ale tole Gaetrelts Rol eee te aor 139 
Chapter VII. General summary and discussion..................0eceeeaee 141 
Net plamktoni. ici. osieieie 5 in go Gaia e 0 Wiest ale aly nce eG tole ct a eee are ee eee cee 142 
INannoplanktom, ie os eee se a hal atc) ee nage arta tone eee ae ee 146 
Total: plan kG . ic. kc Gee ois 00s cole allot selena eile Site la ee a ete gaie ae ee ee 148 
Diseussion oof results css aes! s Ga e ene ee Sy ie aol ee ert eee 150 
Chapter VIII. Chemical analyses of various organisms................... 158 
PPVANGS feiss (eee a cetagteeaver sl ea raries «! 2b: cey & sl eilo estate) MeRle rece dct ee i at Aro a 159 
PANTING 9. Wa ratte oleae A alo oh cash eats ore en SIOQIeIN gael eee ke er 165 
Ash analyses icicle set sik ke a outlet ele pub atten tae ca ec Arenas aes ae et 179 
Appendix —— Tables. tie eats ce: weak MSR eo ones as Soo ee an Gee oe a 181 
ETO ae tao aie hoe Gc coe oie lotic esierios dake) a ch tala eT aru SUT: Goes RA a aN nth Ute 220 


LIST OF FIGURES 


Figure Page 
Peeaimen rand plankfON) Apparatus. ...0 966 sens ols etic ecian eed necuee nes 10 
SePE TUTE ERE Oe TOTO AT VEL SH © 1 s0n ol otes cleo an fale sca yote/a)ichais\ a Jelo\ajtay shea sicyslace 6 3 0 & 6 Sai e265 0% 10 
3. Sketch of lakeside laboratory showing centrifuge.................... 13 
fe scctional view of bow! of centrifuges. oii... bce ncaa c cae c ees Jo os pale 
pe EBVO GAL GEL cA, GOR EO) OL. AGC oi x lois oes oe) vtchin' oar 2) eves cy sreva)ovauese mio. 5 aie seaieie e eee 14 
Gmela Ob LOUr LAKeS, TESTO. 6 5. use eles a bleie ule 0 0 )o's Sock echwceucnoincc umn 21 
7. Organic matter in net plankton of Lake Mendota, 1911-14............ 26 
8. Organic matter in net plankton of Lake Mendota, 1915-17............ 29 
Peeamicocen of wet plankton, TOI I oe i ec eae ik eee ele we wlsletyele eae 33 

Bee atocen of net plankton, 1913-15... 0c... sek wa aim ete edergia wh celyere el 34 

11. Organic matter, crude protein, and ether extract of net plankton of 

Decree Mendota VOUES fase fare wae 5c Saude a eee siden he oh GPS cele. dene vole 36 

12. Organic matter, crude protein, and ether extract of net plankton of 

ire Nemo ticng ih OUR a on Sia ae taelevs aiidran ite se 606! 6. 6. eeu face Blase otaies «6a & 37 
13. Organic matter, crude protein, and ether extract of net plankton of 

ieee ley a > POTS ees irc cca Sse Ries Sinks Hele 8 66 asym win enelwsereveie oe « 38 
14. Organic matter, crude protein, and ether extract of net plankton of 

BEM cera eT Gl gear DBO areas usp nome men ie hie [oi ets eae ahaa) ors (m laiiole\ eon ecdiinta le, w alo ols 39 

15. Organic matter and ash of net plankton of Lake Mendota, 1911....... 46 

16. Organic matter and ash of net plankton of Lake Mendota, 1912....... 47 

17. Organie matter and ash of net plankton of Lake Mendota, 1913....... 48 

18. Organic matter and ash of net plankton of Lake Mendota, 1914....... 49 

19. Organic matter and ash of net plankton of Lake Mendota, 1915....... 50 

20. Organic matter and ash of net plankton of Lake Mendota, 1916-17.... 51 

21. Diagram illustrating construction of spherical type of curve.......... 59 

22. Numerical results for net plankton of Lake Mendota, 1911............ 60 

23. Numerical results for net plankton of Lake Mendota, 1912............ 60 

24. Numerical results for net plankton of Lake Mendota, 1913............ 61 

25. Numerical results for net plankton of Lake Mendota, 1915............ 62 

26. Numerical results for net plankton of Lake Mendota, 1916-17......... 62 

27. Photograph of dry nannoplankton material obtained in 1917......... 66 

28. Organic matter in nannoplankton of Lake Mendota, 1915-17.......... 68 

29. Nitrogen in nannoplankton of Lake Mendota, 1915-17................ 72 

30. Organic matter, crude protein, and ether extract in nannoplankton of 


Aa OMe Ger EO ROS Sac Giteteeiers ctv a ts Sal aie Mee me Hers ete Me atlas & % 76 


vi LIST OF FIGURES 
Figure Page 
31. Organic matter, crude protein, and ether extract in nannoplankton of 
Inake: eviendo tas Oma ieee rvchcnsis clever cleats aed cyemanct a Nokcue ee weoitey -iepertcr at toe tol 
32. Numerical results for nannoplankton of Lake Mendota, 1915.......... 90 
33. Numerical results for nannoplankton of Lake Mendota, 1916-17....... 90 
34. Organic matter in net plankton, nannoplankton, and total plankton of 
Joake:; Mendota VOUS. soe. tise ee Sele mieaenenen ied eet ta mena l ch tag anne aoa 94 
35. Organic matter in net plankton, nannoplankton, and total plankton of 
Lake: Mendota POUG+ET cog ces ON e os ais biotin leeelenoiay cheno el eh ea eee 95 
36. Monthly distribution of total plankton in deep part of Lake Mendota, 
MOTEL aera tered shpat s se elie ve 1g 1b esa a dle wite fo ede tel eile laine bets NEallce store at ence een ae 99 
37. Organic matter in net plankton of Lake Monona, 1913 and 1916, and in 
nannoplanktom) 1n< TO1G ioe. ome eee eee ee encarta eee ae 106 
38. Organic matter and crude protein in nannoplankton of Lake Monona, 
VOD G 5 aio) 5:6 obai a toi Nase ae Ee ile je vale eed does: Ste elcieel stat at outed «eueemea oh arcs ee 116 
39. Organic matter in net plankton, nannoplankton, and total plankton of 
hake |Waubesa ime 3 OUGee wo Uli ees een nee Ue aon eee 125 
40. Organic matter and crude protein in nannoplankton of Lake Waubesa 
BU OG ee ase SR ee ee Ee Ie RO ti ten aan tas oe 133 


PREFACE 


The present bulletin is the third issued by the Wisconsin Survey 
under the general title of ‘‘The Inland Lakes of Wisconsin’’. The 
first bulletin of this series, No. X XII, was published in 1911. It was 
based upon results obtained in a quantitative study of the dissolved 
gases in about 150 lakes situated in different parts of the state; mineral 
and sanitary analyses of the waters of a few of these lakes were made 
also. In addition to the chemical studies, temperature obesrvations 
were made at different depths in the various lakes when the samples 
for the determination of the dissolved gases were secured. This in- 
vestigation, therefore, comprised a general study of the thermal and 
chemical conditions which existed in these lakes at different seasons of 
the year, especially in the summer, together with a consideration of 
the biological significance of these conditions. 

The second bulletin of the series, No. XX VII, was published in 1914. 
It deals with the physiography, hydrography and morphometry of 54 
lakes in southeastern Wisconsin, on which complete hydrographic sur- 
veys have been made; it also contains data regarding 185 other lakes 
in the state, chiefly their length, breadth, and maximum depth. 

The present report presents the results of quantitative and chemical 
studies of the plankton of four Wisconsin lakes, by far the greater 
part of the work being confined to Lake Mendota. Only a single 
observation, in fact, was made on one of these lakes, namely Lake 
Kegonsa. So far, these studies have not been extended to any other 
lakes of the state because large samples of water had to be strained or 
centrifuged in order to obtain enough plankton material for the chem- 
ical analysis. This made it necessary to use large pumping and cen- 
trifuging outfits which could not be readily transported from lake to 
lake. In fact, a permanent lakeside laboratory had to be established 
in order to carry on these studies. Recently, however, a portable type 
of continuous acting centrifuge has been constructed and this instru- 
ment can be used to ascertain the quantity of organic matter in the 
plankton of many other lakes of the state. With this small machine, 
enough plankton material may also be obtained for some chemical 
determinations. 

The authors wish to acknowledge their great indebtedness to Dr. 
H. A. Schuette, assistant professor of chemistry in the University of 


viii PREFACE 


Wisconsin, who has had direct charge of all of the chemical work dur- 
ing this investigation. He has made substantially all of the nitrogen 
determinations, as well as many of the other organic analyses. Some 
idea of the magnitude of the chemical work may be obtained from the 
tables accompanying this report. The results given for nitrogen, ether 
extract, pentosans, and crude fiber in these tables are based on 1,249 
separate chemical determinations; there are 425 quantitative results for 
ash and 519 different determinations of inorganic constituents of the 
ash. This represents a total of 2,193 separate determinations, many 
of which were made in duplicate. In addition to this number, many 
analyses were made which are not shown in the tables, such as ascer- 
taining the quantity of nitrogen in the bowl water and in the filter 
papers of the centrifuge as well as more complete analyses of the water 
remaining in the bowl of the centrifuge at the end of a run; the chem- 
ical results presented in this bulletin, therefore, are based upon a total 
of not less than 2,500 separate determinations, excluding the dupli- 
cates. 


During the period covered by these studies Dr. Schuette has re- 
ceived assistance in the chemical work from a number of individuals. 
Mr. N. A. Bailey, Mr. Geo. M. Bishop, the late Mr. A. J. Duggan, Mr. 
E. A. Hentzen, Mrs. Ethel Hoverson Miller, and Mr. G. G. Town may 
be mentioned in this connection; other persons have also given assist- 
ance for varying periods of time. 


While engaged in this work Dr. Schuette analyzed some special 
catches of plankton and his results were embodied in a paper which 
appeared in the Transactions of the Wisconsin Academy of Sciences, 
Arts, and Letters, Vol. XIX, 1918, pp. 594-613, under the title of ‘‘A 
biochemical study of the plankton of Lake Mendota’’. 

In the collection of plankton material Mr. W. S. Fusch, Mr. EH. H. 
Toole, and several other individuals rendered efficient assistance. 

The arduous task of counting the organisms in the net plankton 
was done chiefly by Mrs. Henrietta Achtenberg Ryall and Miss Dorothy 
von Toerne. The nannoplankton organisms were enumerated by C. 
Juday. 

The junior author is responsible for the preparation of the tables 
and diagrams and for the writing of the report, but the work was 
reviewed in all stages by the senior author. 

The numerical results relating to the net plankton of Lake Mendota, 
shown in figures 22 to 26 and discussed in the latter part of Chapter 
IT, are not presented in detail because they represent only a part of the 
material of this character which has been secured during the past few 
years. In the various investigations that have been made on Wiscon- 
sin lakes since 1905, approximately 10,000 catches of net plankton have 


PREFACE ix 


been taken for the purpose of ascertaining the number and distribution 
of the organisms therein. So far the planktonts in about half of these 
samples have been enumerated; when the counting is completed, a bul- 
letin dealing with this phase of the work in greater detail will be pre- 
pared. 

The United States Bureau of Fisheries granted financial assistance 
toward this investigation and grateful acknowledgment is extended for 
this valuable aid. A more thorough study of the various problems 
was made possible through this assistance. The results are published 
with the permission of the Commissioner of Fisheries. 


INTRODUCTION 


The following bulletin is based upon data obtained in an extensive 
quantitative and chemical study of the plankton of four lakes situated 
in the vicinity of Madison, Wisconsin. (See map, fig. 6.) The in- 
vestigation dealt chiefly with the plankton of Lake Mendota, while the 
observations on the other three lakes were made for purposes of com- 
parison. | 

The investigation was undertaken for the purpose of securing definite 
gravimetric data relating to the size of the plankton crop at different 
seasons of the year and also for the purpose of obtaining some notion 
of the food value of this plankton material. Such data were desired 
not only for the larger organisms which can be readily secured with 
a plankton net but also for the small organisms which easily escape 
through the meshes of the net. These two groups of organisms con- 
stitute the total plankton and it is very important to know just what 
part of the total each group contributes; some of the data presented in 
this report show this relation very clearly. 

The annual cycle of physico-chemical changes which take place in 
the waters of these lakes has a very important bearing upon the crop 
of plankton. This cycle is separated into four phases which correspond 
closely to the four seasons of the year. The disappearance of the ice 
in the spring is followed by an overturning and complete circulation 
of the water. The temperature of the water rises as the season ad- 
vanees and the vernal period of complete circulation is finally termin- 
ated by the rapid warming of the upper water; that is, the upper 
stratum becomes so much warmer, hence lighter, than the lower water 
that the wind can no longer force the former down and mix it with the 
latter. This results in a thermal stratification of the water; evidences 
of this phenomenon appear in the latter part of May, but the strata 
are not very clearly defined until late June or early July. 

Two of these lakes, namely Waubesa and Kegonsa, are so shallow 
that the water is not permanently stratified during the summer. Dur- 
ing the summer period of stratification in Lakes Mendota and Monona 
there is a warm upper stratum, the epilimnion, which is kept in cir- 
culation by the wind and a cool lower stratum, the hypolimnion; be- 
tween these two there is a relatively thin stratum, the mesolimnion or 
thermocline, in which the temperature of the water changes rapidly 
from that of the warm epilimnion to that of the cool hypolimnion. As 
the temperature of the epilimnion falls in the autumn the mesolimnion 
descends to greater and greater depths and this finally results in the 


24 PLANKTON OF WISCONSIN LAKES 


autumnal overturning of the entire body of water. Complete circula- 
tion of the water is again established and it continues until the lakes 
become covered with ice. The ice prevents further circulation of the 
water and this results in indirect stratification; that is, the coldest 
water is at the surface and the warmest at the bottom. Direct stratifi- 
cation is found in the summer when the warmest water is at the sur- 
face and the coolest is at the bottom. 


During the two periods of complete circulation the substances that 
are dissolved in the water are uniformly distributed from surface to 
bottom, but in the intervening periods of stratification the chemical 
conditions in the lower water become very different from those in the 
upper stratum. These differences are especially marked in late sum- 
mer. The hypolimnion, for example, is cut off from contact with the 
air by the epilimnion and the mesolimnion so that its supply of dis- 
solved oxygen is limited to the amount which is held in solution by this 
water at the time stratification takes place. For a certain time this 
stratum is populated by many animals which use up some of the dis- 
solved cxygen in their respiratory processes. Decomposition is a still 
more important factor in exhausting the supply of oxygen in the 
hypolimnion. Through these two agencies, therefore, practically all 
of the free oxygen in this lower stratum is used up by the first of 
August, and a further supply is not obtained by this water until the 
autumnal overturn takes place. During the winter period of stratifica- 
tion, also, the oxygen may be exhausted from a bottom stratum of 
varying thickness. 

Respiration and decomposition not only make inroads upon the 
supply of dissolved oxygen, but they also contribute certain products 
to the water, such as carbon dioxide and nitrogen compounds, for ex- 
ample; the latter remain in the lower strata until an overturning of 
the water takes place. These products may then be taken up by chloro- 
phyl bearing organisms and again play an important role in life proc- 
esses. | 

The following discussion regarding seasonal changes in the crop of 
plankton refers to the results obtained on Lake Mendota because no 
observations were made on the other lakes during the winter and early 
spring. During the vernal period of complete circulation the water 
becomes well aerated at all depths, the dissolved substances are evenly 
distributed, and the temperature of the water gradually rises; the 
plankton organisms respond to these favorable conditions by producing 
a large crop of material at this time. The vernal crop, in fact, proved 
to be the largest one of the year. 

When the water is in complete circulation conditions are favorable 
for the growth of the various organisms at all depths, but as soon as 


INTRODUCTION 3 


the bottom stratum in the deeper water ceases to take part in the gen- 
eral circulation the chlorophyllaceous organisms are at a disadvantage 
because the light is insufficient at these depths for photosynthesis. 
When stratification is finally completed the entire hypolimnion becomes 
unsuitable for these organisms owing to the scarcity of light in this 
stratum; these forms, therefore, are restricted to the epilimnion, or in 
some instances they may extend into the mesolimnion also. 


The disappearance of the dissolved oxygen from the lower stratum 
also restricts the volume of water that can be occupied by some of the 
plankton organisms, such as the rotifers and the crustacea. Substan- 
tially all of the water of the hypolimnion is without free oxygen in the 
month of August so that only those forms which are able to live under 
anaerobic conditions can occupy this region of the lake at that time. 
The hypolimnion, therefore, yields a relatively small amount of plank- 
ton material during this month because the scarcity of hght excludes 
the chlorophyllaceous organisms and a lack of oxygen excludes the 
rotifers and crustacea; the hypolimnion is about twelve meters thick 
at this time so that these organisms are absent from about half of the 
maximum depth of the lake. Following the spring maximum there is 
a gradual decrease in the total quantity of plankton which is correlated 
in time with the thermal stratification and the subsequent disappear- 
ance of the oxygen in the hypolimnion; the minimum amount for this 
season is usually found in August. 

The autumnal overturning and cireulation of the water again re- 
lease the decomposition products which have accumulated in the hypo- 
limnion during the summer and the entire body of water becomes 
aerated, thus making the conditions favorable for the various plank- 
tonts at all depths. These changes are accompanied by an increase in 
the quantity of plankten which culminates in an autumn maximum in 
late September or in October. Following this maximum there is a 
geradual decline to a winter minimum which is reached in February or 
in March; the winter minimum is somewhat smaller than the summer 
minimum. Among the various constituents of the plankton the dia- 
toms seem to respond most vigorously to the favorable conditions that 
obtain in the spring and in the autumn; consequently they are the 
chief forms concerned in the production of the vernal and the autumnal 
maxima. 

Computations based on the area and the total volume of Lake Men- 
dota show that the largest crop of spring plankton yielded 404 kilo- 
crams of dry organic matter per hectare of surface (360 pounds per 
acre), while the largest autumn crop gave 363 kilograms per hectare 
(324 pounds per acre). The smallest summer minimum amounted to 
139 kilograms of organic matter per hectare (124 pounds per acre) and 


4 PLANKTON OF WISCONSIN LAKES 


the smallest winter minimum to 110 kilograms per hectare (98 pounds 
per acre). The average amount of organic matter yielded by the 
entire series of plankton catches from Lake Mendota was 240 kilo- 
grams per hectare (214 pounds per acre) and those from Monona gave 
267 kilograms per hectare (238 pounds per acre). Lake Waubesa 
yielded substantially the same average as Lake Mendota. These figures 
represent the weight of the dry organic matter in the plankton; the 
weight of the organic matter in the living state would be approximately 
ten times as large because water constitutes about 90 per cent of the live 
weight of these organisms, excluding the ash. 

The quantities indicated above represent only the standing crop of 
plankton and not the amount of this material that is produced an- 
nually. A quantitative determination of the annual production of 
plankton is a very complex problem because the factors involved are 
so numerous and so diverse. The plankton itself is an assemblage of 
many different kinds of organisms, ranging from the more simple forms 
such as bacteria, protophyta, and protozoa to the more complex in- 
dividuals such as crustacea and insect larvae; a single catch, for ex- 
ample, may contain as many as fifty or more different kinds of organ- 
isms. It is a very difficult matter, therefore, to determine the relative 
importance of the different forms in this plankton complex. 

The various organisms multiply at very different rates also; under 
favorable conditions the bacteria may pass through several generations 
in the course of a single day, the protophyta and protozoa perhaps not 
more than one or two, while the crustacea may require two weeks or 
longer to pass from one generation to the next. These organisms differ 
enormously in size, ranging from the minute bacteria to forms that are 
approximately two centimeters in length. 

Another complexity is introduced into the problem through the food 
relations of the various plankton constituents. The greater part of 
these organisms possess chlorophyl so that, with the aid of sunlight, 
they are able to manufacture their own food materials out of the sub- 
stances dissolved in the water. Therefore these forms constitute, either 
directly or indirectly, the main source of the food for the other plank- 
ton constituents; the chlorophyl bearing forms also constitute a large 
part of the food of some non-plankton forms, such as the bivalve mol- 
lusks. The rotifers and the crustacea are the chief plankton forms 
which feed upon the chlorophyllaceous constituents and they, in turn, 
are fed upon by fishes; also certain crustacea prey upon others. Com- 
putations based upon the numerical results and upon the average 
weights of the various forms of rotifers and crustacea show that the 
other plankton constituents furnish an abundant supply of food for 
these two groups of organisms; that is, the material which can be used 


INTRODUCTION 5 


as food by them weighs from twelve to eighteen times as much as they 
do. In making an assessment of the annual crop of plankton, there- 
fore, it is necessary to take the destructive processes into account as 
well as the productive processes. 

In order to estimate the annual production of plankton it would be 
necessary to determine the approximate rate of turnover in this ma- 
terial, but the present data show only the actual quantity of organic 
matter yielded by the plankton at the time the various observations 
were made. The results show, however, that the greater part of the 
material was derived from the protophyta and the protozoa; these two 
eroups of organisms reproduce at a rather rapid rate so that the turn- 
over in that part of the stock which they contribute would be equally 
as rapid. ; 

Numerical data were also obtained for the entire series of plankton 
catches, but they can be given only a general consideration at present. 
The average weight per individual has been ascertained for only a few 
of the characteristic plankton forms so that an assessment of the rela- 
tive values of the different constituents cannot be made until further 
determinations of this kind have been made. If the average weight 
per individual were known for all of the different kinds of organisms 
that make up the plankton, the numerical results would make it possible 
to determine roughly, at least, the relative importance of the different 
forms which make up the plankton complex. 

The chemical analyses show that, on an average, the crude protein 
constitutes from a little more than 44 per cent to more than 57 per cent 
of the dry organic matter in this fresh-water plankton; in this respect 
the material compares very favorably with some of the meats that are 
used for human food. Crude protein constitutes about 47 per cent of 
the dry organic matter in the edible portion of the hind quarter of 
beef, for example, and about 37 per cent of that in the hind quarter 
of mutton. This plankton material, therefore, must be given a high 
rank as a source of protein food for other organisms. The edible por- 
tion of fish, however, contains a higher percentage of protein than 
either beef, mutton, or plankton; in the black bass, for example, the 
erude protein makes up about 92 per cent of the dry organic matter of 
the edible portion and in the brook trout about 90 per cent. 

The plankton yields a relatively small amount of fat or ether extract, 
averaging from about five per cent to somewhat more than seven per 
cent. This is comparable to the percentages in black bass and brook 
trout, for example, which are, respectively, eight per cent and ten per 
cent of the dry organic matter in the edible portions. The percentage 
of fat is much larger in beef and mutton; it amounts to 53 per cent of 
the dry organic matter in the edible portion of the hind quarter of 


6 PLANKTON OF WISCONSIN LAKES 


beef and to 63 per cent in the hind quarter of mutton. On a fat free 
basis, then, the beef and mutton would yield a much larger percentage 
of crude protein than the plankton. | 

While this investigation has involved a rather large series of plank- 
ton observations and a fairly detailed quantitative and chemical study 
of the material that was secured, yet it can scarcely be regarded as more 
than an introduction to this fertile field of research. Not only should 
such studies be extended to other types of lakes for purposes of com- 
parison, but it should also be the aim to secure data that would even- 
tually make it possible to estimate the annual production of plankton 
material with some degree of exactness. The first problem presents no 
special difficulties, but the second one is very complicated. The latter 
invelves a study of the rate of reproduction and the length of life of 
the various plankton constituents in their natural environment; such 
studies would also include investigations relating to the effect of the 
physical and chemical factors of the environment upon these organisms. 
The food relations among the plankton organisms themselves are in- 
cluded in this question, as well as those between the plankton and non- 
plankton forms. It is necessary, also, to ascertain the average weight 
of the different kinds of planktonts in order to determine the relative 
values of the different forms. Numerous factors, therefore, are in- 
volved in this problem; some of them are physical, some chemical, and 
some are biological in character. 


METHODS AND APPARATUS Gi 


CHAPTER I 


APPARATUS, METHODS AND PURPOSES OF THE 
INVESTIGATION 


HISTORICAL 


The plants and animals that inhabit the open waters of ponds, lakes, 
streams, and the oceans constitute what is known collectively as 
the plankton, A multitude of organisms is included under ‘this 
term, ranging, in fresh water, from forms as simple as the bacteria to 
forms as complex in structure as crustacea and insect larvae. The 
fresh-water planktonts vary in size from minute bacteria to organisms 
that are 15 millimeters or more in length, but the vast majority of 
them are microscopic in size. The plants float freely in the water and 
are subject to the action of waves and currents. Most of the animals, 
on the other hand, are more or less active swimmers, but they are not 
powerful enough to be independent of the waves and currents so that 
their distribution is also governed chiefly by the general movements 
of the water. 

The insect larvae and some of the plankton crustacea have been 
known to scientists for two and a half centuries, but the smaller plank- 
ton organisms had to await the development of the compound micro- 
scope. Exception must be made of some of the smaller forms, more 
especially some of the algae, since they became familiar to the layman 
as well as to the scientist without the aid of the microscope because they 
frequently appear in great abundance. In the quiet waters of lakes 
these algae may become abundant enough at certain times of the year 
to form a thick scum on the surface of the water; this is the so-called 
‘‘water bloom.’’ 

In spite of the fact that biologists became familiar with many of 
these forms at a comparatively early date, no attempt was made to 
study the plankton from a quantitative standpoint until 1882 when 
Hensen * undertook such investigations on the Baltie and North seas, 
and on the Atlantic ocean. His studies extended from 1882 to 1886 
and the publication of his results in the latter part of 1887 stimulated a 
great interest in both marine and fresh-water investigations of this 
character. As a result a voluminous literature upon this subject has 
appeared in the last three decades. 


*Finf. Ber. Kom. zur wissen. Untersuch. d. deut. Meere, 1887, pp. 1-107. 


8 PLANKTON OF WISCONSIN LAKES 


In obtaining this plankton material Hensen used nets which had 
straining surfaces made of bolting cloth and, in the various nets, bolt- 
ing cloth with different sizes of mesh was used, the finest being the 
No. 20. These nets were lowered into the water to the desired depth 
and were then hauled to the surface at as uniform a speed as possible, 
thus passing through a definite column of water. This is known as 
the vertical haul method. The whole column of water through which 
the net passes is not strained, however, since the straining part of the 
net offers some resistance to the passage of the water and a certain 
portion of it is pushed aside and not strained. This makes it necessary 
to determine the efficiency of the net, or the coefficient, which serves as 
a factor for calculating the total number of organisms in the column 
of water. This method is subject to a serious error in that this 


coefficient varies with the age of the net and with the abundance of the 


plankton. The silk bolting cloth is subject to a certain amount of 
shrinkage and its meshes also become more or less clogged with organ- 
isms which adhere to the net permanently in spite of careful washing. 
During the progress of the haul, the meshes become temporarily 
blocked with organisms, especially when the plankton is abundant, so 
that the efficiency of the net decreases as it approaches the surface. 
Hensen determined the quantity of plankton in his net catches in 
three ways, namely, (a) by allowing it to settle, (b) by enumeration, 
and (c) by evaporating an aliquot part of the catch to dryness, weigh- 
ing it, and then igniting it to ascertain the organic matter. In the 
settling method the material was placed in a graduated cylinder and, 
after standing a definite number of hours, a reading of the volume was 
taken. Such readings, however, have little value since the different 
organisms do not settle with the same degree of compactness. In the 
enumeration method a definite portion of the material was placed in 
a suitable counting tray and the organisms in a certain number of 
definitely marked off squares were counted. From the results thus 
obtained the total number of the various organisms in the catch was 
estimated. The numerical method is still extensively used by plank- 
tclogists and it yields very valuable data concerning the abundance 
and distribution of the various plankton constituents. The method of 
drying, weighing, and igniting is also a valuable one since it gives im- 
portant data respecting the proportion of organic material in the catch. 
As work in this field of science has progressed new types of nets 
have been devised and new methods have been employed for the pur- 
pose of obtaining more accurate results. In order to ascertain more 
accurately the quantity of water that is strained for a catch the pump 
method was introduced. By means of a pump and hose.a definite 
amount of water may be obtained from different depths and strained 


METHODS AND APPARATUS 9 


through a plankton net for a catch. In addition to yielding a definite 
quantity of water this method also enables one to study the vertical dis- 
tribution of the organisms. The chief objection to it is that the cur- 
rents produced in the water at the intake end of the hose tend to drive 
away the more active planktonts that are negatively rheotropic. Sev- 
eral sets of experiments made with a plankton trap and the power 
pumps showed an advantage in favor of the trap of 16 per cent in 
Diaptomus and of 15 per cent in the Daphnias; on the other hand, there 
was a numerical advantage in favor of the power pumps amounting 
to 12 per cent in Cyclops and 27 per cent in the nauplii. While the 
latter forms are smaller than the former, they are usually more numer- 
ous so that the gains and losses probably just about balance each other 
in so far as the weight of the net plankton is concerned. In addition 
to this objection the pump method is not practicable beyond a depth of 
about 75 meters. 

At a comparatively early stage in the development of quantitative 
plankton studies it was found that even the finest meshed nets did not 
retain all of the organisms, but the extent of this loss was not fully 
appreciated until Lohmann? completed extensive investigations along 
this line and published his results in 1908. In studies on marine plank- 
ton he was able to show, by the use of the centrifuge and various filtra- 
tion methods, that many organisms which were present in enormous 
numbers were not represented at all in the net catches. It has since 
been found that the same is true of the fresh-water plankton as well; 
in fact, results presented in this report show that the organic matter 
of the material which is retained by the net is frequently only a small 
percentage of the organic matter in the material which readily passes 
through the meshes of the net. A very important problem in this in- 
vestigation, then, has been to ascertain the relation between the quan- 
tity of plankton material that is retained by the net and that which is 
lost through its meshes. 

In addition to these gravimetric data, some chemical analyses of the 
material were desired. For the latter it was necessary to obtain rather 
large samples of dry material, at least five grams whenever such 
amounts could be secured. It was soon found that this large a sample 
of net plankton made it necessary to strain fairly large amounts of 
water, such as several thousand liters in some instances, and the ap- 
paratus employed had to meet these requirements. 


Pumps AND NET 


For the net plankton, in fact, the quantity of water that was strained 
for each sample varied from a minimum of 2,000 liters to a maximum 


2 Wissenschaftliche Meeresuntersuchungen. Bd. 10, 1908, pp. 131-370. Kiel. 


10 PLANKTON OF WISCONSIN LAKES 


of 38,000 liters, depending upon the abundance of the organisms and 
the size of the sample desired. In the great majority of instances, 
however, the quantity was between 10,000 liters and 20,000 liters. The 
apparatus shown in figure 1 was used in securing these large quantities 
of water from various depths. Figure 1 shows the launch with the 
apparatus in place ready for a run, except that the large plankton net 
suspended in the bow of the launch is hung inside the large can during 
the run. 


Figure 2 gives a more detailed view of the pumping outfit. It con- 
sists of two small vane pumps each having a capacity of about 30 
liters per minute at a speed of 300 revolutions. These pumps are 
operated by a small gasoline engine, the kind used for the ordinary 
milk separator. The engine and pumps are mounted on a substantial 
metal base in order to hold them firm and rigid while in operation. 
The engine is attached to the base with bolts so that it may be readily 
removed for convenience in loading it into the launch. 


The water is obtained from different depths by means of two pieces 
of hose, each 30 meters long and with an inside diameter of 2.5 centi- 
meters. <A calibrated line is attached to the intake end of each hose 
which enables one to lower the hose to the depth from which water is 
desired. The water delivered by the pumps is strained through the 
large plankton net that is suspended inside the large can shown in 
figures 1 and 2. The stream of water from the discharge hose is not 
allowed to strike the straining part of the net directly because this 
would result in many organisms being forced through the meshes. 
When only a net catch is desired the overflow water is discharged over 
the side of the launch through an outlet pipe that is attached near the 
bottom of the can to facilitate the straining process. By means of a 
pet cock attached to the can at about its mid-height samples of the 
strained water from the various depths can be obtained for a study of 
the organisms that escape through the meshes of the net. 


The straining cone of the plankton net is made of No. 20* bolting 
eloth; when thoroughly shrunken this silk gauze possesses more than 
6,000 meshes per square centimeter, with the area of the openings 
varying from 0.001 sq. mm. to 0.003 sq. mm. The straining cone of the 
net is 30 centimeters in diameter at the upper end and it is 70 centi- 
meters long, thus furnishing a large straining surface. 


* Lohmann states (Internat. Revue, Bd. IV, 1911, p. 38) that a new system of 
numbering was adopted by six Swiss manufacturers of bolting cloth in 1907, and 
that the old No. 20 was changed to the new No. 25. Two American dealers had 
not received notice of this change up to 1919, and a sample book of Schindler’s 
‘‘Genuine Swiss Silk’’ obtained in this year still shows the old system of 
numbering. 


gp geno Se A AI IIN CS 


a 


Fig. 1—Launch and plankton apparatus. 


Fig. 2.—Pumping apparatus. 


Se, 


4 
ey 
5 § 

’ 


dy 


METHODS AND APPARATUS. 11 


Work was begun on the net plankton in the latter part of May, 1911, 
and it was continued until July 1, 1914. During the first two winters 
no attempt was made to obtain material while the lake was covered 
with ice, but during the mild winter of 1913-14 catches were made each 
month. Owing to the late freezing of the lake the regular weekly 
observations were continued with the launch until December 24, 1913, 
and mild periods in the following months made it practicable to obtain 
one ecateh each in January and in February, 1914, and two in March. 
Following this the regular weekly observations were begun on April 
18, 1914, and they were continued until July 1. In 1915 work was 
resumed on the net plankton on April 21 and was continued until 
June 1, 1917. In 1916 and 1917, however, only the relatively small 
amounts of water used for the centrifuge catches were strained for 
the net plankton. 

The observations were made at a station where the water of Lake 
Mendota reaches a depth of 23.5 meters, a buoy being placed at this 
point each year to mark the place. Except in late summer the catch 
covered the entire depth of the lake down to 21 meters, either meter 
by meter or at two meter intervals. That is, the intake end of one 
hose was placed a few centimeters below the surface while that of the 
other hose was lowered to one meter. They were kept at these depths 
for a definite interval of time, usually ten to twenty minutes, while 
the pumps were in operation. Then they were lowered to two meters 
and three meters respectively for the same period of time, and so on 
to a depth of 21 meters. Attempts were made to get nearer the bottom 
than 21 meters, but bottom debris was found in the deeper catches so 
frequently that these attempts were finally abandoned. After the 
vernal and autumnal overturns the plankton was uniformly distributed 
throughout the entire depth of the lake for a while and at such times 
the depth was covered at two meter intervals. 

In late summer the lower water, that is below 15 meters, contained 
so little net plawkton that it was omitted from the catches. In all 
such catches, however, correction has been made for this and the results 
given in the tables are based on a depth of 21 meters. The pumps were 
calibrated frequently and their speed was taken during each run at 
the various depths, that is, during each ten to twenty minute period, 
so that the quantity of water strained was readily determined with 
a fair degree of accuracy. 

In most instances the plankton was removed from the net at the end 
of each ten or twenty minute run in order to avoid undue clogging of 
the net and the consequent loss of organisms while the material was 
being concentrated in the bucket of the net. After filtering off as 
much of the lake water as possible the material was transferred from 


12 PLANKTON OF WISCONSIN LAKES 


the plankton bucket to a jar with distilled water from a wash bottle. 
The material was then preserved with a few drops of chloroform and 
the catch, when completed, was placed in a porcelain dish and evapor- | 
ated to dryness on a sand bath at a temperature of about 60° C. The 
dry material was carefully removed from the dish, ground up in a 
mortar, and transferred to a weighing dish. It was then kept in a 
desiccator for twenty-four to forty-eight hours, depending on the size 
of the sample, after which the weight of the entire sample was ascer- 
tained. Thereafter the material was transferred to a bottle and kept 
for further study. 

At the end of the run made at each depth 10 liters of water were 
strained through a small plankton net and this catch was used for a 
numerical study of the various plankton constituents. Once each 
week these small catches were kept separate for the purpose of deter- 
mining the vertical distribution of the organisms. At other times all 
of these small catches were combined into one sample. In order to 
obtain a check on this method of determining the total number of 
organisms in the large catch, samples for counting were taken from the 
large catch itself and the volume of the material was measured when 
the catch was placed in the porcelain dish for evaporation. With the 
exception of a few forms which were not well preserved by the chloro- 
form, the two methods checked as closely as could be expected, the 
differences usually being well within the limits of error of the methods. 


CENTRIFUGE 


As the work progressed on the net plankton it became increasingly 
evident that the material which was lost through the meshes of the net 
should be studied in a similar manner. It was a simple matter to 
take samples of the overflow water, centrifuge them, and enumerate the 
organisms therein, but this gave no results that could be compared 
directly with those obtained for the net plankton. What was desired 
was some method by which these minuter organisms could be obtained 
in sufficient quantities to ascertain their weight per unit volume of 
water, and to permit some studies relative to their chemical composi- 
tion. Such data would thus permit direct comparisons with the results 
obtained for the net plankton. 

Various filtration experiments made during the summer of 1914 
showed that the problem could not be solved by such methods because 
the various substances used for the filtering process soon became 
clogged and thus permitted the use of only a small amount of water, 
not more than two or three liters, usually less. But these experiments 
served to stimulate a greater interest in the problem since the results 
showed that the organic matter in the organisms lost by the net was 


13 


METHODS AND APPARATUS 


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14 PLANKTON OF WISCONSIN LAKES 


three to four times as great as that in the material retained by the net. 

Following these experiments some tests were made with a milk 
clarifier and these proved to be very successful. While this machine 
did not remove all of the organisms, yet it removed such a large part 
of them that the experiments were continued with a larger and more 
powerful centrifuge which gave such satisfactory results that this type 
of machine was selected for the investigation. This centrifuge is the 
De Laval clarifier and filter which is intended for clarifying oils and 
varnishes; the belt style, size A, was used and in this machine the 
water is first subjected to a strong centrifugal action and then filtered 
through filter papers. During the progress of the investigation the 
effluent from this machine was tested frequently in order to ascertain 
what proportion of the organisms that were lost by the net, were being 
recovered by the centrifuge. These tests were made with the small 
centrifuge employed for the sedimentation of the material on which 
enumerations were made; the results obtained in these experiments indi- 
eated that the large centrifuge recovered substantially 98 per cent of 
the algae and protozoa lost by the net. In addition to these organisms, 
plate cultures made on gelatine and agar showed that between 25 per 
eent and 50 per cent of the bacteria were also removed in the centri- 
fuging process. This efficiency is maintained for quantities of water 
up to 1,500 liters, or for as much as 10 grams of dry plankton material. 


Figure 3 is a sketch drawing of the laboratory showing the equip- 
ment used in making these studies. In this sketch C is the centrifuge; 
M is the electric motor by which the centrifuge is driven through an 
intermediate; T is the tank into which the water is pumped from the 
dock by a pump marked P in the figure. 


Figure 4 shows a sectional view of the bowl of the centrifuge. The 
water enters the bowl at A and passes down to the clarifying com- 
partment, B, where some of the material is deposited. Then it passes 
out to the periphery of this compartment, C, where the centrifugal 
force is at a maximum. By far the greater portion is deposited here. 
The water next flows upward and toward the center of the bowl be- 
tween conical dises which divide it into thin layers and subject it to 
further centrifugal action. This removes the last portion of material 
that is obtained in the centrifuging process. This material is de- 
posited on the under side of the dises and most of it passes down and 
out to the pocket at C. 7 

The centrifuged water passes to the center of the bowl and is then 
forced upward and outward to chamber D from which it passes on to 
the filter compartment. The latter is filled with a series of horizontal 
corrugated plates, F, nineteen in number, which possess perforated 
retaining rings at their outer and inner margins. The filter papers, 


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METHODS AND APPARATUS 5 


G, are placed between the corrugated plates and the perforations in 
the plates are so arranged that the water passes through the filter 
papers in its course through this chamber. The water then flows up- 
ward through the passage indicated at I and is discharged from the 
bowl at the point marked J. 

About 5.5 liters of water remain in the bowl at the end of a run 
and this is siphoned off and used to wash off the material that is de- 
posited on the conical dises and on the sides of the bowl. A little 
chloroform is added and the whole is then evaporated in porcelain 
dishes at a temperature of about 60° C. About three quarters of the 
evaporation is done on a sand bath after which the dishes are trans- 
ferred to a Hearson electric evaporating oven for the completion of 
the process. The dry material is then carefully removed from the 
porcelain dishes, ground, and placed in a weighing dish. After stand- 
ing in a desiccator for forty-eight hours it is weighed and transferred 
to a bottle to await the chemical analyses. 

The water used for the centrifuge was obtained from the regular 
station in the deep portion of Lake Mendota and it consisted of a cer- 
tain quantity from each meter down to a depth of 20 meters. It was 
strained through the large plankton net and was caught in containers 
(milk cans) as it flowed from the large can in which the net was sus- 
pended. It was then conveyed to the laboratory dock in the launch 
where it was pumped into the large tank shown in figure 3. This tank 
is made of galvanized iron and has a capacity of about 1,200 liters. It 
is mounted on a framework which rests on a platform scale so that the 
quantity of water used for each run is readily ascertained by weighing. 
The seale also enables one to determine the rate at which the water is 
centrifuged, the usual rate being about 10 liters per minute. The 
framework elevates the tank to such a height above the centrifuge that 
the water flows from the former to the latter through a hose, the rate 
of flow being regulated by a valve. 

By far the greater portion of the work done on the centrifuge plank- 
ton, or nannoplankton, as well as on the net plankton, was done on 
Lake Mendota; but for purposes of comparison some observations were 
also made on Lakes Monona and Waubesa. The quantity of water 
centrifuged at each run varied from about 1,100 liters to a little over 
1,500 liters on Lake Mendota, but from the other two lakes it varied 
from about 700 liters to a little over 1,500 liters. Figure 5 shows the 
launch at the laboratory dock with a cargo of about 500 liters of water. 

During the open season, that is, from April to December, both in 
1915 and in 1916, two centrifuge runs per week were made on Lake 
Mendota when the weather was favorable. In most instances the ma- 
terial obtained in the two runs was combined into one sample for the 


16 PLANKTON OF WISCONSIN LAKES 


chemical analyses. In 1917 only one run per week was made. On 
Lakes Monona and Waubesa observations were not made at regular in- 
tervals in 1915, but in 1916 they were made approximately every two 
weeks. 

CuEmicaL Mrtruops 


The various methods employed in the chemical analyses have been 
described by Schuette* so that they need not be considered in detail 
here. In all instances the samples used for the analyses were dried 
in an electric oven at a temperature of 60° C. for a period of twelve 
hours. After cooling in a desiccator the weights of the samples were 
taken and these constitute the dry weights used in the ealeulation of 
the percentages of the different substances determined. The material 
is more or less hygroscopic and the moisture lost in this drying process 
. varied from a minimum of about 1 per cent to a maximum of almost 
7 per cent; in most cases, however, the loss was between 2 per 
cent and 4 per cent. The material was not dried at a higher tem- 
perature because it was feared that some of the oils might be lost by 
volatilization at higher temperatures. Usually from half a gram to 
a gram of material was used for an analysis and a number of experi- 
ments showed that this amount came to a constant weight under the 
conditions described above, thus showing that it was thoroughly dry. 

In determining the ash the sample was first carbonized at a low 
temperature, after which it was placed in an electric furnace heated 
to a temperature of 500° to 600°. It usually required from 15 minutes 
to 20 minutes to complete the ashing process at this temperature. That 
portion of the material that is consumed may be regarded as the organic 
matter, but a correction is necessary where a considerable amount of 
magnesium carbonate is present, since this substance gives off its 
carbon dioxide at a temperature of 250° to 300°. The regular methods 
were employed in making the quantitative determinations of the vari- 
ous constituents of the ash. 

All nitrogen determinations were made by the Kjeldahl-Gunning- 
Arnold method. 

The ether extract was determined in the Soxhlet extraction ap- 
paratus and the extraction process was continued for a period of 24 
hours. In addition to the oils and fats, chlorophyl is also extracted 
by the ether but the quantity of this substance is regarded as too small 
to affect the results materially. 

In determining the pentosans the material was distilled with hydro- 
chlorie acid, specific gravity 1.06, and the furfurol in the filtered dis- 
tillate was precipitated with phloroglucinol solution. After standing 


*Trans. Wis. Acad. Sci. Arts, and Let., Vol. XIX, 1918, p. 594. 


METHODS AND APPARATUS . 17 


94 hours the precipitate was filtered off, dried, and weighed; the results 
were then calculated to pentosans. 

The amount of crude fiber was determined by digesting the material 
for half-hour periods with sulphuric acid and sodium hydroxide solu- 
tion having a specific gravity of 1.25. The undigested material was 
filtered in a tared Gooch crucible, washed, thoroughly dried at 100°, 
weighed, and then ignited. In the pure crustacean material the crude 
fiber has been regarded as chitin. 

The plankton which was deposited on the filter papers of the centri- 
fuge could not be removed for direct estimation since it adhered too 
closely to the papers. So it was necessary to determine the quantity of 
this plankton by an indirect method; that is, the filter papers were 
dried and the total nitrogen in them was determined. One-eighth of 
each filter paper in the set of nineteen was used for this analysis and 
the nitrogen in excess of that obtained from a corresponding set of 
blanks was regarded as belonging to the plankton. The quantity of 
organic matter in this material was then estimated by multiplying the 
excess nitrogen by the factor representing the ratio of the nitrogen to 
the organic matter in the regular centrifuge catch. This estimated 
organic matter was then added to that obtained from the bowl of the 
centrifuge and the sum of these two constitutes the total organic matter 
in the catch. The quantitative results are based on this total. In most 
instances the nitrogen in the filter papers constituted between five per 
cent and ten per cent of that in the material from the bowl of the 
centrifuge. 

PURPOSES OF THE INVESTIGATION 


These studies were undertaken with a twofold purpose in view; first 
to obtain data on the annual plankton production of a lake with special 
reference to the quantity of organic matter involved as well as the 
chemical composition of this material; second, to obtain similar data 
with respect to the various kinds of organisms making up the plankton. 

Concerning the first problem it may be said that it has been the aim 
to secure results which are comparable in a general way to those that 
have been obtained for the land. The agriculturist knows either exactly 
or approximately the amount of his crops each year in terms of some 
standard unit and he also knows approximately the area of land from 
which these crops have been harvested; thus he can state his season’s 
results quantitatively per unit area of land. Furthermore numerous 
chemical analyses enable one to obtain some idea of the food value of 
the various agricultural products. But we have substantially no infor- 
mation of this character with respect to the productivity of cur fresh 
waters; only estimates based on wholly insufficient data are available. 
Also, with the exception of the fishes, our knowledge of the chemical 
composition of the various fresh-water organisms is very scant indeed. 


18 PLANKTON OF WISCONSIN LAKES 


Thus the present investigation is a step in the direction of supplying 
this much needed information; but it can be regarded only as a begin- 
ning since it will require similar investigations on a considerable num- 
ber of different types of lakes. Also, it applies to only a single element 
of this aquatic life, namely, the plankton. The larger organisms must 
receive similar treatment. Much more work, therefore, both of a 
numerical and of a chemical nature, must be done, not only on the 
plankton but also on the larger aquatic organisms, before we shall 
have sufficient data for drawing any general conclusions as to the 
productivity of our fresh-water lakes. 

The problem of aquatic productiveness involves complexities which 
are not encountered in studies on the productivity of the land. In a 
lake, for example, the productive soil, so to speak, is coextensive in 
depth with the depth of the lake and also includes the bottom to a 
certain depth because many organisms, such as insect larvae and oligo- 
chaets, inhabit the bottom mud. On land the various crops are easily 
kept separate and they mature at a definite time so that they may be 
harvested and their total quantity ascertained. But in a lake the 
plankton erop alone, for example, comprises a considerable number of 
forms of which many are present at the same time. Thus the plankton 
crop, in general, consists of a mixture of forms and only rarely is it 
possible to obtain a pure catch of any form except the larger crustacea, 
which may be sorted out by means of nets having different sizes of 
mesh. The various forms reach their maximum numbers at different 
times of the year, some even in winter, so that there is no definite har- 
vest time at which this material may be collected and the annual pro- 
duction of it thereby ascertained. The plankton, therefore, must be 
considered as a ‘‘standing crop’’ since it is present at all seasons of the 
year and since it does not possess any definite period of maturity; in 
other words, it constitutes a continuous stream of life which presents 
different degrees of abundance during the course of its annual cycle. 
The same is true also of many of the larger aquatic organisms. 

Certain forms of algae frequently appear in a practically pure state 
in the summer as the socalled ‘‘water-bloom’’ and at such times they 
ean be obtained in sufficient abundance for a chemical analysis. The 
smaller forms, however, more especially those found in the nannoplank- 
ton, present a more difficult problem since they do not seem to thrive 
well in laboratory cultures and they rarely appear in the lake in suffi- 
cient abundance and purity for one to obtain enough material of ‘the 
different forms for an analysis. But the problem is not insoluble and 
the final results will justify the expenditure of much time and energy 
in its solution. 


METHODS AND APPARATUS 19 


When several analyses of the various forms, representing the dif- 
ferent seasons of the year and as many lakes as possible, have been 
obtained, the planktologist can then strike an average and obtain, with 
a reasonable degree of accuracy, a factor, a food-value factor if you will, 
for each form. This will serve as an index of the réle played by the 
different organisms in the annual crop of organic matter produced by 
the plankton of a body of water. When such factors are established 
for the various planktonts, not only will it be possible to study the 
productivity of a lake by the numerical method, but it will also enable 
one to consider all numerical studies that have been made in the past 
from this same standpoint, which will add very greatly to the value of 
such studies. In brief, it will put the problem of plankton production 
on a chemical and gravimetric basis; that is, on the same basis as the 
modern investigations with respect to the crop production of the land. 


20 PLANKTON OF WISCONSIN LAKES 


CHAPTER II 


THE NET PLANKTON OF LAKE MENDOTA 


The material for the investigations relating to the quantity and 
chemical composition of the net plankton was obtained from four lakes, 
namely, Mendota, Monona, Waubesa, and Kegonsa. These lakes are 
situated in the Yahara river basin; they occupy local enlargements of 
this valley and they are named in order beginning with the one nearest 
the headwaters of this stream. (See map, fig. 6, p. 21). They have 
a northwest-southeast trend, the general course of the Yahara river 
being southeast. Detailed descriptions of these lakes are given in 
bulletins No. VIII (Second Edition) and No. X XVII of the Wisconsin 
Survey and they need not be repeated here. 

The areas, volumes, and the maximum and mean depths of these 
lakes are given in table 1 (p. 181). Lake Mendota is by far the largest 
and the deepest member of the group. While the maximum depth of 
Lake Monona is only about 12 per cent less than that of Lake Mendota, 
the mean depth of the former is only two-thirds as great, thus showing 
that the basin occupied by the former is relatively much shallower. 
Lakes Waubesa and Kegonsa occupy very shallow basins; their maxi- 
mum depths are distinctly less than half that of Lake Mendota, while 
their mean depths are but little more than a third as great. 


THERMAL STRATIFICATION 


Lakes Mendota and Monona have sufficient depth to become thermally 
stratified in summer; that is, they become separated into three distinct 
strata for a period of three to three and a half months each season, or 
from late June or early July to late September or early October. The 
upper stratum comprises the warm water and is known as the epilim- 
nion. It is kept in circulation by the wind and is thus freely exposed 
to the air so that its supply of dissolved oxygen may be replenished 
should this gas fall below the saturation point at any time. This 
stratum also receives by far the greater portion of the sun’s energy that 
is delivered to the surface of the lake, and it is, therefore, the most 
favorable region for chlorophyl-bearing organisms. These two factors, 
an abundance of oxygen and light, make this the most favorable region 
for the major portion of the net plankton, and it is here that the great 


NET PLANKTON OF LAKE MENDOTA 21 


MENDOTA 


LAKE 
MONONA 


LAKE 
VAUBESA 


Fig. 6—Outline map of the four lakes on which plankton observations were 
made. 


2? PLANKTON OF WISCONSIN LAKES 


majority of the organisms are found most abundantly. The epilimnion 
in these two lakes varies in thickness from five meters to seven meters 
when it is first formed, but it gradually increases in thickness as the 
season advances, extending to a depth of 10 meters to 12 meters by 
the middle of September. 

There is a bottom stratum of cool water which may be regarded as 
stagnant during this period; that is, it is not kept in circulation by the 
wind and is not exposed to the air. Since light does not penetrate to 
this stratum in sufficient amount to permit much activity on the part 
of chlorophyl-bearing organisms, it is cut off from the two sources of 
oxygen supply, namely, the air and photosynthesis, and the quantity 
of this gas is limited to the amount held in solution at the time that 
stratification takes place. This supply of oxygen is drawn upon through 
respiration of the organisms which occupy this region and through the 
decomposition which takes place there, so that, by late July, substan- 
tially the entire hypolimnion of these two lakes is devoid of free oxygen. 
The dissolved gases bave been fully discussed in Bulletin No. XXII of 
this Survey and the reader is referred to that publication for a more 
detailed account. 

The absence of oxygen makes the hypolimnion uninhabitable for the 
plankton crustacea, and those which occupy this region when there is a 
sufficient supply of this gas withdraw when the amount falls below the 
minimum required by the various forms. But this stratum is by no 
means entirely deserted during the period that it possesses no free 
oxygen because various forms of protozoa and some insect larvae thrive 
here even in the absence of oxygen. In fact, one ciliated protozoan 
has been noted which appeared in this stratum only when the oxygen 
was substantially or entirely absent, the largest numbers having been 
found under complete anaerobic conditions. (Juday, Biol. Bul., Vol. 
36, 1919, pp. 92-95.) 

Between the two strata mentioned above there is a definitely marked 
transition zone in which the conditions change from those of the warm 
water above to those of the cool water below. This is the mesolimnion 
or thermocline and it is relatively thin, usually not exceeding three or 
four meters in thickness and frequently not more than two meters. In 
the mesolimnion the temperature of the water rapidly changes from 
that of the warm water of the epilimnion above to that of the hypo- 
limnion below and this decrease in temperature is accompanied by a 
decrease in the quantity of dissolved oxygen from that of the well 
aerated epilimnion to only a trace or none at all in the hypolimnion. 
hese changes do not make this stratum uninhabitable, however, be- 
cause the mesolimnion is generally well populated by various forms, 
some of which, in fact, are more abundant here than at any other depth. 


NET PLANKTON OF LAKE MENDOTA 23 


The area of Lake Kegonsa is so great in proportion to its depth that 
the wind keeps the entire body of water in circulation during the sum- 
mer. Lake Waubesa, while only slightly deeper than Lake Kegonsa, 
is only about a quarter as large, so that the wind is less effective in 
keeping the water in circulation. As a result, the deeper portion of 
this lake, which comprises a relatively small part of its total area, 
possesses a thin bottom stratum of cooler water until about the middle 
ef August; but after this date the lake is substantially homothermous. 

The maximum temperature of the upper water in these four lakes 
during the summer ranges between 25° and 30° C. The bottom tem- 
peratures in Lakes Mendota and Monona in summer vary from about 
9° to 14°. In winter all of the lakes are covered with ice for a period 
of three to three and a half months, sometimes even longer. Usually 
the ice reaches a thickness of about three-quarters of a meter. 

The region in which the lakes are situated is an agricultural district 
so that the dissolved substances carried by the drainage water of this 
basin are subject to the modifications usually found in such an area. 
During the period of these investigations also, Lake Monona received 
some raw sewage from the city of Madison as well as the effluent from 
its sewage disposal plant. This additional material in suspension and 
in solution also affected the waters of Lakes Waubesa and Kegonsa to 
a certain extent, since the Yahara river supplies water from Lake Mo- 
nona to these two lakes. 

The major portion of the work was done on Lake Mendota, on the 
south shore of which the laboratory is situated; but for purposes of 
comparison observations were also made on the other three members of 
this chain of lakes. Lake Kegonsa was visited only once because the 
river between it and Lake Waubesa has not been dredged and is not 
navigable for launches except at an unusually high stage of the water 
and then only with considerable difficulty. The river between Lake 
Mendota and Lakes Monona and Waubesa has been canalized so that 
they can be reached easily by launch from the laboratory. 


QUANTITY OF WATER AND OF PLANKTON 


Table 2 (p. 181) shows the quantity of water that was strained in 
each of the lakes and the amount of net plankton obtained therefrom. 
In all 481 catches were made and a little over two million liters of water 
were strained. Slightly more than 90 per cent of the total quantity of 
water was secured from Lake Mendota. The total volume of water 
yielded a little more than 1,292 grams of dry net plankton, or an aver- 
age of approximately 600 milligrams per cubic meter. The average 
yield of net plankton was much smaller in Lake Mendota than in the 
other lakes ; for 415 catches in this lake the average was 491 milligrams 


24. PLANKTON OF WISCONSIN LAKES 


per cubic meter of water. For 47 catches in Lake Monona the yield 
was 1,499 milligrams; for 18 catches in Lake Waubesa it was 2,182 
milligrams, while in the one catch in Lake Kegonsa the amount was 
6,112 milligrams per cubic meter. The sample from Lake Kegonsa ex- 
ceeded even the maxima of the other lakes. The largest amount found 
in Lake Mendota was only about 1,700 milligrams per cubic meter of 
water, while that of Lake Monona was 3,820 milligrams and that of 
Lake Waubesa 4,600 milligrams. 

Table 8 shows the amount of water that was centrifuged for the 
nannoplankton, together with the quantity of dry material obtained 
therefrom. 

Table 4 (p. 182) gives in detail the quantity of water used for the 
various samples of net plankton and nannoplankton, as well as the 
amount of dry material obtained in each instance. The quantitative 
results shown in the general tables (numbers 48 to 48) are based upon 
the quantities indicated in this table. In computing the amount of dry 
organic matter per cubic meter of water during the period from July to 
September, it was necessary to introduce a correction; that is, during 
these months the hypolimnion of Lakes Mendota and Monona is almost 
or quite devoid of free oxygen and, as a result, the net plankton organ- 
isms are unable to occupy this stratum. From 1911 to 1914, therefore, 
the net samples covered only the inhabited stratum of the lake so that 
it was necessary to apportion the total catch to the entire depth in order 
to get a general average for the whole lake. From 1915 to 1917 the cor- 
rections were very much smaller because material was secured regularly 
down to a depth of 20 meters in Lake Mendota and down to 18 meters 
in Lake Monona. : 

Some of the net plankton catches made in 1915 have been omitted 
because two series of such catches were taken in that year; only those 
are shown which have been subjected to the most complete analysis. 
Attention may be ealled to the fact that the quantity of water is the 
same for a net plankton catch as for the corresponding sample of nanno- 
plankton, the same sample of water being used for both. 

The studies on the net plankton were begun on June 1, 1911, and 
were continued regularly until June 1, 1917, with the exception of the 
period from July, 1914 to April, 1915. During the investigations nine 
sets of observations were made on Lake Mendota in the winter season, 
that is, while the lake was covered with ice. No attempt was made to 
secure winter catches on the other lakes. Work was begun as promptly 
as possible after the ice disappeared from the lake in the spring and it 
was continued until fairly thick ice formed along the shore of the lake 
in early winter and prevented further use of the launch. The latest 
date on which observations were made with the launch was December 


NET PLANKTON OF LAKE MENDOTA 25 


24, 1918; regular trips were usually continued until the first or the 
second week in December. 

Table 5 (p. 186) shows the general distribution of the net plankton 
during the different months of the year. The first column under each 
month indicates the number of runs or catches made during the month 
and the second column gives the average amount of dry net plankton 
per cubic meter of water that was secured in these catches; the latter 
shows, therefore, the monthly average of this matrial. The results for 
1916 and 1917 are based on material obtained from much smaller quan- 
tities of water than in the previous years because net plankton was 
taken only from the water which was centrifuged for the smaller organ- 
isms. In these instances the quantity of water averaged about 1,200 
liters per catch. 

The 415 catches from Lake Mendota were combined in such a way as 
to make 184 samples for chemical analyses. Thirty-eight of these sam- 
ples consist of single catches while the others are combinations of two 
to five catches each. In general it was the plan to make two or more 
catches each week during the open season in order to obtain a better 
average of material as well as a sufficient amount for analysis; usually 
all of the material obtained during a week was combined into one 
sample. Two of the samples from Lake Monona consist of two catches 
each while the others from this lake as well as all of those from Lake 
Waubesa contain but a single catch each. 

Table 5 serves to show only the more general variations in the quan- 
tity of net plankton during the different months of the year; the time 
period is too long a unit to bring out the details. This general table 
indicates that there are marked differences in the amount of net plank- 
ton in the different years and also that the maximum and minimum 
amounts are not found in the same months from year to year. The 
results on the various lakes will now be taken up in greater detail. 


LAKE MENDOTA 
Organic Marrer In Net PLANKTON 


The organic matter of the net plankton consists of that portion of 
the material which is consumed in the ashing process. In general the 
amount of magnesium carbonate in the ash is so small that it is not 
necessary to make a correction for the carbon dioxide removed from it 
during the ashing process. During the greater part of the year from 
70 per cent to 90 per cent or more, of the dry weight of the net plankton 
consists of organic matter; but during periods in which the diatoms 
predominate the organic and inorganic constituents may be almost 
equal in amount. 


LAKES 


PLANKTON OF WISCONSIN 


26 


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NET PLANKTON OF LAKE MENDOTA 27 


The results obtained for the organic matter in the net plankton of 
Lake Mendota are summarized in table 6 (p. 187) in which the maxi- 
mum, minimum, and mean quantities are indicated for the different 
years. The data are given in detail in the general table, No. 48, p. 202. 
During the period of these observations the amount of dry organic 
matter was smallest in 1911; the mean for this year was only 175 milli- 
grams per cubic meter of water, or about half as much as in the other 
full years. Next in order come 1914 and 1917, but in these years the 
observations were discontinued respectively on July 2 and June 1, so 
that data are available for only the first half of these years. The 
results for these two years, therefore, are not comparable with those 
covering a full year. Adding the catches obtained between July and 
December, 1913, to the 14 secured during the first half of 1914 gives a 
mean of 363 milligrams of organic matter per cubic meter of water, 
while the catches taken between June 1, 1916, and June 1, 1917, give an 
average of 324 milligrams. Both of these averages are comparable in 
amount with those obtained during the full-year periods of 1912, 1913, 
1915, and 1916, in which the means range from 343 milligrams to 393 
milligrams per cubic meter of water. Thus, for the four full-year pe- 
riods the largest mean exceeded the smallest by less than 15.0 per cent. 


The seasonal changes in the amount of organic matter in the net 
plankton are shown in two diagrams, figures 7 and 8. The curves 
for 1913-14 (fig. 7) and for 1916-17 (fig. 8) cover all four seasons 
of the year; since they cover the complete annual cycle, they will 
be considered in some detail here. In these diagrams the vertical 
spaces represent the number of milligrams of organic matter per cubic 
meter of water, the scale being indieated in the left margin. The hori- 
zontal spaces represent the months of the year, each month being di- 
vided into four equal parts. In the curves the results obtained in the 
various observations are shown by the circles; the curves themselves 
have been constructed simply by connecting these points without at- 
tempting to round them off into more symmetrical diagrams, These 
diagrams are based on the data given in table 43. 


The first sample obtained in 1918, namely, on April 16-18, yielded 
185.1 milligrams of organic matter per cubic meter of water. Follow- 
ing this the amount rose rapidly to a maximum of 462.3 milligrams on 
May 7-9 and it then remained high until June 2-6. A marked decrease 
followed, a minimum of 126.9 milligrams being found on June 30-July 
3. This was succeeded by a decided rise in July and early August, the 
amount reaching a maximum of 534.0 milligrams on August 4-7. This 
summer maximum was 71.7 milligrams larger than the vernal maxi- 
mum. A very marked and rapid decrease to 87.2 milligrams on Au- 
gust 19-22 came next; this amount proved to be the minimum of the 
year. 


223 | PLANKTON OF WISCONSIN LAKES 


The autumnal increase began early in September and reached its 
highest point on October 22-25, namely, 714.1 milligrams per cubic 
meter, which was the maximum for the year. A marked decrease fol- 
lowed in late October and in early November, while the sample of De- 
cember 16-17 showed a secondary rise to 575.2 milligrams; the sample 
for the following week, however, contained a distinetly smaller amount 
of organic matter. 

This decrease continued during the month of January, 1914, and 
reached a minimum of 123.7 milligrams by the latter part of this 
month. The February sample showed a similar amount, while that 
for March 14-21 contained a larger quantity of organic matter, namely, 
146.6 milligrams. The first sample obtained after the disappearance 
of the ice in April showed a further increase and the organic matter 
continued to rise until it reached 401.8 milligrams on May 5-9. The 
amount then remained high until after a vernal maximum of 426.0 
milligrams was reached on June 2-6. A decrease during the rest of 
June brought the quantity down to 98.8 milligrams on June 30-July 2, 
after which time the catches were discontinued. 

The series of net catches for 1916-17 (fig. 8) began on February 11, 
1916, and continued until June 1, 1917. The sample obtained on the 
former date contained 213.2 milligrams of organic matter per cubie 
meter, while that of March 8, 1916, had only 172.1 milligrams and that 
of April 15, 175.8 milligrams. The curve shows a small secondary peak 
during the last week in April which is followed by a decline to 176.9 
milligrams on May 8-10. It will be noted that the latter amount was 
only a little larger than that of April 15. This was succeeded by the 
regular vernal rise which reached 395.6 milligrams on May 18-19 and 
a maximum of 418.1 milligrams on June 5-10. A rapid decline was 
noted during the next two weeks, which was followed by a slight rise 
during the last week in June; after this a further decrease brought the 
amount of organic matter in the net plankton down to the general 
summer minimum during the second week in July. Between this date 
and the third week in September the quantity of organic matter varied 
between 73.3 milligrams and 134.5 milligrams. The smallest amount 
was found in the sample obtained on August 22-25. The autumnal in- 
crease began during the last week in September and, through a series 
of secondary peaks, it rose to the maximum for the year, that is, 1,135.4 
milligrams, on December 12, 1916. 

By January 18, 1917, the organic matter had fallen to 866.2 milli- 
grams and by February 14 to 187.3 milligrams. It declined still further 
during March and the first half of April, reaching a minimum of 92.4 
milligrams on April 18. The latter was less than one-twelfth as much 
as that of December 12, 1916. During the latter part of April, 1917, 


Zo 


NET PLANKTON OF LAKE MENDOTA 


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i bt mem ee Nn 8 a le ne eae 


30 PLANKTON OF WISCONSIN LAKES 


and through the month of May there was a steady increase in the 
amount of organic matter which culminated in a maximum of 400.2 
milligrams on June 1, 1917; after this date the observations were dis- 
eontinued. 


While the various curves show that there are considerable variations 
in the organic matter of the net plankton from year to year, yet they 
all agree in bringing out the fact that the annual cycle consists of four 
phases which correspond more or less closely to the four seasons of the 
year. Beginning in the spring, there is an increase during the latter 
part of April or early in May, at which time the organic matter rises 
more or less rapidly to a vernal maximum in late May or early June. 
There are considerable variations in the amount and in the duration of 
this increase from year to year. In two of the five years for which 
complete records were obtained, that is, in 1912 and 1915, the quantity 
of organic matter rose to maxima of 574.4 milligrams and 647.1 milli- 
erams per cubic meter of water respectively. In the other three years 
the maxima lay betwen 400.0 and 462.0 milligrams. The period during 
which the organic matter remains near the maximum amount varies 
from a few days, as in 1912, to four weeks, as in 1913 and 1914. That 
part of the curve covering the vernal period in 1912 consists of a sharp 
peak; those for 1915 and 1916 have fairly broad apexes, while those for 
1913 and 1914 have a more or less irregular plateau covering a period 
of about four weeks. 

This vernal pulse is followed by a decrease during June to a general 
summer minimum about the first of July. From this time to the middle 
of September the organic matter may remain uniformly low as in 1911 
and in 1916, or it may show a more or less marked summer increase as 


in 1912, 1913, and 1915. This summer increase was most marked in © 


1913; in fact, the organic matter rose to a higher point at this time 
than it did during the vernal maximum. The curve for 1912 shows two 
secondary peaks during the summer, one the first of August and the 
other the first of September; that for 1915 also shows a secondary 
peak about the first of September. On the whole, however, the period 
from July 1 to September 15 may be regarded as one in which the 
average amount of net plankton is relatively low. 

The third phase of the annual cycle comprises the autumn of the year, 
during which there is a rather rapid increase, beginning about the third 
week in September. This rise terminates in a maximum usually about 
the middle or last of October, but the different years show important 
differences both with respect to the rapidity and the extent of the in- 
crease. In 1916, for example, the autumnal rise was much more grad- 
ual than in the other years and did not reach its maximum height until 
almost the middle of December; this maximum was the largest found 


; 
q 
‘ 
| 


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NET PLANKTON OF LAKE MENDOTA ne | 


during these observations. It exceeded the next in rank, namely, the 
autumnal maximum of 1912 by 80.0 milligrams per cubic meter of 
water. 

The autumnal period is succeeded by a decline to a winter minimum 
which is reached in late January, as in 1914, or by the middle of Febru- 
ary, as in 1917. In 1913, however, the autumnal maximum was fol- 
lowed by a secondary rise which reached its highest point during the 
third week in December; this was then followed by a decline to the 
winter minimum. After reaching the winter minimum the quantity of 
organic matter remains fairly constant until about the middle of April 
and soon after this date the vernal increase begins. 


CHEMICAL RESULTS 
NITROGEN AND CRUDE PROTEIN 


The detailed results for the nitrogen determinations on the net plank- 
ton of Lake Mendota are shown in table 48 (p. 202). There are five 
columns in the general table pertaining to the nitrogen. The first one 
shows the percentage of the total nitrogen in the dry sample; the second 
gives the percentage of nitrogen in the organic matter exclusive of that 
found in the crude fiber; the third indicates the number of milligrams 
of nitrogen per cubic meter of water; the fourth shows the quantity 
of crude protein, that is, the milligrams of nitrogen multiplied by the 
protein factor 6.25; the fifth column indicates the ratio of the organic 
matter to the nitrogen. The curves marked B in figures 11 to 14 show 
the number of milligrams of crude protein per cubic meter of water in 
the net plankton. 

A certain portion of the total nitrogen is found in the chitin of the 
shells of the plankton crustacea ; since these shells pass through the ali- 
mentary tract of fishes without being affected, apparently, by the di- 
gestive processes, it is assumed that this nitrogen compound has no food 
value. It was necessary, therefore, to make a correction for the nitro- 
gen in the chitin since one of the purposes of this investigation was to 
secure data regarding the food value of the plankton. Schuettet found 
that the crude fiber derived from the plankton crustacea yielded from 
5.9 per cent to 6.2 per cent of nitrogen, or substantially the same per- 
centage as reported for chitin by several investigators. So the correc- 
tion was made by determining the percentage of nitrogen in the crude 
fiber of the plankton material and then deducting this amount from the 
total nitrogen. It has been reported that chitin exists in the walls of 
many blue-green algae also, but later investigations do not confirm the 
earlier results ; thus its presence in these forms is still an open question. 


*Trans. Wis. Acad. Sci., Arts, and Let., Vol. XIX, 1918, p. 610. 


32 PLANKTON OF WISCONSIN LAKES 


In the general table, then, column one under nitrogen shows the 
percentage of the total nitrogen, including that in the crude fiber; but 
the nitrogen of the crude fiber was deducted from the total nitrogen 
before calculating the percentage on an ash free basis. In other words, 
the results given in column two under nitrogen in the general table 
represent the crude protein nitrogen in the organic matter. This same 
correction applies to the next two columns, but the last column for 
nitrogen shows the ratio of the organic matter to the total nitrogen. 


Table 7 (p. 187) shows the variations in the percentage of nitrogen in 
the net plankton in the various years. Only about two-thirds of the 
samples obtained in 1916 and 1917 contained enough material for a 
nitrogen determination. In the total nitrogen there was approximately 
a twofold variation in the percentage each year; the ratio of maximum 
to minimum is greater than two for the first three years and less than 
two for the other four years. The greatest difference between maximum 
and minimum was noted in 19138, namely, shghtly more than five per 


eent. The minimum percentage was found in August in each of the 


complete years except in 1913 when it was noted in the month of De- 
cember. In 1914 and 1917 the observations did not cover the latter half 
of the year. In 1918 and 1914 the material which yielded the largest 
percentage of nitrogen was obtained in the month of April; in 1915 and 
1917 it was obtained in May; in 1912 in June, in 1916 in July, and in 
1911 it came in September. Thus in four of the five complete years 
the maximum percentage of nitrogen was found either in the spring or 
in the early summer. ; 

The second part of table 7 shows the variations in the crude protein 
nitrogen when stated on an ash free basis. The ratio of maximum to 
minimum each year is distinctly less than two; the greatest difference, 
almost five per cent, was noted in 1913, while the smallest differences 
were found in 1915 and 1916, excluding 1917 which was not a complete 
year. The maximum percentage of crude protein nitrogen was found 
in 1913 and the minimum in 1916. The mean percentage was smallest 
in 1911 and largest in 1917. Excluding the partial years 1914 and 
1917, the largest mean percentage was found in 1915, but the smallest 
mean, that of 1911, was a little less than one per cent below the maxi- 
mum. Taken as a whole, then, the results for the five complete years 
show a rather striking uniformity in the mean percentage of crude pro- 
tein nitrogen in view of the composite character of the net plankton and 
of the changes in the relative abundance of the various forms during 
the different seasons of the year. 

The third column under nitrogen in table 43 shows the number of 
milligrams of crude protein nitrogen per cubic meter of water. In the 
various net samples on which nitrogen determinations were made, the 


ee 


NET PLANKTON OF LAKE MENDOTA 33 


quantity of crude protein nitrogen varied from a minimum of 3.4 
milligrams per cubic meter of water in one sample collected in 1911 toa 
maximum of 93.0 miligrams in one of the 1916 samples; a minimum of 
5.2 milligrams was noted in one of the 1916 catches, thus giving an 
eighteenfold variation in amount during that year. In 1911 the quan- 
tity varied from 3.4 milligrams to 40.8 milligrams, which represents a 
twelvefold variation in amount. 

The quantity of nitrogen, like that of organic matter, showed vernal 
and autumnal maxima separated by summer and winter minima. The 
autumnal maximum always exceeded the vernal maximum of the same 
year ; the greatest difference between the two was noted in 1912 when the 
vernal nitrogen rose to 49.3 milligrams per cubic meter of water and the 
autumnal to 85.0 milligrams, the difference being 35.7 milligrams. In 
1911 the difference between these two maxima was 22.7 milligrams and 
in 1913 it was 19.4 milligrams, while in 1915 it was only 13.0 milligrams. 

Figures 9 and 10 are graphical representations of the quantitative 
results obtained for nitrogen; they show the number of milligrams of 
erude protein nitrogen per cubic meter of water. These curves serve 


Fig. 9.—The quantity of nitrogen in the net plankton of Lake Mendota in 1911 
and 1912. The vertical spaces show the number of milligrams of nitrogen 
per cubic meter of water. 


OF WISCONSIN LAKES 


PLANKTON 


34 


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SUVIGT[[IM JO LOqUINU OY} MOYS SOAIND OUT, “GIG 9} EIGL WOIZ VJopus~ oOXeT Jo uoj,yueld you oy} ut uesosyru Jo AyryUeNb oy—oOT ‘BLT 


NET PLANKTON OF LAKE MENDOTA 35 


to bring out more clearly the vernal and autumnal maxima of nitrogen 
as well as the summer and winter minima. Many of the net catches 
that were made in 1916 and 1917 did not contain enough material for 
a nitrogen determination so that the data for these years are not com- 
plete enough for the construction of curves covering this period. Thus, 
only one winter season is covered in these curves, namely that of 1913-14 
(fig. 10), but it furnishes a good illustration of the winter minimum 
which extended from late January, 1914, to the latter part of March. 

The various curves reach their greatest heights in the spring in the 
months of May and June, while the summer minima are found in July 
and August. The autumnal maximum in the four years covered by 
these observations was attained in the month of October. These curves 
show clearly that the quantity of crude protein nitrogen was smallest in 
1911, less than 10.0 milligrams per cubic meter of water bemg found 
from the middle of June to the middle of September. The quantity 
of nitrogen remained well above 10.0 milligrams per cubic meter of 
water during the summer of 1915; it fell below this amount only once 
in the summer of 1912 and twice in the summer of 1913. 

Column four under nitrogen in table 43 (p. 202) shows the quantity 
of crude protein per cubic meter of water; that is, it gives the results 
obtained by multiplying the quantity of nitrogen shown in column 
three by the protein factor 6.25. The various proteins do not all pos- 
sess the same percentage of nitrogen, the amount ranging from a mini- 
mum of about 15.0 per cent to a maximum of 19.0 per cent, but this 
factor is the one generally used by food chemists for calculating their 
nitrogen determinations into terms of protein. Thus, the results given 
in this column are more or less conventional, but they serve to give a 
general idea of the quantity of crude protein. 

The range of variation in the quantity of crude protein in the various 
years is shown in the second part of table 8 (p. 188) in which the 
maximum, minimum, and mean amounts are stated in milligrams per 
cubic meter of water ; the ratio of the maximum to the minimum is also 
indicated in the last column of this table. 

The smallest maximum was noted in 1914, but this was due, no doubt 
to the fact that the observations were discontinued on the first of July 
so that the autumn maximum was not obtained. The largest maximum 
was found in 1912, while that of 1911 was somewhat less than half as 
large. The maxima of the other years fell between these two. The 
smallest minimum was noted in 1911 and the largest in 1915. Likewise, 
the smallest mean was found in 1911; it was only about 45.0 per cent as 
large as that for 1913 which was the next smallest of the four complete 
years. The mean for 1915 was the largest, the amount for this year 
being almost two and a half times as large as that for 1911. 


36 PLANKTON OF WISCONSIN LAKES 


The first part of table 8 indicates some of the general relations of 
the quantity of crude protein to the quantity of organic matter. It 
shows the maximum, minimum, and mean percentages of the former in 
the latter. It will be noted that the amount of crude protein fell as low 
as 84.5 per cent of the organic matter and it reached a maximum of 70.0 
per cent, thus giving slightly more than a twofold variation. The seven 
maxima range from 56.2 per cent to 70.0 per cent, a difference of 13.8 
per cent, while the minima vary from 34.5 per cent to 52.6 per cent, 
a difference of 18.1 per cent; the mean percentages show a variation of 
15.1 per cent. The organic matter contained the lowest percentage of 
crude protein in 1912 and the highest in 1913; the highest mean per- 
centage was found in 1915. The high mean percentage of 1915 is ac- 
counted for by the fact that the crude protein fell below 50.0 per cent 
of the organic matter in only four of the 34 samples obtained in this 
year. 

Nitrogen determinations were made on 166 samples and, in this num- 
ber, the crude protein constituted from 45.0 per cent to 60.0 per cent 
of the organic matter in 123 samples; that is, the proportion of crude 
protein in the organic matter does not vary more than 15.0 per cent in 
about three-quarters of the samples. Lowering the minimum to 40.0 
per cent adds 16 more samples to this list. 

The ratio of the crude protein to the organic matter is shown graph- 
ically in figures 11 to 14, inclusive. The curves marked A in these dia- 
erams represent the organic matter and those marked B the crude pro- 


AUGUST SEPTEMBER OCTOBER 


+ 


Fig. 11—The amount of dry organic matter, of crude protein and of ether 
extract in the net plankton of Lake Mendota in 1911. Curve A represents 
the organic matter, curve B the crude protein and curve C the ether 
extract. The curves show the number of milligrams per cubic meter of water. 


NET PLANKTON OF LAKE MENDOTA 37 


tein. The space between the base line, that is, the zero line, and the 
curve B in each case indicates that portion of the organic matter which 
the crude protein comprises, while the area between the two curves 
shows the remainder of the organic matter, such as the chitin, the carbo- 
hydrates, and the fats. 

In general these curves show that a marked increase or decrease in 
the organic matter is accompanied by a similar change in the crude 
protein. Thus during corresponding periods, the curves representing 


NOVEMBER 


Fig. 12—The amount of dry organic matter, of crude protein, and of ether 
extract in the net plankton of Lake Mendota in 1912. Curve A represents 
the organic matter, curve B the crude protein and curve C the ether ex- 
tract. The curves show the number of milligrams per cubic meter of water. 


the latter possess the same general form as those for the organic mat- 
ter. In some instances, however, there are minor differences between 
the two sets of curves. In 1912, for example (fig. 12), there was a 
perceptible increase in the organic matter between the last week in 
June and the last week in July, but the crude protein remained almost 
constant in amount during this time; in fact, it showed a slight de- 


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NET PLANKTON OF LAKE MENDOTA 39 


erease. The secondary peak in the curve for organic matter appearing 
the first of August, 1912, has a sharp apex while that of the protein 
curve at this time is broad. The same difference between the two curves 
also appears during the autumnal maximum. In the vernal peaks of 
1915 (fig. 14) the protein reached its highest point about a week later 
than the organic matter so that the former curve has a distinctly 
broader summit. With the exception of these few minor differences, 
however, the curves for organic matter and crude protein show a close 
correlation during the period of time covered by these observations. 


JULY AUGUST SEPTEMBER OCTOSER NOVEMEER 


fuaaaadaaeoaee aaa seeee ee 


Fig. 14.—The amount of dry organic matter, of crude protein and of ether ex- 
tract in the net plankton of Lake Mendota in 1915. Curve A represents 
the organic matter, curve B the crude protein and curve C the ether extract. 
The curves show the number of milligrams per cubic meter of water. 


Column five under nitrogen in the general table (No. 48, p. 202) 
shows the ratio of the organic matter in the net planktcn to the total 
nitrogen; that is, it is a numerical expression of the relation of the 
former to the latter. As might be expected from the results given for 
the percentage of total nitrogen in column one, this ratio is subject to 
a twofold variation and it is relatively low because the percentage of 
nitrogen is fairly high. In 91 out of 134 samples the ratio falls between 
9 and 12; that is, in these samples the nitrogen constitutes from one- 
ninth to one-twelfth of the dry organic matter. In two samples the 
ratio falls below 9 and in 41 samples above 12, the highest being 17.4 
and the lowest 8.7. 


40 PLANKTON OF WISCONSIN LAKES 


Quantitative determinations of four forms of nitrogen were made by 
Schuette? on five samples of plankton; the results of his analyses are 
shown in table 9 (p. 188). With respect to the material which he used 
for these analyses it may be said that it was collected in 19138, the first 
four samples in October and November of that year, and the fifth in 
December. A and B were regular pump catches so that they contained 
the same organisms as the net plankton of this period; that is, diatoms 
and crustacea were the chief constituents. The material in C and D 
was obtained with tow nets so that only the larger organisms were se- 
cured, hence it contained a larger proportion of plankton crustacea. 
Sample E consisted almost entirely of two algae, namely, Aphani- 
zomenon and Anabaena. 


It will be noted that by far the greater portion of the total nitrogen 
in these samples was found in the form of mono-amino nitrogen, while 
di-amino nitrogen was next in importance. These two forms of nitrogen 
constituted from 77.0 per cent to 85.0 per cent of the total nitrogen in 
these samples. From 9.0 per cent to 12.0 per cent of the total was found 
in the form of ammonia nitrogen and about half as much in the form 
of ‘‘humic’’ nitrogen. The highest percentage of ‘‘humic’’ nitrogen 
was found in sample E which consisted of the two blue-green algae. 


ETHER EXTRACT 


The ether extract from the net plankton contains the fats, more or 
less chlorophyl according to the relative abundance of the algae in the 
sample, and probably other substances, such as lecithin, which are 
widely distributed in plant and animal material and which are soluble 
in ether. The presence of chlorophyl] is clearly indicated by the green 
color which it imparts to the extract, but no attempt has been made to 
ascertain the quantity of the various substances, in addition to the fat, 
which are extracted by the ether. It is believed, however, that such 
materials constitute only a very small part of the whole extract. 


There are marked differences in the percentage of ether extract ob- 
tained from the different organisms as well as differences in the same 
organism at different seasons of the year. In general the extract from 
the algae varies from slightly more than one per cent to about 4.5 per 
cent of the dry weight. On the other hand a much larger percentage is 
obtained from the crustacea, ranging from a minimum of about 4.0 per 
cent to a maximum of approximately 40.0 per cent. The highest per- 
centage was obtained from the copepod Limnocalanus which usually 
contains a large globule of oil in the thoracic region of the body. An- 
other copepod, Cyclops, yielded 20.0 per cent of fat. There is a rather 


* Trans. Wis. Acad. Sci., Arts, and Let., Vol. XIX, 1918, p. 604. 


NET PLANKTON OF LAKE MENDOTA Al 


wide range in the percentage of fat in Daphnia; this variation appears 
to be more or less closely correlated with their stage of development. 
Those which are in an active stage of reproduction and are carrying 
many embryos in their brood chambers, contain the largest amount of 
fat; immature individuals or adults with few embryos yield a smaller 
percentage of extract. In the former individuals the amount of fat 
may exceed 21.0 per cent of the dry weight while in the latter it may 
fall as low as 3.9 per cent. 

Thus the percentage of ether extract in the net plankton depends 
upon three factors, namely, (a) the relative proportion of plant and 
animal material present, (b) the predominant form of crustacean, and 
(c) the developmental stage of the Daphnias present. 

The range of the variation in the percentage of the ether extract is . 
shown in the first part of table 10 (p. 188) where the results are stated 
on an ash free basis; the second part of this table gives the variations 
in terms of milligrams per cubic meter of water. The maximum per- 
centage in the net plankton was found in 1911; in one sample obtained 
this year, the ether extract constituted 26.58 per cent of the total or- 
ganic matter. The minimum for this year was also higher than in any 
of the other years; the maximum percentage for 1911 was only about 
three and a half times as much as the minimum. The lowest maximum 
percentage was noted in 1914 and the lowest minimum in 1913. The 
maximum percentage in the latter year was nearly seven times as large 
as the minimum; in the other years the maxima were from three to four 
times as large as the minima. The highest mean percentage was found 
in 1914 and the lowest in 1913. 

When stated in terms of milligrams per cubic meter of water, as in 
the second part of the table, the largest amount of ether extract was 
found in 1912 and the next largest in 1915. A sample obtained in 1913 
showed the smallest amount per cubic meter and the maximum quantity 
for this year was a little more than twenty-five times as large as the 
minimum ; this was the largest annual difference noted during the pe- 
riod of these observations. In the other instances, the maxima were 
from eight to twelve times as large as the minima of the corresponding 
years. The mean amount for 1911 was far below those of the other 
years, being only about half as large as three of the others. The 
largest average amount for a whole year was found in 1915, namely, 
46.6 milligrams per cubic meter of water. 

The curves marked C in figures 11 to 14, inclusive, show graphically 
the quantities of ether extract in milligrams per cubic meter of water 
for the different years ; they also indicate the relative proportion of this 
material in the organic matter as well as the relation of the amount of 
ether extract to that of crude protein. 


42 PLANKTON OF WISCONSIN LAKES 


In general the curves for ether extract exhibit the four annual phases 
that have been noted for the organic matter and the crude protein, that 
is, spring and autumn maxima and summer and winter minima. Thus 
the form of these curves is similar to that of the curves representing 
the organic matter and crude protein. Attention may be called to some 
minor differences, however; in two of the four series of observations 
covering the open season of the lake, the curves for ether extract reach 
their highest points for the year during the vernal maxima, while in 
one year, namely, 1918, the highest point of the year is reached during 
the autumn maximum. In 1915 the maximum height of the curve is 
found at the end of the first week in December. On the other hand, the 
curves for organic matter.and crude protein reach their highest points 
during the autumnal periods in all four years. 

Some idea of the character of the ether extract of net plankton 
which consists chiefly or entirely of crustacea, may be obtained from 
table 11 (p. 189). Samples C and D were tow net catches from Lake 
Mendota and both contained mainly crustacean material. By far the 
greater part of the former consisted of Daphnia, Diaptomus, and Cy- 
clops, with a smaller portion of algal material, chiefly the diatom Fragi- 
laria; the major portion of the latter consisted of Daphnia, only a few 
copepods and very little plant material being present. Sample No. 403 
was a tow net catch from Lake Monona and contained nothing but 
Daphnia, chiefly Daphnia pulex. The extracts possessed a fishy odor, 
especially that of No. 403; the physical and chemical constants indi- 
cate also that they should be classed with the fish oils. Upon standing 
for twelve hours crystals of glycerides were deposited and the extract 
from No. 403 solidified upon exposure to the air. 


CARBOHYDRATES 


Quantitative determinations indicated that only small amounts of 
sugars were present in the net plankton and no systematic study of 
them was undertaken because it was not practical to obtain sufficient 
material in the regular catches for such determinations. The results 
obtained by Schuette? are summarized in the following statements. 

1. Aqueous and alcohol extracts of the crustacean samples did not 
show the presence of reducing sugars. 

2. Material in which the algae greatly predominated possessed re- 
ducing sugars that were soluble in 50.0 per cent alcohol. 

3. A sample of blue-green algae and one consisting of a mixture of 
diatoms and crustacea contained carbohydrates that were soluble in hot 
water. No pentoses were present but the extracts showed a slight re- 
ducing action toward Fehling’s solution. 


* Trans. Wis. Acad. Sci., Arts, and Let., Vol. XIX, 1918, p. 605. 


NET PLANKTON OF LAKE MENDOTA 43 


4. The aqueous extract of the sample consisting of diatoms and crus- 
tacea eave an amorphous brown precipitate when treated with phenyl- 
hydrazine hydrochloride and sodium acetate. This precipitate did not 
contain an osazone that was soluble in hot 60.0 per cent alcohol or in 
pyridine. Neither did it contain disaccharides or glucosides. 

5. No substance comparable to the algin of marine algae was found. 

The samples of net plankton yielded measurable amounts of furfurol 
when distilled with hydrochloric acid; the furfurol was collected as 
_ the phloroglucide, weighed, and the result calculated to pentosans. 
Such quantitative determinations were made on 95 of the 134 net 
samples that were collected between 1911 and 1915. Table 12 gives a 
summary of the results obtained on the material from 1912 to 1915; 
only three determinations were made on the material collected in 1911 
and none at all on that of 1916 and 1917, so that these three years do 
not appear in this table. All of the analyses are shown in the general 
table, however (No. 43, p. 202). The pentosans constitute a relatively 
small proportion of the organic matter in the net plankton. In only 
two samples out of the 95 that were analyzed did the pentosans amount 
to as much as 5.0 per cent of the organic matter, while the annual 
means range from a minimum of 1.7 per cent in 1914 to a maximum 
of 3.4 per cent in 1915. 

The total quantity of the pentosans was relatively small; it exceeded 
30 milligrams per cubic meter of water in only two of the samples on 
which determinations were made. One sample collected in 1912 yielded 
36.1 milligrams and another obtained in 1915 gave 33.6 milligrams per 
cubic meter of water; both of these amounts were found during the au- 
tumnal maxima of organic matter. The annual means for the pentosans 
range from a minimum of 5.4 milligrams in 1914 to a maximum of 15.3 
milligrams per cubic meter of water in 1912, almost a threefold varia- 
tion. Since the observations were discontinued on July 2, 1914, the 
results cover only the first half of this year; therefore, it can not be 
compared fairly with the full years. The minimum of the complete 
years is 10.9 milligrams which is two-thirds as large as the maximum of 
1912, thus showing a very much smaller range of variation when the 
three complete years only are considered. 


CRUDE FIBER 


The term ‘‘crude fiber’’ is applied to the organic material which 
remains undissolved after the plankton is digested for half-hour periods 
with sulphuric acid and sodium hydroxide solutions having a specific 
gravity of 1.25. In the mixed plant and animal material of the net 
plankton, the crude fiber consists of carbohydrates derived chiefly from 
the former and of chitin from the shells of the crustacea. A certain 


44 PLANKTON OF WISCONSIN LAKES 


part of the crude fiber of forage crops is digested by the ruminants, but 
no data are available to indicate whether any portion of the carbohy- 
drates in the plankton erude fiber is utilized by the organisms which 
feed upon this material or not. The chitinous portion of the crustacean 
shells passes through the alimentary canal of fishes without being 
affected by the digestive processes, so that this part of the plankton 
crude fiber may be regarded as having no food value, and it constitutes 
a considerable portion of the total crude fiber at times. Since the crude 
fiber constitutes a relatively small part of the organic matter in most 
instances, and since the chitinous part of it has no food value, it appears 
that the crude fiber of the net plankton plays a comparatively unimpor- 
tant role from the food standpoint, even if some of the carbohydrates 
in it are utilized by the organisms which consume the various plank- 
tonts. 

In table 13 (p. 189) it will be noted that the crude fiber of the net 
plankton varied from a minimum of 2.6 per cent of the organic matter 
in one sample obtained in 1913 to a maximum of 20.2 per cent in a 
sample collected in 1912. Thus, in one sample out of the 119 on which 
determinations were made the crude fiber amounted to as much as one- 
fifth of the organic matter, while in three-quarters of the samples it 
was 10.0 per cent or less. The mean percentage for the different years 
varied from 6.3 per cent in 1915 to 10.6 per cent in 1911. 

In terms of milligrams per cubic meter of water the amount of crude 
fiber ranged from a minimum of 3.7 milligrams in 1911 to a maximum 
of 67.0 milligrams in 1912, an eighteenfold: difference. The greatest 
difference for a single year was found in 1911 in which the maximum 
amount was fourteen times as large as the minimum. The mean quan- 
tity for the different years fell between 17.5 milligrams in 1914 and 30.8 
milligrams in 1912. 

NiTROGEN FREE EXTRACT 


The various constituents which have been considered thus far, such 
as the crude protein, the ether extract, the crude fiber, and the ash, 
do not comprise the whole of the net plankton; that is, if expressed 
in percentages of the dry sample they do not constitute 100.0 per cent 
of the material. In some instances, in fact, these four items account for 
only about two-thirds of the dry matter. This is due to the fact that 
there are various carbohydrates present which are not included in these 
items. More or less carbohydrate material is found in the erude fiber, 
especially in samples containing large numbers of algae, but, in general, 
only a relatively small proportion of the total amount of carbohydrates 
appears in the crude fiber. It is customary to designate all of the ecar- 
bohydrates not included in the erude fiber as nitrogen free extract and 
the quantity of this extract is determined by difference; that is, the 


NET PLANKTON OF LAKE MENDOTA 45 


percentages of crude protein, ether extract, crude fiber and ash are 
deducted from 100 and the remainder constitutes the percentage of 
nitrogen free extract. 

There is a wide variation in the percentage of this extract in the 
net plankton of Lake Mendota, ranging from a minimum of about 5.0 
per cent to a maximum of 35.0 per cent; in the great majority of the 
samples, however, it falls between 10.0 per cent and 25.0 per cent. The 
average for the entire series of net samples is a little more than 20.0 
per cent of the dry weight of the material. 

Analyses of substantially pure catches show that the nitrogen free 
extract in ten samples of blue-green algae constitutes from 25.0 per cent 
to 52.0 per cent of the dry weight of the material; in most of the sam- 
ples it falls between 30.0 per cent and 40.0 per cent. Two samples of 
diatoms show approximately 23.0 per cent and 34.0 per cent, respec- 
tively. 

Table 49 shows that the crude protein, ether extract, crude fiber, and 
ash constitute from 75.0 per cent to 95.0 per cent of the dry weight of 
the plankton crustacea; the former was noted for a sample containing 
Daphmia pulex and the latter in one consisting of Cyclops. These four 
items, therefore, account for a much greater proportion of the crusta- 
cean material than they do of the algal material. 

A quantitative study of only one of the carbohydrate compounds of 
the net plankton was made, namely, of the pentosans. The analyses 
show that the pentosans constitute but a relatively small part of the 
carbohydrate material that is present in the samples. The average for 
all of the pentosan determinations, for example, is about 2.6 per cent, 
while that of the nitrogen free extract is 20.8 per cent, the former 
being only about one-tenth as large as the latter. 


AsH 


The ash content of the net plankton was relatively large, more espe- 
cially when diatoms were abundant. In only 12 samples out of a total 
of 184 did the ash fall below 10.0 per cent of the dry weight and in only 
79, or less than half of the total number, did it fall below 20.0 per cent. 
In 68 out of 100 samples obtained from Lake Mendota between 1911 and 
1914 the ash fell below 20.0 per cent, while only 11 out of 84 secured 
between 1915 and 1917 fell below this percentage. Table 14 gives a 
summary of the ash and silica determinations of the different years, 
while all of the determinations are indicated in the general table (No. 
43, p. 202). The differences between maximum and minimum percen- 
tages of ash in the various years range from less than twofold to almost 
sevenfold. The former was noted in 1914, in which year the observa- 
tions were discontinued on July 1, so that a full season was not repre- 


46 PLANKTON OF WISCONSIN LAKES 


sented; the latter was found in 1912. The highest maximum per- 
centage, nearly half of the dry weight, was noted in 1915 and the lowest 
in 1914. The maximum percentage of ash each year was correlated 
with the appearance of large numbers of diatoms; in four of the years 
in which the observations continued from spring until late autumn or 
early winter, the highest percentage of ash was found in September and 
October, but in 1911 it came in August, being correlated with a crop 
of diatoms which flourished during this month. The very low maxi- 
mum in 1914 was due to the seareity of diatoms during the vernal 
period of that year and the observations did not cover the other diatom 
season, namely, the autumn. 

Thus the mean percentage for 1914 is by far the lowest of all, but 
that for 1917, in which year the observations also covered only the 


JULY AUGUST SEPTEMBER OCTOBER 
ile lsiale els ely eles) ie Sie eee 


300 


Ore, 
J 
Seu a 
If 4 
“Oss O= Om 2 
= 
. ees Ce me ee i 


Fig. 15.—The quantity of dry organic matter and of ash in the net plankton 
of Lake Mendota in 1911. Curve A represents the organic matter and curve 
B the ash. The amounts are indicated in milligrams per cubic meter of 
water. 


vernal period, is exceeded by only two other years. Excluding the two 
parts of years, the mean percentage of ash shows a gradual increase 
from 1911, the lowest, to 1915 and 1916, the highest, the last two years 
being substantially the same. This is in accordance with the fact al- 
ready pointed out that most of the samples having less than 20.0 per 
cent of ash were collected during the period 1911 to 1914. The mean 
percentage of ash for the entire series of 184 net samples is 23.5 per eent 
of the dry material. 

The relative amounts of organic matter and of ash or imorganie 
matter in the net plankton of the various years are shown graphically 
in figures 15 to 20, inclusive. Both are indicated in milligrams per 


NET PLANKTON OF LAKE MENDOTA 47 


eubic meter of water and the space between the two curves represents 
the excess of organic matter over the inorganic matter. In these dia- 
erams the curves for the former are marked A and those for the 
latter B. 

The curves representing the ash possess, in a general way, the same 
configuration as those of the organic matter; that is, they show spring 
and autumn periods of maxima and summer and winter periods of 


Fig. 16—The quantity of dry organic matter and of ash in the net plankton 
of Lake Mendota in 1912. Curve A represents the organic matter and curve 
B the ash. The amounts are indicated in milligrams per cubic meter of 
water. 


minima, which have already been noted for the organic matter. The 
ash is subject to certain irregularities in summer, however, just as 
noted.in the organic matter. A closer comparison of the curves for the 
various years shows that more or less marked differences exist. In 
1911, for example (fig. 15), the curve for ash does not reach its highest 
point in August until nearly a week after the organic matter reaches 


48 PLANKTON OF WISCONSIN LAKES 


its maximum. Likewise in early October of that year the rise in the 
ash was not proportionately as rapid as in the organic matter. 

The marked vernal rise in the organic matter in 1912 (fig. 16) was 
not accompanied by an equally marked increase in the ash. Between 
the first of May and the end of the first week in June, the former rose 
from 83.6 milligrams per cubic meter of water to 574.4 milligrams, 
almost a sevenfold increase, while the ash, during the same interval of 
time, increased from 18.1 milligrams to 63.7 milligrams, the latter being 
only three and a half times the former. The increase in organic matter 
at this time was due largely to an increase in the number of copepods 
and usually these organisms possess a low percentage of ash. During 
the last week of July in this year there was also a distinct rise in the 


| noerecr | oecemece_| 


| seprenece | ccrosce | 
sfrlelste|s[elsis| | elsleislelste! 


AUGUST 


100 


Fig. 17.—The quantity of dry organic matter and of ash in the net plankton 
of Lake Mendota in 1913. Curve A represents the organic matter and curve 
B the ash. The amounts are indicated in milligrams per cubic meter of 
water. 


organic matter which was not accompanied by a corresponding increase 
in the ash; in this ease, the rise of the former was due chiefly to a crop 
of Ceratium and the percentage of ash in this organism is low. The rise 
in organic matter in late August and early September was due to dia- 
toms and it was accompanied by a similar rise in the ash. During the 
remainder of this year the two curves are similar, but the ash showed 
a proportionately greater decrease in November than the organie 
matter. | 

The vernal increase of ash in 1913 (fig. 17) did not begin as early 
as that of the organic matter and the curve for the former does not 


NET PLANKTON OF LAKE MENDOTA 49 


reach its maximum height until the last of May, while the latter reaches 
its highest point about three weeks earlier. Between the last week in 
August and the first of October, there was a thirty-fivefold increase in 
the ash while the organic matter on the latter date was only a little more 
then seven and a half times as much as that on the former date. During 
the rest of October the ash showed a decline; the organic matter showed 
a decline at the end of the first week in October and then rose to the 
_ highest point for 1913 at the end of the third week in October before it 
began to decline in amount. Thus, the curve for organic matter shows 
two peaks for the autumnal period of this year, while that for the ash 
shows only one. During the latter half of November and the first half 
of December, the ash showed approximately a fivefold increase while the 
change in organic matter during this interval was less than twofold. 


ha Feanuae 


4{e felts lel slo [ole ly ets lel sje |e [4] fe | ola 


Fig. 18.—The quantity of dry organic matter and of ash in the net plankton 
of Lake Mendota in 1914. Curve A represents the organic matter and curve 
B the ash. The amounts are indicated in milligrams per cubic meter of 
water. 


In 1915 (fig. 19) the vernal increase of ash did not begin as early 
as that of the organic matter and it declined more promptly following 
the maximum; as a result the curve for ash possesses a sharp-pointed 
peak and that for the organic matter has a peak with a much broader 
outline as well as a small secondary peak in the middle of May. Up to 
the middle of May the increase in organic matter was due largely to 
copepods and this was followed by a marked rise in the number of dia- 
toms with a consequent increase in ash. Following this the diatoms 
declined while the organic matter remained high as a result of the 
inerease in the number of crustacea. 

During the latter part of April, 1916 (fig. 20), there was a small 
increase in the organic matter which was accompanied by a decrease 


50 PLANKTON OF WISCONSIN LAKES 


in ash. This increase in the organic matter was due to a rise in the 
number of Cyclops and Microcystis, both of which possess a relatively 
low percentage of ash. The ash and organic matter rose simultaneously 
about the middle of May and the former declined rapidly during the 
latter part of the month, thus making a sharp peak in this curve; but 
the organic matter remained high during this time and rose to a maxi- 
mum at the end of the first week in June, so that this curve is character- 
ized by a broad apex covering this period. One other difference may 
be noted here; there was a marked decline in the ash during the second 


week of October to which no similar decrease in organic matter cor- 
responded. 
Aa ae 


JULY AUSUST SEPTEMBER OCTOBER NOVEMBE Re. 


BOOOGUSGRGGRGEE Gc 


Fig. 19—The quantity of dry organic matter and of ash in the net plankton 
of Lake Mendota in 1915. Curve A represents the organic matter and curve 
B the ash. The amounts are indicated in milligrams per cubic meter of 
water. 


In 1917, the curve for ash shows a prominent peak about the first of 
May, but the organic matter shows a steady increase following this date 
instead of a decline similar to the ash. The increase in the organic 
matter during May was due chiefly to an increase in the crustacea and 
Aphanizomenon; these forms yield a much smaller percentage of ash 
than the diatoms which produced the rise in late April and early May. 

In general, then, whenever the increase in organic matter is due to 
the crustacea, the rotifers, and the algae, exclusive of the diatoms, the 
accompanying rise in the ash is relatively small; but when the diatoms 


"CP O1V} OOG “1OJVM JO IOJOU OIQnD Jod SUIVASIT[IUL UL PoYVoIpUI Iv SJUNOWIS OY, “YSe OY} | OAINO pue 1044eU JIUvSIO OYY S}ueserder W 
eAIND “LTEL ‘ABI 03 ‘OTET ‘ArenIGa,, WoT eyopuceTT oye Jo uojyueld you oy} UL YSe JO pue 104yvM oIuvs10 AIP Jo AyYUeNbD OY T,— 0g ‘ST 


BORE OGEE odes ooo ooo ooo odo ooo ce 


52 PLANKTON OF WISCONSIN LAKES 


play the predominant rdle, the increase in ash is correspondingly 
marked because their silicious shells make a very substantial contribu- 
tion to the ash. 

CONSTITUENTS OF THE ASH 


Silica (SiO,). Quantitative determinations of the silica were made 
for substantially all of the samples and the results are indicated in 
the general table (No. 48, p. 202); a summary of the results is given 
in table 14 (p. 190). The latter table shows that the maximum per- 
centages of silica for the five full years ranged from 31.6 per cent of 
the dry weight of the sample in some material collected in 1912 to 37.2 
per cent in another sample secured in 1915; that is, in three of the five 
complete years samples were obtained in which the silica constituted 
more than a third of the dry weight of the net plankton, while in the 
other two years the maximum percentages fell only a little below one- 
third. In 1911 the highest percentage of silica was found in August, at 
which time a crop of diatoms predominated; in 1912 and in 1915 the 
highest percentage was found in material collected in October, while in 
1913 it was noted in December and in 1916 in September. The smallest 
percentage of silica noted for the entire series of net catches from Lake 
Mendota was found in sample No. 314 which was collected on June 30- 
July 3, 1918. The mean percentage for the complete years was lowest 
in 1911 and highest in 1915. 

In 1914 and in 1917 the observations were discontinued on July 2 
and June 1, respectively, and they show much smaller maximum per- 
centages of silica than the complete years, more especially the former. 
This is accounted for by the fact that the largest crops of diatoms come 
in late summer as in 1911, or in the autumn or early winter as in the 
other four full years. The mean percentage of silica for 1914 is very 
much lower than those of the complete years, but that of the part year 
1917 is somewhat higher than that of 1911, but distinctly lower than 
those of the other four full years. 

A comparison of the mean percentages of ash and silica for the com- 
plete years shows that the latter is the most important constituent of 
the ash. In general the mean percentage of the silica comprises about 
half or more of the mean percentage of ash. This does not hold true for 
the individual samples, however, because there is a wide variation in 
the proportion of silica in the ash; that is, the former comprised as little 
as one twenty-fifth of the ash in one sample and more than four-fifths 
in another. It is derived mainly from the diatoms of the net plankton 
so that the amount is smallest when the diatoms are scarcest and largest 
when they are most abundant. The silica, in fact, serves as a good index 
of the relative abundance of diatoms in the samples. Thus, with the 
exception of 1911 when the maximum amount of silica was found in 


NET PLANKTON OF LAKE MENDOTA 53 


August, the highest percentages of this substance were found in spring 
and in autumn, the latter always exceeding the former, and the lowest 
percentages were noted in summer and in winter. 

The difference between the mean percentage of ash and the mean per- 
centage of silica for the various years is shown in the last column of 
table 14. This difference represents the other inorganic constituents 
of the ash; it was smallest in 1912 and largest in 1917. The maximum 
range of variation was from 3.7 per cent in one sample of 1914 to 27.0 
per cent in one sample of 1915. The general range, however, was much 
smaller; in only four samples out of 184 did the difference between 
the percentage of ash and the percentage of silica fall below 5.0 per cent 
and in only 238 did it go above 15.0 per cent. Of the latter, 12 were 
found in 1916 and 7 in 1917. 

Further analyses were made on the ash of 27 samples of net plankton 
and the results of these analyses are shown in table 15, p. 190. 

Iron and Alumina (Fe,O, and Al,O,). Quantitative determinations 
of the iron and alumina were made on the ash of 16 samples. An at- 
tempt was made to obtain quantitative results for them separately but 
their amounts were so small in the material available for the work 
that the analyses did not yield concordant results; hence this effort was 
discontinued and they have been recorded together in the table. The 
iron and alumina constituted a minimum of 0.26 per cent of the dry 
weight of the sample in one instance and a maximum of 2.18 per cent 
in another sample. In 10 samples the amount did not exceed one per 
eent while the mean for the 16 analyses is a little less than one per cent. 

Manganese (Mn,0,). The manganese was determined in the ash of 
12 samples for the purpose of ascertaining how extensively the plankton 
organisms make use of this element. Bradley* found that the tissues 
of fresh-water mollusks belonging to the genera Anodonta and Unio 
contain, on an average, slightly more than one per cent of manganese. 
Since some of the plankton organisms included in these net catches 
serve as food for these bivalves a few analyses were made in order to 
determine the relative importance of this material as a source of this 
element. No measurable amount was found in 5 of the samples; in 5 
others the amount of Mn,O, ranged from 0.30 to 0.89 per cent of the 
dry weight of the material, while in the other two it was larger, namely, 
0.75 and 0.77 per cent. Some of the net plankton, therefore, may serve 
as a source of manganese for other organisms, but the amount available 
in this material is relatively small. 

Phosphorus (P,0;). The phosphorus was determined in 21 samples 
of ash. It was present in all of them and varied in amount from a 
minimum of 0.25 per cent of the dry weight of the material to a maxi- 


‘Jour. Biol. Chem., Vol. 3, pp. 151-157; Vol. 8, 1910-11, pp. 237-249. 


54 PLANKTON OF WISCONSIN LAKES 


mum of 2.54 per cent, when calculated as P,O,. The average percentage 
for the 21 determinations is 1.44 per cent. Schuette® reported the ele- 
mentary phosphorus as varying from 0.91 to 1.57 per cent in 6 samples 
of plankton which he analyzed. 

Sulphur. The quantity of sulphur was determined by Schuette in 
8 samples of plankton; 5 of these samples consisted of a mixture of 
plant and animal material, two were blue-green algae, and one was the 
crustacean Daphnia pulex. One of the mixed samples contained only 
0.42 per cent.of sulphur, while the other 7 samples possessed substan- 
tially the same percentage, namely, from 0.60 per cent to 0.64 per cent. 

Calcium (CaO). Calcium determinations were made on the ash of 
23 samples. The amount varied from a minimum of 1.03 per cent to a 
maximum of 6.8 per cent; the average for the 23 samples was 3.2 per 
cent. The plankton crustacea, more particularly the Daphnias, show 
a rather large variation in the amount of calcium which they possess 
and this probably accounts for the marked differences in the net plank- 
ton. . 

Magnesium (MgO). The analyses of 24 samples show that the mag- 
nesium is smaller in amount than the calcium. The magnesium in 
these samples varied from a minimum of 0.17 per cent to a maximum of 
1.95 per cent. The average for the series was 0.81 per cent, or only 
about a quarter as large as the calcium. 


RESULTS OF OTHER INVESTIGATORS 


Apstein® made ash determinations on the net plankton of several 
fresh-water lakes. In the material from Dobersdorfersee he found a 
marked variation in the percentage of ash during the different months 
of the year; it ranged from a minimum of 6.9 per cent of the dry ma- 
terial in a catch which was obtained on August 31, 1891, to a maximum | 
of 45.0 per cent in a catch taken on October 4, 1891. In Ploner See a 
minimum of 7.1 per cent was found on September 25, 1891, while on 
November 6 a net catch yielded 66.6 per cent of ash. The lowest per- 
centage of ash was found in net material collected in Molfsee on Au- 
cust 18, 1895, namely, 2.9 per cent. 


The variations in the percentage of ash in the net plankton of Lake 
Mendota were of about the same order of magnitude as those noted by 
Apstein for Dobersdorfersee, that is, from a minimum of 5.6 per cent 
to a maximum of 48.3 per cent. The net plankton of Molfsee yielded 
a smaller percentage of ash than that of Lake Mendota and the material 
from Ploner See gave a larger percentage. 


°Trans. Wis. Acad. Sci., Arts, and Let., Vol. XTX, 1918, p. 602-3. 
*Das Suesswasserplankton. Kiel und Leipzig, 1896, p. 200. 


NET PLANKTON OF LAKE MENDOTA 55 


In some samples of marine net plankton which were obtained during 
the winter months when diatoms were abundant, Hensen’ found that 
the ash constituted from 52.1 per cent to 54.7 per cent of the dry ma- 
terial. The ash of Ceratium amounted to 3.9 per cent of the dry weight; 
one sample of copepods contained only 0.45 per cent of ash and another 
consisting of Calanus yielded 3.78 per cent. Salpa runcinata gave 14.6 
per cent of ash. 

Brandt® gives results for the chemical analyses of nine samples of 
marine net plankton obtained in Kiel bay between September 21, 1892, 
and September 28, 1893. His data are given in table 51 (p. 218) 
in the series marked II to X. The nitrogen in these samples varied 
from a minimum of 1.8 per cent to a maximum of 5.6 per cent of the 
dry material; thus, the crude protein ranged from 11.2 per cent to 35.0 
per cent, representing a little more than a threefold variation in per- 
centage. This marine material yielded from 1.5 per cent to 8.7 per 
cent of ether extract and from 8.5 per cent to 61.4 per cent of ash; the 
ash contained from 4.5 per cent to 51.2 per cent of silica. 

In the net samples from Lake Mendota the minimum percentage of 
nitrogen was 3.9 per cent of the dry sample (table 43) and the maxi- 
mum was 9.9 per cent. In terms of crude protein these results repre- 
sent 24.5 per cent and 62.3 per cent respectively. The minimum for 
nitrogen and crude protein, therefore, is more than twice as large in the 
Mendota material as in the marine catches recorded by Brandt, while 
the maximum in the former is nearly twice as large as that in the latter. 
The ether extract in the net samples from Lake Mendota ranged from 
2.7 per cent to 20.0 per cent; both of these percentages are larger than 
the minimum and maximum given by Brandt. The minimum and maxi- 
mum percentages of both ash and silica were larger in the marine ma- 
terial than in the Mendota net samples. 

On the basis of these chemical analyses, then, the net plankton of 
Lake Mendota represents a better class of food material than the ma- 
rine net plankton, in so far as protein and fat are concerned, because 
the former contains a larger percentage of these two excellent food 
substances ; in addition also, the percentage of ash is not as large in the 
fresh-water as in the marine net plankton. 


ORGANISMS. RESPONSIBLE FOR PERIODIC INCREASE 


During the five years of these obesrvations the vernal increase in 
the organic matter of the net plankton was due to a rise in the number 
of diatoms. In three of these years all of the limnetic forms, namely, 
Melosira, Tabellaria, Fragilaria, and Asterionella, showed a distinct 


*Finf. Ber. Kom. z. wissen. Untersuch. d.d. Meere, 1887, pp. 1-107. 
*Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, pp. 45-90. 


56 PLANKTON OF WISCONSIN LAKES 


increase in number at this season; but in the other two years the rise 
was due chiefly to a single form, such as Tabellaria in 1913 and Aste- 
rionella in 1914, while the other forms showed only a relatively small 
increase at this time. 

Following the vernal period the green and blue-green algae become 
the predominant phytoplanktonts and they usually hold this position 
until autumn. In 1913, however, the marked rise in the organic mat- 
ter in late July and early August (fig. 7) was due to two organisms, 
one of which was a diatom; both Ceratium and Melosira began to in- 
erease in numbers during the first week in July. The former reached 
a maximum of 25,800,000 per cubie meter of water on July 21 and the 
latter rose to 29,900,000 filaments per cubic meter on August 4. After 
the latter date both declined rapidly in numbers, corresponding to the 
decline in the organic matter. 

In the curve for 1912, Ceratium was responsible for the peak which 
appeared in late July (fig. 7). Its average number in the upper 10 
meters was 15,665,000 individuals per cubic meter of water on July 30. 
The peak shown for the first week in September was produced by an 
increase in the diatoms, Melosira being the chief form while Fragilaria 
was second in importance. The former rose from 1,077,000 per cubic 
meter on July 30 to 17,639,000 on September 2 and the latter increased 
from 74,000 to 4,548,000 filaments per cubic meter during the same 
period. The secondary peak shown in the first week of September, 
1915, was produced by an increase of two diatoms, namely, Fragilaria 
and Tabellaria, the former being the more important factor. 

The five autumnal maxima shown in figures 7 and 8 were due chiefly 
or wholly to marked increases of the diatoms. In 1911 there was a 
large increase of Melosira in late September and of Fragilaria in Oc- 
tober corresponding to the rise in the quantity of organic matter. 
Among the blue-green algae also there was a distinct rise in the number 
of colonies of Microcystis from the last of September to the middle of 
October. The Cyclops population was also larger during the latter 
half of October. 

In 1912 inereases in Melosira, Tabellaria, and Fragilaria were corre- 
lated with the autumn rise of the curve for organic matter. Coelo- 
sphaerium also increased in numbers during the second half of Sep- 
tember and the number remained fairly high until the last of October. 
The first peak in the autumnal increase of organic matter in 1913 was 
correlated with an increase in the numbers of Fragilaria and Melosira, 
but the second peak came at a time when these two forms were declining 
in numbers. In fact, it is difficult to account for the second peak in 
October by means of the numerical data because the only increases in 
numbers at that time were relatively small ones in Microcystis, Coelo- 


NET PLANKTON OF LAKE MENDOTA 57 


sphaerium, and Daphnia hyalina. The December peak of 1913 was 
correlated with a rise in Fragilaria. 

In 1915 the peak noted in October was correlated with a rise in Fragi- 
laria, while the increase in organic matter during November and De- 
ecember corresponded to a gradual increase in the numbers of Tabellaria 
and Stephanodisecus. Asterionella and Stephanodiscus were respon- 
sible for the October peak in the curve for 1916. 

The summer and winter minima correspond to relatively small num- 
bers of the various organisms in the net plankton. In late spring or 
early summer the diatoms decline to a minimum number and the green 
and blue-green algae then become the predominant phytoplanktonis. 
While the light and temperature conditions are favorable for these 
forms during the summer, yet they do not develop in sufficient abun- 
dance in Lake Mendota to produce a maximum comparable to the max- 
ima of the diatoms in the spring and in the autumn. In winter both tem- 
perature and light conditions are unfavorable for the growth of the 
phytoplankton; the latter is especially unfavorable after the lake be- 
eomes covered with ice and snow. As a result the numbers of these 
organisms decline as the winter season advances. 

The numerical results obtained in this investigation show more or 
less pronounced maxima of crustacea and rotifers in spring and autumn 
and Birge ® also noted these two periods of maxima, with an additional 
one in July, in his studies on the crustacea of Lake Mendota. Just how 
prominent a part these two groups of organisms play in increasing the 
quantity of organic matter in the net plankton is difficult to determine. 
When allowed to settle, the net samples taken during the spring and 
autumn maxima show a distinct preponderance of algal material; con- 
sequently these maxima have been attributed chiefly to the latter forms. 
Thus, the large growths of phytoplankton at such times tend to mask 
the increases shown by crustacea and rotifers. 

Numerically, of course, the phytoplanktonts are more abundant, but 
a comparatively small increase in the number of the larger crustacea is 
equivalent to a very marked increase in the algae so far as weight is 
concerned. Several determinations show that it takes 225 individuals 
of Cyclops of mixed sizes to yield one milligram of dry material; of the 
other plankton crustacea it takes 135 Diaptomi, 140 Daphma retro- 
curva, and 75 Daphma hyalina to yield this amount of dry material. 
Among the rotifers it takes about 125 colonies averaging one hundred 
individuals each, or 12,500 individuals of Conochilus volvox, to yield 
one milligram of dry material, while it requires only about one-tenth 
as many, or 1,250, large specimens of Asplanchna for this amount. 
About 650 colonies of Volvox of mixed sizes yield a milligram of dry 


*Trans. Wis. Acad. Sci., Arts, and Let., Vol. XI, 1898, pp. 274-448. 


58 PLANKTON OF WISCONSIN LAKES 


material, but it requires about one million individuals of Euglena. One 
milligram of dry material contains about 30 million cells of Micro- 
eystis, while Whipple and Jackson '° found that it takes about 2.8 mil- 
lion cells of Asterionella to weigh this amount. These results show 
clearly that a relatively small rise in the number of crustacea is equiv- 
alent to a very marked increase in numbers in the smaller forms so far 
as the yield of dry material is concerned. 


The numerical results show a wide variation in the numbers of the 
different forms, ranging from two or three individuals per liter of 
water in some forms to several thousand in others, or even to more than 
30 million in one of the members of the nannoplankton. Such a wide 
range in numbers makes it impossible to construct diagrams by the 
usual methods for the purpose of illustrating the distribution of the 
various organisms, so that it has been necessary to use the spherical ~ 
type of curve in order to get all of the forms on the same diagram. 
Lohmann ™ used this type of curve for the graphical expression of some 
of his results on marine plankton and he has discussed the method of 
constructing such curves. He prepared a table, based on a value of 
1 = 0.25 mm., showing the radii of spheres by half millimeters, which 
represent numbers from 82 individuals up to 864 million individuals; 
his table is incorporated in this report as table No. 52 (p. 219). The 

3:/ wa 
formula for determining the radius in a given instance is R = \/—— 


4.19 
in which V equals the volume of the sphere, or in this case the number 


of individuals to be represented by the sphere. In order to simplify 
the formula, Lohmann has used 4 as a denominator instead of 4.19 
since the omission of the fraction 0.19 makes only a very small differ- 
ence, amounting to but 1.5 per cent in a number as large as 800 million. 


Lohmann’s diagram illustrating the method of construction of the 
spherical curve is shown in figure 21. The time element is platted 
along the abscissa, which also serves as the equatorial plane of the 
series of spheres. The radii of the spheres representing the various 
numbers of a given form are platted as ordinates at the proper time 
intervals so that the central point of each sphere is situated at the in- 
tersection of the radius and the abscissa. A circle of proper radius 
drawn around this point of intersection represents a cross section of 
the sphere. In order to complete the curve the outer ends of the radii 
representing ordinates are connected by lines. Only the radii above 
the abscissa may be used in constructing a curve, or both those above 
and those below are connected if a symmetrical figure is desired. 


Jour. N. E. Waterworks Assoc., Vol. 14, 1899, pp.1-25. 
1 Wissensch. Meeresuntersuch. K. Kom., Abt. Kiel, Bd. 10, 1908, pp. 192-194. 


aid 


NET PLANKTON OF LAKE MENDOTA 59 


Fig. 21.—Diagram illustrating the construction of the spherical type of curve. 


The curves in figures 22 to 26 represent the number of individuals 
or colonies of the various organisms in the net plankton per cubic 
meter of water; this is a rather large unit of volume, but a smaller one 
would make it difficult to show some of the forms which are present 
in relatively smaller numbers, such as Diaptomus and Daphnia, for 
example. The scale used in these curves is the same as that employed 
by Lohmann, namely, 1—0.25 mm., and his table has been used to 
ascertain the lengths of the radii. For purposes of comparison with 
the enumerations of the net planktonts, the results obtained for the 
organic matter are also shown by this type of curve; it was necessary 
to make use of a larger volume of water for the organic matter in order 
to bring out more clearly the various changes in quantity. These 
curves for the organic matter, therefore, show the number of milli- 
grams of dry organic matter per 10,000 cubic meters. Figures 7 and 
8 indicate the variations in the quantity of the organic matter much 
better and these diagrams should be considered along with figures 22 
to 26. The printed figures showing the numerical results for the net 
plankton are approximately one-fourth as large as the original dia- 
grams. 

In 1911 (fig. 22) numerical work was begun on the net plankton in 
April but the gravimetric and chemical study did not begin until the 
first of June. The increase of organic matter in the second week of 
June was correlated with an increase in Ceratium and slight increases 
in the number of diatoms, while the August rise of the organic matter 
corresponded to marked increases in Ceratium and Melosira. In the 
autumn of 1911, there was a marked rise in the number of diatoms, 
especially Melosira and Fragilaria, with an increase in the organic 
matter at this time. 

In 1912 (fig. 23) there was no marked rise in any one form corre- 
lated with the increase in organic matter during the month of April, 
but several forms were present in considerable abundance at this time. 
A rise in the organic matter about the first of June accompanied an 


60 PLANKTON OF WISCONSIN LAKES 


inerease in the numbers of Ceratium and Melosira, and a second rise 
during the first week of August corresponded to increases of Melosira 
and Fragilaria. The September rise of organic matter accompanied 
a marked increase in the diatoms as well as a distinct rise in the num- 
ber of Coelosphaerium. 

The diagram for 1913 (fig. 24) shows that a large crop of Tabellaria 
in May yielded a large amount of organic matter, while increases in 
Ceratium and Melosira during the last week in July and the first week 
in August correspond to an expansion in the curve for organic matter 
at this time. Fragilaria was the chief organism responsible for the 
autumnal rise. 

Increases in the numbers of diatoms and of Microcystis in May, 1915 
(fig. 25), were accompanied by an increase in the organic matter, while 
a large increase of Aphanizomenon was found in June, which helped to 
keep the organic matter high when the other two forms declined in 
numbers. The October maximum of this year corresponded to a rise 
in Fragilaria. In 1916 (fig. 26) the peak during the last half of 
May and the first half of June was accounted for largely by the in- 
crease in the diatoms during this interval. The rise in the amount of 
organic matter in the autumn of 1916 was correlated in time with 
increases in the numbers of the various diatoms; some of these forms 
together with Aphanizomenon prolonged the increased quantity of 
organic matter until the middle of December. 


EXPLANATION OF THE NET PLANKTON DIAGRAMS 


Figures 22 to 26 show the numerical results obtained for the samples 
of net plankton; the spherical type of curve has been used in these 
diagrams. The curves show the number of individuals or colonies per 
cubic meter of water. In order to bring out the variations in the 
quantity of organic matter more clearly by this type of diagram, that 
curve was platted on a different scale; it shows the number of milli- 
grams of dry organic matter in 10,000 cubic meters of water. 

The species of Cladocera and Rotifera were enumerated separately, 
but it was not practicable to indicate each species in the diagrams. The 
following abbreviations have been used for the different forms: 

DI = Diaptomus, CY —Cyclops, NA=nauplii, DA = Daphnia, 
RO = Rotifera, CHK = Ceratium, MI = Microcystis, CO == Coelosphae- 
rium, AP = Aphanizomenon, AN = Anabaena, LY = Lyngbya, SM = 
Staurastrum, ME = Melosira, TA = Tabellaria, FR = Fragilaria, AS 
== Asterionella, ST = Stephanodiseus, OM== organic matter. 


elele[+ [ole lel [ofelel [+[elels[elelets [> lele[ [olele, 
190 | 4096 | anv | Amr | mar | srw | wav | 


= - a ee a a Did Pt ae et le J i 7 ate 

us on fe Sr : E —s ~ him oe 

3 ee meas - z 7 we eal! So kes =a = 
‘ = ey 2 t ~ _—S 
= ~ = mi 
~ 
r 
= 
<= 
: 
- 
a 
‘ 

s 

= =; ‘ 


Me 


& 


= 


N 


APR. MAY | JUNE | SULY SEPT 
jlelsl4)vl2lsl4l, s[4(7[2[s]4[7] els [4[7)2[s]4] 
| | Sixties 


Fig. 23.—Diagram showing the numerical results for the net plankton of Lake 
Mendota in 1912. The various forms of plankton organisms are indicated 
in the explanation on page 60. 


* 


ne ae 


fi 


| Se faaeaee 2lal4|/|e fs | 


Sige 
(Ue im: crt eN ii a 
TI nee 


Fig. 24.—Diagram showin sults for the face nkton of Lake 
We ndota a 1913. ay various a plankton organ indicated 


ra ete ete feta tals Eb felafet Le lotale 4 |s | BBBRAAEE 2[s «| 


Jee] PCEFCOMTET TELE 
| LEPLITT OE ee a 
TT PR ee 


QD ee a 
i | LTT ETE} 
ees ae ) 


Fig. 25.—Diagram showing the numerical results for the net plankton of Lake 
Mendota in 1915. The various forms of plankton organisms are indicated 
in the explanation on page 60. 


FEB | MAR APR MAY SLIME _| SOL) 


AVE SEP. ocr | Wov |- DEC | AN | FEB MAR _|_APR MAY 
elslelej2isi(4i 414 4/7 leis lel [els [4{Jele 2hal4|elelole|sle|slele leis lel jel[slel|sielsiels sil< ely l-{7[e[2]-|7Jel2/< 
jal [ee cat se | | ci 

YQ)! Gayo SS SSS SS Se a aS SSS SSS SaaS oS SSS SSS SaaS SSS SoSSsSSSs— 

e CUS SSS EO See Ee eee Seas 

n ACCC Sa 

A) = a 


RO LLL So | 
e arto 5 
H/ 

J 
cO 


| 


Fig. 26.—Diagram showing the numerical results for the net plankton of Lake 


Mendota in 1916 and 1917. 
explanation on page 60. 


The various planktonts are indicated in the 


NANNOPLANKTON OF LAKE MENDOTA 63 


CHAPTER IIT 
THE NANNOPLANKTON OF LAKE MENDOTA 


The term nannoplankton, meaning dwarf plankton, was employed by 
Lohmann ‘ in 1911 to designate the very minute plankton organisms. 

He set an arbitrary maximum diameter of 25y for the members of 
this group. Many of the fresh-water organisms which are so small 
that they readily pass through the meshes of the finest bolting cloth, 
exceed this dimension, so that the usefulness of the term would be 
very greatly restricted if it should be applied only to forms whose 
maximum diameter is not greater than 25y. Therefore, since these 
organisms are grouped together on a purely artificial and arbitrary 
basis, a broader meaning is applied to the term nannoplankton in this 
report in order to make it more useful to planktologists. As used here, 
it is applied to the assemblage of plankton organisms which are so 
small that they readily pass through the meshes of the finest silk bolt- 
ing cloth and are thus lost regularly by the net. Such an extension of 
the meaning makes the term much more useful and gives, at the same 
time, a practical basis for the separation of the total plankton into 
two definite classes, namely, the net plankton and the nannoplankton. 
As used in this report, the term nannoplankton is the equivalent of 
what has been called the centrifuge plankton, but the word nanno- 
plankton is a more convenient term as well as a more euphonious one. 

In the Wisconsin lakes that have been investigated so far, the assem- 
blage of organisms which passes through the meshes of the net is made 
up of rhizopods, flagellates, and ciliates among the animals, and of 
various algae, such as Ankistrodesums, Oocystis, Sphaerocystis, Aphano- 
capsa, and species of diatoms belonging to the genera Stephanodiscus, 
Cyclotella, and Coceconeis. Certain forms are lost by the net acci- 
dentally, such as rod-shaped organisms which strike the net endwise 
and so are enabled to pass through, young individuals or colonies, and 
fragments of the colonial forms. In general, however, the great bulk 
of the material which was obtained with the centrifuge in these in- 
vestigations consisted of the minute individuals that were small enough 
to be lost regularly by the net. 

Quantitative studies on the nannoplankton of Lake Mendota were 
begun with the large De Laval centrifuge on April 21, 1915, and were 


1Internat. Revue, Bd. IV, 1911, pp. 1-38. 


64 PLANKTON OF WISCONSIN LAKES 


bors o: ei oa : 
contd aa J une 1, 1917. With the exception of January, 1916, 


observations were made every month during this period, and runs or 
catches were made twice a week in most instances during the time that 
the lake was free of ice, 1. e., from April to December. In 1915 the 
centrifuge runs totaled 65 in number, in 1916 there were 69, and in 
1917 there were 11 up to June 1 when the observations were discon- 
tinued. The number of runs and the amount of water centrifuged each 
year are shown in table 3 (p. 182). The total for Lake Mendota for 
the three years was 145 runs in which 179,506 liters of water were cen- 
trifuged, an average of about 1,240 liters for each run. 


Errect oF CENTRIFUGING 


The centrifuge bowl has an inside diameter of 24 centimeters and the 
maximum speed at which it can be run with safety is 6,000 revolutions 
per minute. For all of the runs made in these investigations the speed 
was kept a little below the maximum so that it ranged from 5,600 to 
5,800. Experiments showed that this high speed was necessary to 
remove the organisms effectively; only about 70.0 per cent as much 
material was obtained at 4,000 revolutions as at 5,800. Taking 5,700 
revolutions per minute as the average speed of the centrifuge, the 
inner surface of the bowl, on which most of the material was deposited, 
moved at a rate of about 4,300 meters per minute, or 71.6 meters per 
second. 


The material was frequently examined with a microscope in order to 
ascertain whether the various organisms showed any signs of serious 
injury as the result of being centrifuged. In general those forms which 
possess a fairly firm cell wall did not show any evidences of serious 
injury. Colonies of Pandorina were found intact, for example, and 
they promptly renewed their normal activities after being centrifuged ; 
while Euglena showed no signs of physical injury, it remained in a 
contracted state for about an hour before it became active. None of the 
algae showed any evidences of injury to the cells. 


The more delicate forms, such as Amoeba and the monads, appeared 
in the material in good condition but their numbers were not as large as 
expected from enumerations made previous to centrifuging. It seems 
probable, therefore, that some of these organisms suffered more or less 
damage, but it is not believed that this was sufficient to affect appre- 
ciably the total quantity of the centrifuge material nor its chemical 
composition. Such an injury might result in the loss of some of the cell 
fiuids which have about the same density as water, but the protoplasmic 
portion would be retained in the centrifuge. <A slight loss as the result 
of more or less injury to these forms, however, would have little signifi- 


NANNOPLANKTON OF LAKE MENDOTA 65 


cance in the final results because they constitute a relatively small part 
of the bulk of the centrifuged material. 

Three sets of experiments were made in 1915 to determine what effect 
the centrifuging process had on the chemical composition of the ma- 
terial. The results of these tests are shown in. table 16, p. 191. Net 
plankton, consisting largely of algae, was used for the experiments. A 
fairly large catch of plankton was obtained by means of a net and the 
material was placed in about two liters of water. The catch was then 
stirred thoroughly in order to distribute the organisms as evenly as 
possible and it was divided into two equal parts. One portion was 
evaporated to dryness directly and used as a standard sample. The 
other portion was put into about 900 liters of centrifuged water and the 
material was then recovered with the centrifuge; its subsequent treat- 
ment was the same as that given to the regular centrifuge catches. 


In samples No. 5108 and No. 5109 Coelosphaerium was the predomi- 
nant form with Ceratium second; a relatively small portion of the ma- 
terial consisted of Microcystis and several other algae. In samples No. 
5120 and 5121 Coelosphaerium was also the most abundant form but 
there was a very large element of Lyngbya and Microcystis. The bulk 
of the material in samples No. 5154 and No. 5155 consisted of diatoms, 
chiefly Melosira and Stephanodiscus. In these catches the blue-green 
algae were represented mainly by Microcystis with an occasional fila- 
ment of Lyngbya. These catches also contained a larger proportion of 
zooplankton than the other two sets; Chydorus and Anuraea cochlearis 
were the leading representatives of this group. 

In total organic matter the uncentrifuged material in sample No. 
5109 exceeded that in the centrifuged half of the catch by 311.6 milli- 
grams. This difference in favor of the uncentrifuged portion was due 
to two factors. With such a large amount of material, namely, about 
22 grams dry weight, concentrated into a little less than two liters of 
water, it was difficult to distribute the organisms evenly so that the 
catch could be divided into two equal portions ; experiments have shown 
also that there is danger of a slight loss of material from the centrifuge 
whenever the catch exceeds about 10 grams in dry weight. In spite of 
these unfavorable factors, however, it will be noted that the difference 
is less than three per cent. In samples No. 5120 and No. 5121 there 
was a small difference, less than two per cent, in favor of the centri- 
fuged portion of the catch. Sample No. 5154, the centrifuged portion, 
yielded just 34.0 milligrams less than No. 5155, a difference of less than 
one-half of one per cent in favor of the uncentrifuged material. 

In table 16 the nitrogen and ether extract are indicated in percent- 
ages of the dry organic matter in the various samples. The centrifuged 
and uncentrifuged portions of each set of experiments show substan- 


66 PLANKTON OF WISCONSIN LAKES 


tially the same percentages of these two substances; the differences in 
each case are no greater than those usually shown by duplicate deter- 
minations on a single sample. These results indicate, then, that the 
proteins and fats in these organisms are not affected quantitatively 
by the centrifuging process. 


QUANTITY OF MATERIAL 


The amount of material obtained in the various centrifuge catches 
in 1917 is shown in figure 27. After the samples were evaporated to 
dryness and the residues were ground up in a mortar, they were placed 
in vials until needed for the chemical analyses. When arranged in a 
series these catches illustrated, in a very interesting manner, the 
changes in the quantity of nannoplankton during the vernal pulse of 
this year and a photograph of them was taken. Since these vials were 
of approximately the same diameter, the height to which they were 
filled by the different samples indicated roughly these changes in quan- 
tity. The samples in the first two vials were obtained from 904 and 909 
liters of water respectively, while all of the others were obtained from 
a larger quantity of water, namely, from 1,168 liters to 1,183 liters; 
the former, therefore, ought to be a little more than 25.0 per cent larger 


_ than they are for a direct comparison with the other samples. 


Attention may also be called to the fact that this material contains 
a large proportion of ash, ranging from a minimum of 51.7 per cent in 
sample No. 718 to a maximum of 64.2 per cent in sample No. 708; that 
is, somewhat less than half of the dry material consists of organic 
matter. 

For the purpose of indicating more clearly the differences in height 
two scales marked off in centimeters were included in the photograph. 
The samples are arranged in chronological order beginning with sample 
No. 704 (vial No. 1) obtained on February 14, 1917, and ending with 
sample No. 722 (vial No. 10) secured on June 1, 1917; the intervening 
numbers and dates are shown in the general table for the nannoplank- 
ton catches, No. 44, p. 207. The series includes all of the centrifuge 
samples collected in 1917, except No. 702 and this one was omitted be- 
cause some of it had already been used for a chemical analysis when the 
photograph was taken. 

Vial No. 2, containing sample No. 706 collected on March 9, 1917, 
shows the smallest quantity of material and increasing its volume 25.0 
per cent would not make it as large as that of vial No. 3 (sample No. 
708), so that there was a distinct increase in material by the latter 
date, namely, April 18. There followed, then, a substantial increase 
during the succeeding week, but the most marked increase came between 
April 25 (vial No. 4) and May 2 (vial No. 5); the material in the 


Fig 


can) 


. 27—Photograph of dry nannoplankton material obtained from Lake Men- 
dota in 1917. Nannoplankton catches 704 to 722, inclusive, table 44. 


$ 


NANNOPLANKTON OF LAKE MENDOTA 67 


former reached a height of about 3 centimeters and in the latter about 
6 centimeters. The maximum height, fully 6.5 centimeters, was. at- 
tained in vial No. 6 (sample No. 714); this was succeeded by a rapid 
decline to a height of only about 3.5 centimeters in vial No. 7 (sample 
No. 716) during the following week. By the first of June (vial No. 10, 
sample No. 722) the amount of material was substantially the same as 
that obtained on April 18 (vial No. 3, sample No. 708). 


OrGANIC MATTER IN THE NANNOPLANKTON 


In the regular observations on Lake Mendota the material obtained 
from the two runs of the same week were usually combined into one 
sample for the chemical analyses. In all there were 35 samples of cen- 
trifuge material in 1915 and 41 in 1916; only one run per week was 
made in 1917, or 11 im all, and these were not combined so that the 
total number of centrifuge samples amounted to 87. The results of the 
chemical analyses of the various samples are shown in detail in table 
44, p. 207. 

The variations in the amount of dry organic matter in the centrifuge 
plankton are shown in table 17, p. 192. The largest amount, 3,151.5 
milligrams of dry organic matter per cubic meter of water, was ob- 
tained in sample No. 604 on April 15, 1916, and the smallest amount, 
795.2 milligrams, in No. 706 on March 9, 1917. It will be noted that 
this minimum is distinctly below the minima of the other two years, but 
the mean amount for the three years shows a striking uniformity, the 
differences being only a little more than one per cent. 

The results for the dry organic matter are shown graphically in 
figure 28 in which the curves represent the number of milligrams per 
cubic meter of water that were obtained from the various samples. The 
first sample obtained in 1915 contained the largest amount of organic 
matter for that year. It is uncertain whether a larger amount was 
present before this date or not, but the first sample consisted of two 
runs, one made on April 21 and the other on April 23, 1915, and the 
latter produced a larger amount than the former, which indicates that 
the first sample covered the highest point attained by the nannoplank- 
ton in 1915. 

A sharp decline in the amount of organic matter was noted during 
the last week in April and the first week in May, 1915, and the quantity 
thereafter remained fairly uniform until the middle of June. During 
the last half of June there was a marked decrease, which was followed 
by a distinct rise in July. The increase culminated in a peak about the 
end of the first week in August after which the amount fell rapidly, 
reaching the lowest point of the year about the middle of August. Dur- 
ing the succeeding two months there was a gradual increase and there- 


YIGNIIIG 


YIFDWFTAON 


YIGOLIO 


aIOWILITS 


Lgnony 


eee eee ee es el 
WwW 


Av" 


[2] 


Phere Er 


The curves show the number of milligrams of 


TOPS) opel Se 
per cubic meter of water. 


organic ma 


Fig. 28—The quantity of dry organic matter in the nannoplankton of Lake 
Mendota, 


tter 


NANNOPLANKTON OF LAKE MENDOTA 69 


after the quantity of organic matter showed considerable variation, 
but remained relatively high until the last run was made on December 
1915. 

Between December 6, 1915, and February 11, 1916, the organic mat- 
ter decreased from 1,719.0 milligrams to approximately 995.0 milli- 
grams per cubic meter of water, a decline of 43.0 per cent. By March 
8, 1916, the amount was almost double what it was on February 11, and 
it rose to more than treble the amount on the latter date by April 15. 
During the succeeding three weeks there was a marked decline so that 
the vernal peak in the curve for this year is a very prominent one and 
reaches the highest point of the entire series. 


A secondary peak covers the period from the second week in May to 
the middle of June, 1916, and another reaches its maximum height 
about the end of the first week in July. Following this a gradual de- 
cline continued until the end of the first week in September; during 
the remainder of this month there was a rapid rise which was followed 
by an equally rapid decline in October. Thus, the curve for this period 
possesses a sharp and prominent peak which gives it a very different ap- 
pearance from the curve of 1915. This decline continued in 1916 until 
the minimum amount of the year was reached on November 1. The fol- 
lowing six weeks were characterized by two minor increases in the 
organic matter which were succeeded by relatively small decreases, so 
that the curve presents two small peaks covering this period of time. 

A decrease of about 8.0 per cent in the organic matter of the nanno- 
plankton was noted between December 12, 1916, and January 18, 1917; 
this decline continued until March 9, 1917, when the minimum of the 
entire series was found. By April 18 the organic matter had nearly 
doubled in amount and the increase continued until the vernal maxi- 
mum was reached on May 7; this maximum was a little more than three 
times as much as the minimum of March 9. There was a marked decline 
between May 7 and May 16 so that the curve presents a well defined 
vernal peak; this decline was followed by a prominent increase on May 
23 and this was followed by a decrease on June 1, 1917, when the ob- 
servations were discontinued. 

In a general way the curves in figure 28 show that the annual cycle 
of the nannoplankton is separated more or less definitely mto four 
phases corresponding to the four seasons of the year; the same fact has 
been noted for the net plankton. It will be observed, however, that the 
autumnal phase may not be as prominent in the nannoplankton in some 
years, such as 1915 for example, as it was each year in the net plankton. 
A distinct vernal maximum was noted for each of the three years cov- 
ered by these cbservations and it proved to be the maximum of the year 
in both of the complete years, namely, 1915 and 1916. 


70 PLANKTON OF WISCONSIN LAKES 


The curves for 1916 and 1917 show that there are considerable vari- 
ations in the extent and duration of the vernal pulse of nannoplankton. 
The highest point of the former, for example, is almost 20.0 per cent 
above that of the latter. In the former year also a marked increase of 
organic matter took place during the latter part of February and early 
in March, while in 1917 it was decreasing at this time; the late April 
samples of 1917, in fact, contaimed a smaller amount than that of 
March 8, 1916. Thus, the vernal peak of 1916 represents very much 
more organic matter than that of 1917. Following the vernal maximum 
the curves for both 1915 and 1916 show two secondary peaks during 
the summer period. with an extended minimal region in August and 
September. 

In the autumn the organic matter rose to a decided maximum in 
1916, but in 1915 it rose only to a median level and then remained 
fairly uniform in amount during this season. The winter season was 
characterized by a decline in the amount of organic matter. The rela- 
tion of the various organisms to the changes in the quantity of organic 
matter is discussed on a subsequent page of this chapter. 

On August 7, 1915, two samples were obtained for the purpose of 
studying the vertical distribution of the nannoplankton material. One 
sample was taken from the lower water of Lake Mendota, or from the 
14-20 meter stratum, and the other from the upper water, or the 0-13 
meter stratum. The sample of nannoplankton from the upper stratum 
yielded 1,837.0 milligrams of dry organic matter per cubie meter of 
water, while the one from the lower stratum contained only 854.0 
milligrams; that is, the former yielded a little more than twice as much 
organic matter per cubic meter of water as the latter. 


CHEMICAL ANALYSES OF THE NANNOPLANKTON 
NITROGEN AND CRUDE PROTEIN 


Owing to the presence of a large amount of organic matter in the 
centrifuge material, the percentage of nitrogen in the dry sample was 
relatively low. It varied from a minimum of 1.27 per cent in one 
sample obtained in 1916 to a maximum of 5.18 per cent in one sample 
secured in 1915. (See general table, No. 44, p. 207). The mean per- 
centages ranged from 2.37 per cent in 1916 to 3.63 per cent in 1917. 

Calculated on an ash free basis, the maximum percentages of the dif- 
ferent years vary from 8.77 per cent of nitrogen to 10.66 per cent, 
while the minima are about half as much, namely, from 4.24 per cent 
to 5.91 per cent. The results for the different years are summarized in 
table 18, p. 192. The mean percentage was lowest in 1916, namely, 
6.27 per cent and highest in 1917, or 8.27 per cent, the difference being 


NANNOPLANKTON OF LAKE MENDOTA TL 


just two per cent. While the average amount of organic matter was 
somewhat higher in the nannoplankton of 1916 than in that of the other 
two years, it contained a smaller percentage of nitrogen. 


A comparison of the percentage of nitrogen in the net planktcn 
(table 7, p. 187) with that in the nannoplankton shows that the latter 
is much lower in the dry sample, but when both are calculated to an 
ash free basis the difference is relatively small, although the nitrogen 
in the nannoplankton material is appreciably smaller than in the net 
plankton. In 1915 when the observations covered all seasons of the 
year, the mean percentage of nitrogen in the nannoplankton was less 
than half of one per cent below that in the net plankton of 1911, but 
it was almost one and a half per cent below that of the net plankton of 
1915. The mean percentage of nitrogen in the nannoplankton in 1916 
was almost two and a half per cent below that in the net plankton of 
1915, 


The annual variations in the quantity of nitrogen in the nannoplank- 
ton of Lake Mendota are shown in figure 29; the curves in this diagram 
indicate the number of milligrams of nitrogen per cubic meter of water. 
It will be noted that the general form of these curves is similar to that 
of the curves representing the organic matter of the corresponding 
years in figure 28; they show vernal and autumnal maxima with sum- 
mer and winter minima. In 1915; however, there was an August maxi- 
mum in both the organic matter and the nitrogen which exceeded the 
autumnal maximum of that year. The curves for organic matter and 
nitrogen are very similar in configuration in 1915, but the curves for 
the other years show more or less marked differences. In 1916, for 
example, the nitrcgen did not rise to its maximum height in April 
until a few days after the organic matter had passed its maximum 
point; in the rise which took place during the latter half of June and 
in early July the nitrogen reached its highest point during the last 
week in June while the organic matter reached its highest point a week 
later. The nitrogen curve has a well rounded form during November 
and December, 1916, while the curve for organic matter presents two 
well defined peaks at this period. In 1917 the nitrogen curve possesses 
two equally prominent peaks in the month of May, but in the curve for 
organic matter the second peak is much lower than the first. 

In 1915 the ratio of the organic matter to the total nitrogen in the 
nannoplankton of Lake Mendota varied from 9.1 to 18.8, thus showing 
slightly more than a twofold difference. (See table 44.) In 1916 the 
range was from 11.4 to 23.5, indicating a smaller proportion of nitrogen 
in the organic matter obtained this year than in the previous year; the 
material collected in 1917 showed the smallest range of variation, 
namely, from 9.4 to 16.9. For the entire series of nannoplankton sam- 


‘ 


PLANKTON OF WISCONSIN LAKES 


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NANNOPLANKTON OF LAKE MENDOTA 73 


ples from Lake Mendota, the ratios varied from 9.1 to 23.5; that is, the 
nitrogen constituted from one-ninth to about one-twenty-third of the 
organic matter in the centrifuge material. This was a larger variation 
than was found in the net plankton of Lake Mendota in which the 
extremes were 8.7 and 17.4. 


In 1915 several determinations of the nitrogen content of the water, 
both before and after being centrifuged, were made for the purpose of 
ascertaining how closely the loss of nitrogen during the centrifuging 
process corresponded with the amount of nitrogen in the nannoplankton 
recovered by the centrifuge. It was found that the individual deter- 
minations showed considerable variation; in some instances the loss of 
nitrogen by the water was in excess of that recovered in the nanno- 
plankton and in other samples the reverse was true. These differences 
were due, apparently, to slight inaccuracies in the method of determin- 
ing the quantity of nitrogen in the water when it is present in such 
relatively small amounts. These variations are about evenly balanced, 
however, so that the differences become comparatively small in the aver- 
ages for a number of determinations. 


In twelve analyses, for example, the average quantity of nitrogen 
lost by the water in the centrifuging process was 106.3 milligrams per 
cubic meter of water, and the nannoplankton material recovered from 
this water yielded 103.5 milligrams per cubic meter; the two amounts 
correspond as closely as could be expected when the two very different 
methods of treatment are taken into consideration. That is, the dis- 
crepancy is only 2.8 milligrams, or a difference of 2.6 per cent. A 
larger number of determinations would, doubtless, have reduced this 
difference materially. 

In these twelve analyses the average quantity of organic nitrogen in 
the water after it was centrifuged was 758.0 milligrams per cubic meter ; 
in comparison with this the nannoplankton yielded 103.5 milligrams 
and the net plankton 21.5 milligrams, thus making a total of 125.0 milli- 
grams of organic nitrogen per cubic meter of water for the total plank- 
ton. On this basis the water of Lake Mendota contained somewhat 
more than six times as much organic nitrogen as the total plankton. In 
addition to this the water contained an average of 4.0 milligrams of 
nitrite and 392.0 milligrams of nitrate so that the total nitrogen 
amounted to 1,154.0 milligrams per cubie meter of water, all of which 
would be available for the nitrogenous element in the food of the va- 
rious plants of the lake. According to these results, then, the water 
contained a little more than nine times as much nitrogen as the total 
plankton. 

In view of the abundance of the dissolved nitrogen compounds, 
Piitter’s theory regarding the nutrition of aquatic animals may be con- 


74. PLANKTON OF WISCONSIN LAKES 


sidered here. Through computations based on data obtained by him- 
self and by others, Piitter? became convinced that there is not enough 
organized food present in most bodies of water to support the animal 
population ; he concluded, therefore, that the animals are able to make 
use of the organic compounds that are held in solution by the water, 
as well as of the organized food. Putter thought he obtained con- 
firmatory evidence for his theory in a series of experiments on several 
kinds of aquatic animals, including plankton crustacea and fishes; in 
spite of these results, however, his theory is not generally accepted by 
cther investigators. 

The quantitative results obtained on Lake Mendota show that there 
was always an abundance of food for the rotifers and crustacea and 
these two groups of animals are the chief consumers of the other plank- 
ton forms. Computations based upon numerical data and upon the aver- 
age weight of the different kinds of rotifers and crustacea indicate that 
from twelve to eighteen times their own weight of available food was 
present in the plankton of Lake Mendota at the time of this investiga- 
tion; it must be concluded, therefore, that these two groups of animals 
always had an ample supply of organized food. Puttter’s conclusion 
regarding the lack of food for plankton crustacea was based upon data 
obtained from net catches only; all of the nannoplankton, however, may 
be used for food by the crustacea and the quantity of nannoplankton 
in Lake Mendota was several times as large as the available crustacean 
food in the net plankton. 

The results for crude protein (N X 6.25) are summarized in table 
19. The largest percentage of crude protein was found in a sample col- 
lected in 1917, while the maximum, minimum, and mean percentages 
were lower in 1916 than in the other two years. On the other hand, the 
largest quantity of crude protein per cubic meter of water was found in 
1916, namely, 1,560.0 milligrams; this was more than 43.0 per cent 
higher than the maximum amount of 1915. The largest minimum 
amount was found in 1915 and the smallest in 1917, the latter being just 
a little more than 75.0 per cent of the former. The maximum quantity 
of crude protein in 1915 was less than three times as large as the mini- 
mum of that year, but in 1916 there was a fivefold difference, with more 
than a fourfold difference in 1917. The mean quantity was smallest in 
1916 and largest in 1917, the former amounting to about 77.0 per cent 
of the latter. The average amount of crude protein in the nannoplank- 
ton was three times as large as that in the net plankton in 1916, three 
and a half times as large in 1915, and a little more than five times as 
large in the 1917 samples. (Compare tables 8 and 19.) 


*Die Ernahrung der Wassertierc, p. 168. Jena, 1909. 


NANNOPLANKTON OF LAKE MENDOTA WS) 


The crude protein constituted 45.0 per cent of the organic matter, or 
more, in three-quarters of the net plankton samples, but in only 30 of 
the 87 centrifuge samples, or about a third of them, did it reach this 
large a proportion; in only 50 samples did the crude protein equal 40.0 
per cent or more of the organic matter in the nannoplankton. 

The details of the relation of the crude protein to the organic mat- 
ter are shown in figures 30 and 31. In these diagrams the quantities 
are indicated in milligrams per cubic meter of water. In 1915 the 
curve representing the crude protein, marked B in the diagram, pos- 
sesses an outline similar to that representing the organic matter, curve 
A, but attention may be called to two minor differences. The small peak 
in the former which appears in the last week of July has a valley cor- 
responding to it in the latter, the organic matter having reached a small 
maximum during the previous week. Also during the latter half of 
November and the first week in December the changes in the quantity 
of crude protein were not closly correlated in time with similar changes 
in the organic matter. 

In 1916 the vernal maximum for crude protein came a few days later 
than that of the organic matter and a rise of the latter during the first 
part of July was correlated in time with a decrease in the former. The 
curve for crude protein in November and December, 1916, presents a 
rounded form while that for the organic matter shows two peaks at 
this time. In 1917 the second peak in the curve for organic matter 
during April is much lower than the first, while the two peaks in the 
curve for crude protein at this time are substantially equal in altitude. 

Column 5 under nitrogen in the general table (No. 44) gives the ratio 
of the organic matter to the total nitrogen in the various samples. In 
60 of the 87 samples of nannoplankton this ratio varies between 12 and 
18; that is, the nitrogen constituted from one-twelfth to one-eighteenth 
of the organic matter in these samples. The ratio fell below 12 in 10 
samples and above 18 in 18 samples. The sample containing the largest 
proportion of nitrogen gave a ratio of 9.1 and the one having the small- 
est proportion 23.5. These figures again serve to show that the pro- 
portion of nitrogen in the nannoplankton, in general, was distinctly 
smaller than in the net plankton. In a little more than two-thirds of 
the net plankton samples, for example, the ratio fell between 9 and 12, 
while in the nannoplankton samples only about one-ninth of them gave 
a ratio as low as 12 or less. 


ETHER EXTRACT 
A summary of the results of the determinations of the ether extract 


in the nannoplankton is given in table 20, p. 192; the figures indicate 
the percentage of the organic matter and the number of milligrams per 


76 PLANKTON OF WISCONSIN LAKES 


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Fig. 30.—The quantity of organic matter, of crude protein and of ether extract 
in the nannoplankton of Lake Mendota in 1915. Curve A shows the amount 
of dry organic matter, curve B the crude protein and curve C the amount 
of ether extract in milligrams per cubic meter of water. Compare with 
figure 14. See table 44. 


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78 PLANKTON OF WISCONSIN LAKES 


cubic meter of water. The details of the analyses are shown in the 
general table, No. 44, p. 207. 

The highest percentage of ether extract, calculated on an ash free 
basis, was found in a sample obtained in 1915, namely, 15.77 per cent. 
In 1917 the maximum was only 7.07 per cent, less than half of the 
preceding maximum, but the latter is not representative of an entire 
year since the observations were discontinued on June 1, 1917. The 
minimum percentage was highest in 1915 and lowest in 1917 and the 
same was true of the mean percentage. There was slightly more than 
a fivefold difference between the maximum and minimum of 1915, al- 
most sixfold in 1916, and more than threefold in 1917. 

When stated in terms of milligrams per cubic meter cf water, as in- 
dicated in the second part of this table, the largest quantity of ether 
extract was found in 1915 and the smallest in 1917. The mean quantity 
was lowest in 1917 and highest in 1915, the latter being about 40.0 per 
cent higher than the former. 

The curves marked C in figures 30 and 31 give a graphic representa- 
tion of the quantity of ether extract in the various samples. These 
curves show that, in general, the extract constituted a relatively small 
proportion of the organic matter. The largest amount was found dur- 
ing the vernal pulse of the nannoplankton, after which there was a 
gradual decline to the summer level, which was reached about the first 
week in June. Thus, the decrease was more gradual than in either 
the organic matter or the crude protein. With two exceptions the quan- 
tity remained well below 100 milligrams per eubiec meter of water 
during the summer season. In 1915 the curve shows a distinct peak 
during the early part of August which is correlated with similar peaks 
in the organic matter and the crude protein at this time. In 1916 the 
quantity rose slightly above 100 milligrams about the middle of July, 
and again during the autumnal rise in late September and early Octo- 
ber. In the latter part of 1915 there is no marked rise corresponding 
to the early autumnal increase of organic matter, but a prominent peak 
covers the second part of November which is correlated with a peak 
in the organic matter. 

In comparison with the net plankton (table 48) the nannoplankton 
shows a distinctly lower percentage of ether extract; the mean of the 
latter averages only about a half to two-thirds as high as the former. 
This is probably due to the larger proportion of fat in the crustacea of 
the net plankton. Owing to the presence of a larger quantity of nanno- 
plankton, however, the total amount of ether extract in it is markedly 
ereater than that in the net plankton. In 1915, for example, the mean 
percentage of ether extract in the net plankton is one and seven-tenths 
times as much as that in the nannoplankton, yet the mean quantity of 
the latter is two and a half times that of the former. 


NANNOPLANKTON OF LAKE MENDOTA 79 


PENTOSANS 


The pentosans were determined in 32 nannoplankton samples which 
were collected in 1915. The amount in them varied from a minimum 
of 2.84 per cent of the organic matter to a maximum of 7.35 per cent; 
the mean percentage for these samples was 4.94 per cent. (Table 21, p. 
193.) This represented a distinctly larger amount than was found 
in the net plankton of this year, the mean percentage of the latter 
being 3.41 per cent. With a larger amount of organic matter and a 
higher percentage of pentosans in the nannoplankton than in the net 
plankton the differences in favor of the former are still more striking 
when the amounts are expressed in terms of milligrams per cubic meter 
of water. In 1915, for example, the average quantity of the pentosans 
in the nannoplankton was a little more than six times as much as that 
in the net plankton. The maximum quantity in the former was about 
three and a half times as large as the maximum of the net plankton, 
while the minimum of the nannoplankton was more than fourteen times 
as large as the minimum of the net plankton. 

Only eight determinations were made on the samples collected in 1916 
and but two on those of 1917. The samples of 1916 yielded a higher 
mean percentage of pentosans than those of 1915, but the two of 1917 
were both lower than the mean of 1915. When stated in milligrams per 
cubic meter of water the mean quantity in 1916 was about 10.0 per 
cent higher and that of 1917 about 8.0 per cent lower than the mean 
of 1915. 


CRUDE FIBER 


The percentage of crude fiber in the organic matter of the nanno- 
plankton showed considerable variation during the period of these ob- 
servations. The largest percentage was found in 1917 and the smallest 
in 1915. (See table 22, p. 193.) The mean percentage for 1917 was 
a little larger than that of 1916, while that of 1915 was less than half 
as much as that of 1916. Crude fiber determinations were made on only 
17 of the 41 samples obtained in 1916 and on only 4 of the 11 samples 
of 1917, and this fact must be taken into account with reference to the 
mean percentages. The differences between the mean quantities are of 
similar magnitude when the results are expressed in terms of milligrams 
per cubic meter of water. 

The mean percentage of the crude fiber in the nannoplankton in 1916 
and 1917 was substantially the same as that of the net plankton for 
1913 to 1915 inclusive, but it was much higher in the latter in 1911 and 
1912. (See table 18, p. 189 for net plankton.) In 1915 the mean per- 
centage of crude fiber in the net plankton was a little more than twice 
as large as that of the nannoplankton; in this year also the mean quan- 


20 PLANKTON OF WISCONSIN LAKES 


tity was 49.0 milligrams per cubic meter of water in the latter and 22.0 
milligrams in the former, somewhat more than a twofold difference. 
The mean quantity of crude fiber in the samples of net plankton ob- 
tained between 1911 and 1915 was only about a quarter to a sixth as 
much as that of the nannoplankton material collected in 1916 and 
1917. This difference was due mainly to the presence of a larger 
quantity of nannoplankton than of net plankton. 

The crude fiber of 20 samples of nannoplankton were analyzed for 
their nitrogen content, but it was found that the fiber contained at most 
only a trace of nitrogen. This indicates that the organisms in this 
material are practically free from chitin. 


NITROGEN E'REE EXTRACT 


The nitrogen free extract, or that part of the organic matter which 
is left after deducting the crude protein, the ether extract, the crude 
fiber, and the ash, ranges from a minimum of a little less than 10.0 per 
cent to a maximum of about 32.0 per cent of the dry weight of the 
nannoplankton; that is, it shows a little more than a threefold varia- 
tion as compared with sevenfold in the net plankton. The largest per- 
centage of nitrogen free extract was found in a nannoplankton sample 
collected in 1915 and the smallest percentage was noted in a sample ob- 
tained in 1916. The mean percentage for the 87 samples of nanno- 
plankton from Lake Mendota is approximately 19.0 per cent, or just 
a little less than the mean for the net plankton. In the nannoplankton 
the mean percentage of the pentosans is only a little more than one- 
tenth as large as the mean of the nitrogen free extract and this is 
substantially the same as noted for the net plankton. 


ASH 


The centrifuge material from Lake Mendota yielded a very high 
percentage of ash, the range being from a minimum of about 35.0 per 
cent to a maximum of about 75.0 per cent of the dry sample. In the 
corresponding samples of net plankton the ash varied from about 9.0 
per cent to a little more than 48.0 per cent of the dry material. Thus 
the minimum of the nannoplankton was about four times as large as 
the minimum of the net plankton, while the maximum of the former 
was somewhat less than twice as large as that of the latter. 

The inorganic constituents of the nannoplankton were derived from 
three sources, namely, (1) the nannoplankton organisms, (2) the silt 
removed from the lake water by the centrifuge, and (3) the water re- 
maining in the bowl of the centrifuge at the end of arun. In the latter 
instance about 5.5 liters of lake water are involved; that is, at the end 
of each run this quantity of water remains in the bowl of the centri- 


NANNOPLANKTON OF LAKE MENDOTA 81 


fuge and large numbers of organisms are suspended in it. During 1915 
all but about 250 cubic centimeters of this water was siphoned out of 
the bowl and kept separate from the material which was deposited on 
the side of the bowl. The amount of organic matter represented by 
the organisms suspended in the bowl water was estimated by making a 
quantitative determination of the organic nitrogen in this water. The 
quantity of nitrogen was then multiplied by the factor representing the 
ratio of the organic matter to the nitrogen in the sample. By this method 
of treatment only about a quarter of a liter of lake water was added to 
the sample, distilled water being used to wash the catch from the bowl; 
this small quantity of lake water did not add materially to the ash of 
the sample. 

In 1916 and 1917, on the other hand, all of the bowl water was added 
to the sample and the entire quantity was evaporated; this made a very 
substantial addition to the ash. Several determinations showed that 
an average of 119.0 milligrams of morganic material was obtained from 
a liter of lake water, so that the 5.5 liters contributed about 655.0 milli- 
grams of ash to the sample. Ash determinations on aliquot portions of 
18 bowl waters in 1915 gave an average of 652.0 milligrams of inorganic 
material for 5.5 liters, thus corresponding closely with the former 
amount which was ascertained by another method. On the basis of 
these results it has been assumed that the bowl water contributes 655.0 
milligrams of ash to the sample, or twice this amount where two runs 
are combined into one sample. 


The percentage of ash in the various organisms of the nannoplank- 
ton, with the exception of Euglena, has not been determined, but such 
results have been obtained for various constituents of the net plankton 
and these may be used as a basis for estimating the ash of the nanno- 
plankton. In some of the blue-green algae the ash varied from 4.3 
per cent to 7.8 per cent of the dry weight, while in Euglena it amounted 
to 5.1 per cent. In the diatoms, of course, there is a much larger pro- 
portion of ash, ranging from 40.0 per cent to approximately 50.0 per 
cent. Since the diatoms constitute a relatively small portion of the 
centrifuge material, except in the spring and in the autumn, the ash 
derived from the organisms in the summer and winter catches may be 
estimated as about 10.0 per cent; that is, the organic matter constitutes 
90.0 per cent of the dry material in these organisms. On this basis, 
then, the ash derived from the nannoplankton organisms ranged from a 
minimum of about 100.0 milligrams to a maximum of approximately 
200.0 milligrams per cubic meter of water during June, July, and Au- 
gust, while in January, February, and March the amount varied from 
90.0 milligrams to about 158.0 milligrams per cubic meter. In the 
spring and in the autumn small diatoms and the fragments of the larger 


29 PLANKTON OF WISCONSIN LAKES 


forms are more plentiful in the centrifuge material so that it is impos- 
sible to give a fair estimate of the percentage of ash in the nannoplank- 
ton organisms during these seasons. 

While the water of Lake Mendota does not show any turbidity due to 
silt, except after unusually heavy rains, yet it contains regularly a 
larger or smaller amount of such material in suspension; without ex- 
ception, silt was noted in all of the centrifuge samples that were used 
for the enumeration of the organisms. No attempt was made to meas- 
ure the quantity of this silt directly, but the amount removed from the 
lake water by the large centrifuge is represented roughly by the quan- 
tity of inorganic matter left after the ash of the bowl water and that 
of the organisms are deducted from the total ash of the sample. The 
results obtained in this way are indicated for a number of samples in 
table 23, p. 194. The samples represented in this table include only 
those in which the diatoms constitute but a relatively small part of the 
material as shown by the enumerations. Where these organisms are 
abundant in the centrifuge material the ash of the nannoplankton would 
exceed 10.0 per cent of the dry weight and the samples given in the 
table have been selected with a view of keeping the ash content within 
the 10.0 per cent limit. 

No bowl ash is shown for the samples of 1915 because the bowl water 
was not added to the samples of that year. The sixth column of table 
23, shows the amount of dry silt in milligrams per cubic meter of water. 
During the summer months, the quantity of silt varies from about 370.0 
milligrams (sample No. 5124-26) to 1,349.0 milligrams (sample No. 
579-81) per cubie meter of water, representing almost a fourfold varia- 
tion. Expressed in other terms, these quantities are 0.37 and 1.35 parts 
per million, respectively. The particles of silt have not been meas- 
ured, but they are so minute that even these surprisingly small amounts 
represent an enormous number of individuals. 

The specific gravity of the more common rock-forming minerals aver- 
ages about 2.6, so that this may be assumed as approximately the spe- 
cific gravity of the silt also. On this basis the volume of 370.0 milh- 
grams of silt is about 140 cubic millimeters and that of 1,349.0 milli- 
grams equals 520 cubic millimeters, thus giving a range of 0.14 cubic 
millimeters to 0.52 cubic millimeters of this material per liter of water. 

The last two samples given in the table show the results for the Jan- 
uary and March eatches of 1917. In both of these samples the amount 
of silt obtained by difference, namely, 235.5 milligrams and 112.5 milli- 
grams, is well below the summer minimum; the March catch, in fact, 
shows only about a third as much as the summer minimum. The Janu- 
ary sample was taken about a month after the lake became covered with 
ice and the comparatively small amount of silt found on that date seems 


NANNOPLANKTON OF LAKE MENDOTA 83 
to indicate that some of the suspended material gradually settles out 
of the water under these conditions because the ice keeps the water from 
being disturbed by the wind. The results obtained on March 9 show a 
further decrease in the quantity of silt thus indicating that the process 
of settling continued during the intervening period of time. In some 
winters, however, there is a very substantial contribution to the 
amount of silt during this season. 

Thaws accompanied by rain sometimes occur in late January or in 
February and much drainage water, well laden with silt, may reach the 
lake at such times. This drainage water contains very little dissolved 
material so that its specific gravity is less than that of the lake water. 
For this reason the drainage water does not mix readily with the lake 
water, but tends to spread out and form a layer of turbid water just 
under the ice. The thickness of this stratum ranges from half a meter 
to a meter, or perhaps a little more at times. In some years the spread- 
ing continues until the entire lake becomes covered with this stratum of 
turbid water, which results in a wide distribution of the suspended silt. 

The second part of table 23 gives the results of some analyses of the 
ash of the centrifuge material. It will be noted that the addition of the 
bowl water to the samples in 1916 and 1917 increased the percentages 
of CaO and MgO very materially, but that this made practically no dif- 
ference in the percentage of silica nor in that of iron and alumina. 
Analyses of the solids of 14 samples of bowl water in 1915 showed that 
CaO constituted from 16.2 per cent to 19.5 per cent of the total solids, 
while MgO ranged from 10.7 per cent to 17.4 per cent. The percentage 
of silica in the solids of the bowl water, as well as that of iron and 
alumina, was relatively low; the former varied from 1.8 per cent to 3.4 
per cent with one sample unusually high, namely, about 9.0 per cent, 
and the latter ranged from 1.1 per cent to 1.7 per cent. These results 
indicate that the diatoms and silt are pretty thoroughly removed from 
the bowl water in the centrifuging process. Since the ash of the cen- 
trifuge catches had a complex origin, any further discussion of its 
chemical composition would not be profitable from a biological stand- 
point. 

THE NUMBER OF ORGANISMS 


After being pumped into the tank, the water used for a centrifuge 
run was thoroughly stirred and a representative sample of about two 
liters was taken out for a numerical study of the organisms; about 75 
cubic centimeters of this sample were used for the enumerations and the 
remainder was then returned to the tank. A few of the forms were 
usually present in sufficient numbers to be counted directly without any 
concentration; for the enumeration of the others it was necessary to 
concentrate the material with a centrifuge. An electric centrifuge ear- 


84. PLANKTON OF WISCONSIN LAKES 


rying two 15 cubic centimeter tubes and having a speed of about 4,000 
revolutions per minute was used. The enumerations were made in 
the usual manner and, in general, they were made in duplicate; in most 
instances it was found that the two counts checked very closely for most 
of the organisms, but whenever the differences for the common forms 
seemed too large, a third count was made. In the more abundant forms 
also, the counts were frequently checked by counting them in the cen- 
trifuged as well as in the uneentrifuged material. The mean of the 
different counts has been regarded as representing the number of the 
various organisms. 

Samples of water were also obtained at different depths of the lake 
for a study of the vertical distribution of the various forms noted in 
the nannoplankton, but the data seeured in the enumerations will not be 
discussed here. 

The results of the counts made on the samples of water taken from 
the tank are shown graphically in figures 32 and 33. As already indi- 
eated for the net plankton, the numbers of the various organisms differ 
so widely that it has been necessary to use the spherical type of curve 
for the nannoplanktonts also, in order to show all of them on the same 
diagram. The curves in these two figures indicate the number of indi- 
viduals in 10 liters of water. The number of individuals per unit 
volume of water is so much larger in the nannoplankton forms than in 
the net plankton forms, that a much smaller volume of water has been 
used for the diagrams of the former than for those of the latter ; that is, 
10 liters in the nanneplankton as compared with one cubic meter in the 
net plankton. The organic matter of the nannoplankton has been 
platted on the same basis as in the net plankton, namely, the number 
of milligrams of dry material per 10,000 cubic meters of water. In 
spite-of being based on a much larger volume of water, this type of 
curve does not show the results for the organic matter nearly as well 
as the curves in figure 28. It will be best to compare the two types 
of diagrams in this connection. 

The various forms of organisms appearing in the nannoplankton of 
Lake Mendota are shown in figures 32 and 33. The rhizopods were 
represented by four different forms. One of them was a small Amoeba 
which was irregular in its seasonal distribution; large numbers were 
found at all depths in some instances. An average of 255,000 indi- 
viduals per liter of water was noted in a sample obtained in August, 
1916; in most of the samples in which this form was found, the numbers 
ranged frcem 1,000 to 6,000 per liter. Three unidentified rhizopods were 
noted; they were present much more frequently than Amoeba, but the 
number rarely exceeded 1,000 to 2,000 per liter of water. 


NANNOPLANKTON OF LAKE MENDOTA 85 


Minute flagellates were found in the nannoplankton of Lake Mendota 
at all seasons of the year. Most of them, perhaps all of them, belonged 
to the form described by Lewis’? in 1913 under the name of Chloro- 
chromonas minuta. Lewis’ description is based cn material obtained 
from Lake Mendota, but the form has been noted in lakes in Iowa and 
New York as well, which indicates that this flagellate is rather widely 
distributed. In Lake Mendota it was found in very considerable num- 
bers at times, especially during the month of May. During the third 
week in May, 1916, the number reached two and a quarter million per 
liter of water in two sets of observations. The average number for 137 
samples in which this form was found, was 237,000 per liter. 

Cryptomonas was found pretty regularly in the various counts, but 
the breaks in the curves show that it was not found at certain periods 
both in winter and in summer. It was most abundant in November, 
1915, when more than 300,000 individuals per liter of water were noted 
in two samples. The average for 112 samples in which this form was 
noted, was 45,000 individuals per liter of water. Another fiagellate, 
Euglena, was found in 1915; it appeared chiefly during the months of 
September and October, but the numbers were relatively small, ranging 
from about 1,000 to 13,000 individuals per liter of water. 

Ciliated protozoa of various sizes were found in the majority of the 
catches, but they were rather irregular in their distribution as indicated 
by the breaks in the curve representing them. A fairly large anaerobic 
ciliate appeared in the lower water each year during the latter part 
of the summer. * 

Several genera of the blue-green algae (Cyanophyceae or Myxophy- 
eeae) and of green algae (Chlorcphyceae) were represented in the ma- 
terial as indicated in the diagrams. A few other forms appeared from 
time to time, but they were found in such small numbers and so irre- 
cularly that they have not been shown in the figures. 

The most important alga belonging to the blue-green or green group 
of forms is an Aphanocapsa with very minute cells, probably Aphano- 
capsa delicatissuma West. It was found at all seasons of the year and 
at all depths of the lake. This alga was obtained at a depth of 170 
meters (558 feet) in Seneca Lake, New York, and it seems to have a 
wide range geographically, since it has been noted in the nannoplank- 
ton of various Wisconsin lakes, in Seneca and Cayuga Lakes, New York, 
and in West Okoboji Lake, Iowa. Usually the colonies are rather small, 
having only 15 to 50 cells, but sometimes a colony is found which has 
100 cells or more. This Aphanocapsa was most abundant in Lake 
Mendota in April and May, but the numbers were fairly large during 


* Archiv fiir Protistenkunde, Bd. 32, 1913, pp. 249-256, 1 pl. 
“See Juday, Biological Bulletin, Vol. 36, 1919, pp. 92-95. 


86 PLANKTON OF WISCONSIN LAKES 


the other months of the year also. In one set of observations in April, 
1915, a maximum of somewhat more than 8 million colonies per liter 
was noted. 

The other forms of blue-green and green algae were found chiefly 
in the summer and autumn. Coelosphaerium and Oocystis were the 
most regular in their appearance, but breaks in the curves representing 
them show that they were not found at times. With respect to the 
breaks in the various curves it may be said that a break does not mean, 
necessarily, that this particular form had disappeared entirely for the 
period covered by the break in the curve, but it does mean that the 
form was so scarce that it was not noted in the samples for this period 
of time; by centrifuging a larger sample of water for the enumeration, 
say 100 or perhaps 200 cubic centimeters, some of the breaks might 
have been eliminated, but in all probability not many of them. Coelo- 
sphaerium was more abundant as well as more regular in its appear- 
ance than Oocystis; the other forms of blue-green and green algae were 
still more irregular in their appearance in samples as indicated by the 
more numerous breaks in the curves representing them. 

Three genera of diatoms were found in the nannoplankton, namely, 
Cocconeis, Cyclotella, and Stephanodiscus. Cocconeis appeared at ir- 
regular intervals during the period from March to December, 1916, but 
the number was relatively small. A small form of Cyclotella was 
found in all of the 1915 samples and it was present in the 1916 catches 
from February to October with the exception of one set of observations 
during the latter part of August. In November and December it was 
noted as irregular and it was not observed in any of the samples be- 
tween the middle of January and the last week of April, 1917. 

Stephanodiscus astraca * showed a most interesting periodicity dur- 
ing this series of observations as indicated in the diagrams. ‘This dise- 
shaped diatom is very small, the diameter ranging from 6.5» to 9.0u 
with an average of approximately 8.0u. It appeared about the middle 
of April in 1915, rose to a maximum during the second week in May, 
and then declined in numbers, rather rapidly at first and then more 
eradually until it finally disappeared about the first of July. In 1916 
this diatom was found during the latter part of March, increased 
rapidly in numbers in early April, and reached its maximum point in 
the third week of this month. There was a rapid decrease in the last 
week of April and a more gradual decline in May, the form disappear- 
ing entirely by the first of June. In 1917 this diatom again made its 
appearance in the latter part of March, but the number rose more 
slowly so that the maximum was not reached until the end of the first 
week in May. This was followed by a rapid decrease in numbers, but 


* We are indebted to Dr. Albert Mann for the identification of this diatom. 


NANNOPLANKTON OF LAKE MENDOTA 87 
the form was still present in considerable numbers when the observa- 
tions were discontinued on the first of June. S. astraea yielded a 
larger number of individuals per liter of water than any other organism 
that was found in the nannoplankton. The mean of two counts made 
on a sample taken from the tank on April 18, 1916, was approximately 
35 million individuals per liter of water. In 1917 the maximum num- 
ber of S. astraea was a little more than 27 million per liter, while in 
1915 it was only about 8.5 million. 

Another species of Stephanodiscus, a distinctly larger form than the 
preceding, was found each year in the spring and in the autumn. It 
was never present in very large numbers, however ; the largest number 
was observed in the first week of October, 1916. 

Fragments of the colonial diatoms, such as Asterionella, Fragilaria, 
and Tabellaria, were found in the centrifuge material and they were 
enumerated. As indicated in the diagrams, these fragments were most 
abundant in spring and in autumn. 

Filaments of the blue-green alga Aphanizomenon were found in most 
of the samples, but usually relatively small numbers of them were 
noted. 

The vernal increases in the organic matter of the nannoplankton 
were correlated in time with the increase in the numbers of certain 
forms; in 1915, for example, the maximum for organic matter was 
found at the same time that Aphanocapsa reached its maximum point 
for the entire series of observations. Stephanodiscus astraea was also 
increasing in numbers at this time, but its increase was not sufficient 
to counterbalance the decline of Aphanocapsa in the last week of April 
and the first week of May. Thus, there was a decrease in the organic 
matter at this time corresponding to an increase of this diatom, but its 
maximum number this year was only about a quarter as large as in 
1917. 

The vernal maxima of organic matter in 1916 and in 1917 were both 
correlated in time with the maximum number of Stephanodiscus 
astraea. There was also a marked increase in Cyclotella and in Apha- 
nocapsa at this period in the former year and a distinct rise in the 
number of monads was noted at this time in the spring of 1917. 

The marked rise in the organic matter in the latter part of July and 
during the first week of August, 1915, was correlated in time with in- 
creases in the numbers of monads and of Coelosphaerium. In Septem- 
ber there was an increase in Aphanocapsa and in Coelosphaerium at 
the time of the rise in the organic matter, but there was no well marked 
maximum of the latter during this autumn and neither was there any 
unusual rise in the number of organisms. A fairly well defined in- 
crease in the monads in late October and in November was accom- 


28 PLANKTON OF WISCONSIN LAKES 


panied by a decrease in Aphanocapsa and Coelosphaerium, the two 
changes just about balancing each other according to the results for 
organic matter. 

In 1916 the rise in organie matter in late June and in early July 
corresponded to increases in the numbers of monads, Coelosphaerium, 
Aphanocapsa, and Oocystis. The prominent September peak in the 
eurve for organic matter in 1916 was correlated in time with a marked 
rise in Cryptomonas, Stephanodiseus, and the fragments of diatom 
colonies. 


ROLE OF BACTERIA IN THE NANNOPLANKTON 


No quantitative observations were made on the bacteria of Lake 
Mendota during this plankton investigation, except to ascertain by 
means of several series of plate cultures that the centrifuge removed 
about one-third of the bacteria normally present in the lake water. A 
quantitative study was begun in July, 1919, and the work is still being 
continued, March 1, 1922. The results obtained during this interval 
of time furnish enough data for a fair estimate of the role of the 
bacteria in the plankton complex of the lake. 

In this quantitative study the number of bacteria has been deter- 
mined both by direct counts and by plate cultures. For the plate 
method Nahrstoff-Heyden agar was selected for the culture medium 
because it gave the highest counts and apparently the largest number 
of different types of colonies. Several series of direct counts have 
yielded an average of ten times as many bacteria as the plate cultures, 
so that it is necessary to multiply the plate counts by the factor ten in 
order to ascertain approximately the number of bacteria per cubie 
centimeter of water by the plate method. 

On the basis of the direct counts, the total number of bacteria in 
Lake Mendota from July to October, 1919, averaged about 3,000 per 
cubic centimeter of water from surface to bottom in 23.5 meters of 
water. In late autumn and early winter the number decreased to a 
minimum average of 1,500 per cubic centimeter. In the spring and 
early summer of 1920 the number rose steadily to a maximum average 
of 30,000 per cubic centimeter; the number remained near the maxi- 
mum until the latter part of August, after which there was a gradual 
decline to a winter minimum of 2,000 bacteria per cubic centimeter of 
water in January, 1921. In the following spring and summer, the 
number averaged from 3,000 to 5,000 per cubic centimeter, but in Sep- 
tember the number rose rapidly. A maximum of 60,000 individuals 
per cubic centimeter was obtained at a depth of 10 meters on September 
22, 1921. Following this the number declined to a winter minimum 
of about 3,000. 


NANNOPLANKTON OF LAKE MENDOTA 89 


The direct counts also show that about two-thirds of the total number 
of bacteria in Lake Mendota are rod-shaped forms and one-third spher- 
ical or coceus forms. The rods range in length from 1.2y to 10.0u and 
their diameters vary from 0.2 to 2.754. The shorter individuals are 
more abundant than the longer ones, so that a large number of meas- 
urements gave an average length of 2.54 and a mean diameter of 0.9p. 
The coccus forms vary from 0.22, to 0.754 in diameter, with a mean 
of 0.44u. 

These results, together with those obtained by other investigators, 
constitute a basis for the computation of the live weight and the quan- 
tity of dry organic matter in this crop of bacteria. The volume of a 
rod-shaped individual of mean size, that is 2.54 long and 0.9» in 
diameter, is 1.5904 cubic micra, or 0.0000000015904 cubic millimeter. 
The volume of a spherical individual with a diameter of 0.44y is 0.0446 
cubie micron, or 0.0000000000446 cubic millimeter. 

The maximum summer average in 1920 was 30,000 bacteria per cubic 
centimeter, or 30 billion per cubic meter of water. Two-thirds of them, 
or 20 billion, were rod-shaped individuals and one-third, or 10 billion, 
were spherical forms. On the basis of the mean size the volume of the 
rod-shaped bacteria in a cubic meter of water at that time was 31.808 
cubic millimeters and of the spheres 0.446 cubic millimeter, making a 
total of 32.254 cubic millimeters for the two groups. Rubner’® found 
that the specific gravity of water bacteria averaged about 1.05 so that 
the live weight of the bacteria in a cubic meter of Mendota water dur- 
ing the summer maximum of 1920 was approximately 33.9 milligrams. 

Rubner ® and Nishimura’ state that about 84.0 per cent of the live 
weight of aquatic bacteria consists of water; on this basis the maximum 
erop of bacteria in Lake Mendota in 1920 represented 5.4 milligrams 
of dry material per cubic meter of water. Nishimura also found 11.2 
per cent of ash in a water bacillus which he analyzed; deducting this 
percentage of ash from the Mendota material leaves 4.8 milligrams of 
dry organic matter per cubic meter of water for the maximum summer 
erop of bacteria in 1920. The average number from July to October, 
1919, was 3,000 bacteria per cubic centimeter, or one-tenth as many as 
the summer maximum of 1920; the crop of the former year, therefore, 
represented only about 0.48 milligram of dry organic matter per cubic 
meter of water. The winter minimum of 1,500 bacteria per cubic 
centimeter was only one-twentieth as large as the summer maximum of 
1920, so that it amounted to 0.24 milligram per cubic meter. 


° Archiv fiir Hygiene, Bd. 11, 1890, pp. 365-395. 
*Arehiv fiir Hygiene, Bd. 46, 1903, pp. 1-63. 
“Archiv fiir Hygiene, Bd. 18, 1893, pp. 318-333. 


90 PLANKTON OF WISCONSIN LAKES 


The maximum number of 60,000 per cubic centimeter noted at 10 
meters on September 22, 1921, represented twice as much material as 
the maximum of 1920: that is, wet weight 67.8 milligrams per cubic 
meter of water, dry weight 10.8 milligrams, and organic matter 9.6 
milligrams. The 1921 maximum, however, is not the average for all 
depths, but the number at 10 meters; the numbers noted at other depths 
on this date were considerably smaller, but they would still give an 
average above that of 1920. 

On the basis of their ability to produce living matter in the course 
of the year, Lohmann ® estimated that one volume of bacteria is equal 
to six volumes of protista (protophyta and protozoa) and to three 
hundred volumes of metazoa. These results obtained on Lake Mendota 
show a very much larger proportion of protista to bacteria. The 
nannoplankton alone, exclusive of the protophyta and protozoa of the 
net plankton, yielded an average of 1,472.0 milligrams of dry organic 
matter per cubic meter of water during the three summer months of 
June, July, and August in 1915, and 1,507.0 milligrams during the 
same months in 1916. For the same periods the net plankton averaged 
337.0 milligrams of organic matter per cubic meter in 1915 and 181.0 
milligrams in 1916. Computations based upon numerical data show 
that the crustacea and rotifers contribute an average of one-third of 
the organic matter in the net plankton; thus, it seems safe to estimate 
that about half of the net plankton during the summers of 1915 and 
1916 was derived from the protista. Adding half of the organic matter 
of the net plankton to that of the nannoplankton gives an average of 
about 1,640.0 milligrams of dry organic matter in the protista in the 
summer of 1915 and of 1,589.0 milligrams per cubic meter in 1916. 
These amounts represent more than three hundred thirty times as much 
organic matter as the maximum crop of bacteria in 1920 and more than 
one hundred sixty times as much as the maximum number of bacteria 
noted at 10 meters on September 22, 1921. They represent more than 
three thousand times as much organic matter as the late summer crop 
of bacteria in 1919. 

The value of the bacteria in the plankton economy of Lake Mendota 
is by no means as small as these figures seem to indicate, because they 
multiply at a much faster rate than the protista; the bacteria may 
pass through a number of generations in the course of a day under 
favorable food and temperature conditions, while the protista may not 
average more than one or perhaps two divisions per day under similar 
conditions. In spite of this marked difference in reproductive capacity, 
it appears from the foregoing results that the bacteria do not play 
nearly as important a role in the plankton complex of Lake Mendota 
as Lohmann’s estimate might lead one to expect. 


* Internationale Revue, Bd. 4, 1911, pp. 1-38. 


APR | MAY | SUWE | JuLy_ | AUG | SEPT 2) 
JHU 35 (cae gee i 


i om 


OM 


ie 


ZEB APR MAS 
2lalel/lelol-|7l2lole<]7l2lsl<- [<]7] 


Bear Sie eee SR 


NANNOPLANKTON OF LAKE MENDOTA 91 


EXPLANATION OF THE NANNOPLANKTON DIAGRAMS 


Figures 32 and 33 show the numerical results obtained for the samples 
of nannoplankton from Lake Mendota; the spherical type of curve 
has been used. The curves show the number of individuals or colonies 
in 10 liters of water. In order to bring out the variations in the quantity 
of organic matter more clearly by this type of diagram, that curve was 
platted on a different scale; it shows the number of milligrams of dry 
organic matter in 10,000 cubic meters of water. 

The following abbreviations have been used for the different organ- 
isms: RH=Rhizopoda, CH=—Chlorochromonas, CR=Cryptomonas, 
EUEuglena, CI=Cilates, AP=Aphanocapsa, AR—Arthrospira, CC 
=Chroococcus, CL==Closterium, CO=Coelosphaerium, CM=Cosma- 
rium, OO—Oocystis, SC=Scenedesmus, SP=Sphaerocystis, CS=—Coe- 
coneis, CY=Cyclotella, SA=Stephanodiscus astraea, ST—Stephano- 
discus, OM=organic matter, FD=fragments of diatoms, FA—frag- 
ments of Aphanizomenon. 


OL ee kN eel: 


92 PLANKTON OF WISCONSIN LAKES 


CHAPTER IV 
THE TOTAL PLANKTON OF LAKE MENDOTA 


The term total plankton is used here to designate the sum of the net 
plankton and the nannoplankton. Since the ash of the centrifuge 
material contains a certain amount of silt, and is therefore abnormally 
high, the discussion of the results is necessarily limited to the organic 
matter of the net plankton and of the nannoplankton. Only those 
catches of the former which correspond to the samples of nannoplank- 
ton are taken into consideration in this chapter. That is, the results 
given for net plankton in this chapter cover only those samples which 
were obtained between April 21, 1915, and June 1, 1917. 


VARIATIONS IN THE QUANTITY 


The distribution of the dry organic matter of the net plankton and 
of the nannoplankton of Lake Mendota by months for the different 
years is shown in table 24, in which the average amount of each is in- 
dicated for the different months as well as the sum of the two, or the 
organic matter of the total plankton. The months of January and 
February are represented by single catches, but the results for the 
other months are averages of two to eight or more catches per month. 

The large crop of Aphanizomenon which developed late in 1916 held 
over into January, 1917, and gave a large catch of net plankton in this 
month. By February the net plankton had decreased to less than a 
quarter of the January amount, even being below that of February, 
1916. In both years the March samples were smaller than those of the 
previous month. The average of April, 1917, was about 10.0 per cent 
below March, but in 1916 there was an increase in the net plankton 
during this period. The average for May showed an increase in all 
three years; June yielded a distinctly larger average than May in two 
years, but it was somewhat smaller than May in 1916. July and 
August were characterized by declines in the amount of net plankton, 
but September ushered in the autumnal rise both in 1915 and in 1916; 
the maximum point was reached in December of both years. 

The difference in the amount of organic matter in the two February 
samples of nannoplankton was only a little over 5.0 per cent, the quan- 
tity being somewhat larger in 1916 than in 1917. The catch in Febru- 
ary, 1917, showed a decrease of somewhat more than 20.0 per cent over 


TOTAL PLANKTON OF LAKE MENDOTA 93 


that of January of this year. In 1916 the amount of organic matter in 
the nannoplankton was almost twice as large in March as in February, 
but in 1917 the quantity was smaller in March than in February. April 
showed the maximum average of the year both in 1915 and in 1916, 
but the maximum of 1917 was not reached until May. The average 
amount of organic matter for June showed a variation of only about 
8.0 per cent in the three years covered by these observations, but there 
was a much greater difference in July. The summer minimum was 
reached in August and the averages were substantially the same in 
1915 and in 1916. The September averages were higher than those of 
August and the amount remained at this level or somewhat higher 
during the last three months of the year. __ 


In the total plankton, that is the net plankton plus the nannoplank- 
ton, the smallest amount of organic matter was found in February or 
March and the largest average in April or May. The average for De- 
cember, 1916, was higher than that of May, 1917. From June until 
September the monthly averages of organic matter in the total plankton 
ranged from a little more than 1,350 milligrams to about 2,000 milli- 
grams per cubic meter of water; October fell within these limits in 
1916, but for October, 1917, and for the last two months of both years 
the amount varied from about 2,200 milligrams to substantially 2,500 
milligrams. 

The smallest amount of organic matter in the total plankton was 
found in March, 1917, and the largest amount for the entire series was 
noted in April, 1916; the latter was approximately three times as large 
as the former. 

Figures 34 and 35 show graphically, in more detail, the relations be- 
tween the quantity of organic matter in the net plankton and that in 
the nannoplankton; they also show the total organic matter or the 
sum of the two. The amounts are indicated in milligrams per cubic 
meter of water. The curve marked A in the diagrams represents the 
organic matter of the total plankton; the one marked B indicates that 
of the nannoplankton, while C represents the organic matter of the net 
plankton. The nannoplankton curve, B, is distinctly higher than the 
one for net plankton, C, for the entire series of observations, thus show- 
ing clearly that the organic matter in the former always exceeds that 
in the latter. The two curves are most widely separated in April, 
1915, and 1916, and in May, 1917; they approach each other most 
closely in December, 1916. 

The fact that the curve for total plankton, A, is, in general, very 
similar in form to that for the nannoplankton, B, is further evidence of 
the predominance of the latter over the net plankton. The most prom- 
inent difference between these two curves is shown in October, 1915, 


94 PLANKTON OF WISCONSIN LAKES 


sr 


ee 


Fig. 34.—The amount of dry organic matter in the net plankton, the nanno- 
plankton and the total plankton of Lake Mendota in 1915. Curve A repre- 
sents the total plankton, curve B the nannoplankton, and curve C the net 
plankton. The curves show the number of milligrams per cubic meter of 
water. 


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96 PLANKTON OF WISCONSIN LAKES 


sents a conspicuous peak in October corresponding to a similar peak 
in C, but which is not represented in B; in the latter year, curve A 
possesses a broad, prominent peak in November-December, 1916, and 
in January, 1917, which more closely resembles the one found in C at 
this time. 

Expressing the relation of the amount of organic matter in the net 
plankton to that in the corresponding sample of nannoplankton in the 
form of a ratio also serves to bring out more clearly some of these 
quantitative differences. Taking the samples collected on December 
12, 1916, in which the organic matter of the two is most nearly equal in 
amount, the following ratio is obtained, net plankton: nannoplank- 
ton==1:1.1; on the other hand, the samples which show the greatest 
difference, those collected on April 21-23, 1915, give a ratio of 1:24.6. 
Each year the largest differences were found during the vernal maxi- 
mum of the nannoplankton; the nannoplanktonts thus appear to re- 
spond more promptly to the more favorable conditions which obtain 
after the ice disappears than the organisms in the net plankton. That 
is, the small forms multiply and develop more rapidly than the larger 
ones. 

The ratios of the mean quantities of organic matter in the net plank- 
ton and in the nannoplankton are the same for 1915 and 1916, namely, 
1 :4.8, while that for the samples collected in 1917 is somewhat higher, 
that is, 1:5.9. The latter year, however, is incomplete; if the January 
samples, in which the net plankton is unusually large, are omitted, the 
ratio then becomes 1:7.8. In comparison with this, the samples ob- 
tained between February and June, 1916, give a ratio of 1:7.7, or sub- 
stantially the same as that of the same period in 1917. Thus, it ap- 


pears that, with the exception of the January samples, the normal ratio - 


of the organic matter in the net plankton to that in the nannoplankton 
is shown by the material collected in 1917. In general, then, it may 
be said that, for the entire year, the nannoplankton of Lake Mendota 
yields an average of about five times as much organic matter as the 
net plankton; but, at certain times, the amounts may be almost equal, 
while at other times the nannoplankton may yield about twenty-five 
times as much organic matter as the net plankton. 

Moore, Edie, Whitney, and Dakin! obtained from three to six times 
as much organic matter from sea water with a Chamberland filter as 
with No. 20 silk bolting cloth. 


* Biochemical Journal, Vol. 6, 1912, pp. 255-296. 


TOTAL PLANKTON OF LAKE MENDOTA 07 


MEAN QUANTITY AND CHEMICAL COMPOSITION OF TOTAL PLANKTON 


Table 25 gives a general summary of the results obtained for all of 
the samples of net plankton and of nannoplankton from Lake Mendota, 
as well as those from Lakes Monona, Waubesa, and Kegonsa, which 
are discussed in subsequent chapters. The results obtained by com- 
bining the net plankton and the nannoplankton into what has been 
called the total plankton are also indicated in this table. 

The average amount of organic matter in the 184 samples of net 
plankton from Lake Mendota secured between 1911 and 1917 was 332.5 
milligrams per cubic meter of water; in the various samples the quan- 
tity ranged from a minimum of 42.0 milligrams to a maximum of 
1,135.0 milligrams per cubic meter of water. The average of 332.5 
milligrams of organic matter contained 28.7 milligrams of nitrogen 
which was equivalent to about 180.0 milligrams of crude protein, 39.8 
milligrams of ether extract, 11.4 milligrams of pentosans, and 22.8 
milligrams of crude fiber. These four items account for 76.2 per cent 
of the organic matter. Since some of the pentosans may be derived 
from carbohydrates in the crude fiber and thus duplicate a certain 
amount of the latter, the pentosans may be omitted from this computa- 
tion. The crude protein, ether extract, and crude fiber together give 
an average of 242.6 milligrams per cubic meter of water which is ap- 
proximately 73.0 per cent of the organic matter; this leaves 27.0 per 
cent of the average quantity of the organic matter in the net plankton 
as nitrogen free extract. 

For direct comparison with the nannoplankton samples, a summary 
of the corresponding samples of net plankton is included in the table. 
In making the computations for the total plankton, only the 84 samples 
of net plankton which cover the same period of time as the nannoplank- 
ton observations have been used, and not those of the entire series of 
net catches, namely, 184 samples. The average amount of organic 
matter in the 84 samples of net plankton collected between 1915 and 
1917 is somewhat larger than that of the entire series of net catches, 
or 343.5 milligrams per cubic meter of water as compared with 332.5 
milligrams; this represents a difference of only 11.0 milligrams, or 
slightly more than 3 per cent. The nitrogen, ether extract, pentosans, 
and crude fiber were slightly larger in the average of the 84 samples 
than in the whole series, the difference ranging from a fraction of a 
milligram to 2 milligrams per cubic meter of water. The crude protein, 
ether extract, and crude fiber constituted 70.1 per cent of the organic 
matter of these samples of net plankton; this is about 3 per cent below 
the average for the entire series of net catches. Approximately 30.0 
per cent of the organic matter in these samples, therefore, consisted of 
nitrogen free extract. 


98 PLANKTON OF WISCONSIN LAKES 


The average quantity of dry organic matter in the entire series of 
nannoplankton samples from Lake Mendota is 1,630.5 milligrams per 
cubic meter of water, or about four and three-quarters times as much 
as that in the net plankton. The amount varied from a minimum of 
about 795.0 milligrams to a maximum of 3,151.0 milligrams per cubic 
meter of water. The average quantity contained 111.5 milligrams of 
nitrogen (equivalent to 697.0 milligrams of crude protein), 106.5 milli- 
grams of ether extract, 78.6 milligrams of pentosans, and 84.6 milli- 
grams of crude fiber. The crude protein, ether extract, and crude 
fiber together constituted 888.4 milligrams, or 54.5 per cent of the 
average quantity of organic matter. The remainder, 45.5 per cent, 
consisted of nitrogen free extract; the latter, therefore, was 15.5 per 
eent larger in the nannoplankton than in the net plankton. 

The total plankton, that is, the net plankton plus the nannoplankton, 
yielded an average of 1,974.0 milligrams of dry organic matter per 
cubic meter of water from April, 1915, to June, 1917. In this material 
there were 878.2 milligrams of crude protein, 148.7 milligrams of ether 
extract, 90.2 milligrams of pentosans, and 105.0 milligrams of erude 
fiber. The crude protein, the ether extract, and the crude fiber 
amounted to 1,131.9 milligrams, or 57.3 per cent of the average quan- 
tity of organic matter, leaving 42.7 per cent as nitrogen free extract. 


Tota PLANKTON Per Unit AREA 


The total plankton may now be considered from the standpoint of the 
quantity per unit of area; the averages for the deep water are pre- 
sented first and those for the entire lake later. Since the observations 
were made in the deeper part of Lake Mendota and extended to a 
depth of 20 meters, the results apply more particularly to this portion 
of the lake. For this part of the discussion, then, the 20 meter con- 
tour line may be taken as the boundary of the particular portion of the 
lake under consideration. This comprises an area of 6,641,000 square 
meters, or 16.8 per cent of the total area of the lake. The volume of 
water in this area down to a depth of 20 meters is 132,820,000 cubic 
meters and below this depth it is 13,449,000 cubic meters, giving a total 
of 146,269,000 cubic meters within this area, or 30.6 per cent of the 
total volume of the lake. The quantitative computations for the plank- 
ton of the deep water are thus based on this area and on this volume. 


TotaL PLANKTON IN DEEP WATER 


The monthly averages for the entire set of observations on the total 
plankton have been ascertained from table 24 and these averages have 
been used to compute the quantity of organic matter per unit of sur- 


TOTAL PLANKTON OF LAKE MENDOTA 99 


face. The results for the deep water are given in the first part of table 
26 in which the quantities found for the different months of the year 
are expressed in kilograms per hectare and in pounds per acre. The 
average quantity of dry organic matter in the total plankton varies 
from a minimum of 257.7 kilograms per hectare (230 pounds per acre) 
in February to a maximum of 521.5 kilograms (465 pounds per acre) 
in December. While the largest single catches were obtained in April 


Wan [Fee man arr [war une] vuby [aval serr] ocr [Now Bec 


| | : fl i. 
jj i] | | il } | | 


Fig. 36.—Diagram showing the monthly distribution of the total plankton in the 
deep part of Lake Mendota, 1915 to 1917. This diagram is based on the re- 
sults given in table 24; it shows the average number of kilograms of dry 
organic matter per hectare of surface for the area bounded by the twenty 
meter contour line. 


Oo 


in 1915 and in 1916 and in May, 1917, the averages for these two 
months fall below that of December; this is especially true of the 
month of May. 

The averages for the different months of the year are shown graph- 
ically in figure 36 in which the various columns indicate the average 


100 PLANKTON OF WISCONSIN LAKES 


nega? of organic matter in the total plankton in kilograms per hec- 
tare of surface. The shortest column appears in February, while the 
next in length is found in August, and the third in March. The longest 
column is found in December, with April second and November third. 
Only two columns rise above the 500 kilogram line, namely, December 
and April, while February-March and August-September fall well be- 
low the 400 kilogram line, more especially the February average. 


The figures given in table 26 and the columns indicated in diagram 
36 show only the amount of dry organic matter per unit of area. In 
the living state, this material weighs about ten times as much as shown 
here because most of the organisms found in this material contain at 
least 90.0 per cent of water when alive, some of them in fact as much 
as 97.0 per cent. On this basis, then, the quantity of living organic 
matter in the total plankton in this portion of Lake Mendota ranges 
from a minimum of 2,577 kilograms per hectare (2,300 pounds per 
acre) in February to a maximum of 5,215 kilograms per hectare (4,652 
pounds per acre) in December. An additional 10.0 per cent to 15.0 
per cent of the dry weight would be contributed to this total by the ash 
of the various organisms. 


Attention may be again called to the fact that these figures show 
only what has been referred to as the standing or permanent crop, that 
is, the amount of material that is present constantly during the differ- 
ent months of the year. Therefore, they do not show how large a 
quantity of organic matter is produced in the course of a year. Each 
plankton form that appears during the year plays its particular role 
in the production of this material; each comes on, passes through its 
regular cycle when conditions are most favorable for it, and then de- 
clines to a minimum number or disappears entirely for a certain por- 
tion of the year. When some forms are on the wane, others are usually 
ready to take their places to a greater or less extent, so that, while there 
are marked variations in quantity during the year, there is no period 
during which all forms are absent. This results in a complex over- 
lapping of the crops of the various planktonts which continues through- 
out all seasons of the year. 

The problem of ascertaining the amount of plankton material pro- 
duced by the lake annually is made extremely difficult by this over- 
lapping of the various forms and also by the fact that production is 
a continuous process. There are no definite breaks in the stream of 
plankton life which might serve to mark off one season’s or one year’s 
production from another. Concomitant with the process of production, 
also, is the process of destruction; some of the planktonts are con- 
stantly being consumed as food by other organisms. Even some mem- 
bers of the group, such as the plankton crustacea, feed upon others, 


TOTAL PLANKTON OF LAKE MENDOTA 101 


such as the algae and flagellates. The crustacea, in turn, are preyed 
upon by the larger forms. In addition to these losses, a certain portion 
of the plankton is constantly dying and sinking to the bottom. Thus, 
the quantity of plankton material maintained by the lake is the result- 
ant of the productive processes and of the various destructive proc- 
esses. In spite of the various changes in quantity in the course of the 
year, the amount of material found during a particular month does not 
differ so very greatly from year to year. 


In the spring the water is kept in fairly complete circulation by the 
wind from the time of the disappearance of the ice, which usually takes 
place about the first week in April, until early June; the same is true 
for the autumn and early winter, or from the autumnal overturn about 
the first week in October until the lake becomes covered with ice, usually 
in December. As a result of this extensive circulation of the water at 
these seasons, the plankton becomes fairly uniformly distributed from 
surface to bottom for about four to five months each year. Since the 
mean depth of the water within the 20 meter contour is 22 meters, the 
quantity of plankton in the upper 11 meters would be half as large 
as that shown in table 26 during these periods of general circulation 
and one-quarter as great in the upper 5.5 meters. 


In summer and in winter, however, the plankton organisms do not 
have such a uniform distribution; the upper strata in summer are more 
densely populated than the lower, while in winter after the lake be- 
comes covered with ice there is a tendency for the chlorophyl-bearing 
erganisms to come nearer the surface where light conditions are more 
favorable and for the crustacea to be more numerous in the lower strata 
where the water is somewhat warmer. 

Two special runs made on August 7, 1915, will serve to illustrate the 
unequal distribution of the plankton at this time of the year. The re- 
sults for organic matter are shown in table 27. Samples No. 593 and 
No. 594 represent the organic matter in the nannoplankton and in the 
net plankton respectively, from the 0-13 meter stratum ; this stratum in- 
eluded the epilimnion and the mesolimnion at this time. Samples No. 
591 and No. 592 are the catches of nannoplankton and net plankton 
from the 14-20 meter stratum and they represent the hypolimnion. 
The nannoplankton obtained from the 0-13 meter stratum yielded 
nearly two and a half times as much organic matter per cubic meter 
of water as that from the 14-20 meter stratum. The difference was 
still more striking in the net plankton; the upper stratum yielded al- 
most twelve times as much organic matter per cubic meter as the lower 
stratum. 

EKnumerations were made for the purpose of ascertaining the vertical 
distribution of the various plankton organisms at the time these catches 


102 PLANKTON OF WISCONSIN LAKES 


were secured and, on the basis of these counts, the amount of organic 
matter in the 0-5 meter and in the 0-10 meter strata have been esti- 
mated. These estimates together with the results obtained in the ob- 
servations are given in table 28; they are indicated in kilograms per 
hectare and in pounds per acre. According to the estimates nearly 
37.0 per cent of the organic matter in the total plankton came from 
the upper 5 meters, while a little more than two-thirds of it was ob- 
tained from the 0-10 meter stratum. The catches show that 79.5 per 
cent of the total quantity of organic matter came from the 0-13 meter 
stratum, leaving only 20.5 per cent for the 14-23 meter stratum. 

Some data were obtained on the horizontal distribution of the plank- 
ton by taking one of the two catches of a week about four kilometers 
(2.5 miles) west of the regular station. The total plankton obtained 
at the regular station on July 10, 1916, yielded 1,968.7 milligrams of 
organic matter per cubic meter of water, while the catch taken in the 
western part of the lake on July 14, 1916, gave 1,972.9 milligrams. The 
difference is only 4.2 milligrams per eubic meter which indicates a 
pretty uniform horizontal distribution of the material. 


PLANKTON OF ENTIRE LAKE PER Unit or AREA 


The monthly averages for organic matter may now be considered on 
the basis of the amount per unit area when the surface of the whole 
lake is taken into account. In general, the amount of plankton under 
a unit of surface is dependent upon the depth of the water since the 
volume of water occupied by the organisms is proportional to the depth. 
Unfavorable factors, such as the absence of dissolved oxygen, insuffi- 
cient light, and low temperature, may exclude many organisms from 
the deeper strata, still certain forms occupy the lower water even when 
such conditions exist. The population of this region may be scant in 
comparison with the upper strata, as shown in table 27, yet those forms 
which oceupy the lower strata contribute their quota to the total quan- 
tity of plankton under a unit of surface. Thus, while the increase in 
plankton would not be directly proportional to an increase in depth 
during periods of unequal vertical distribution, still an increase in 
depth at such times would show a well marked increase in some of the 
organisms. 

It must be borne in mind, therefore, that, in stating the results in 
averages for the entire lake, a larger quantity of plankton than is 
actually present there, is attributed to the water which is shallower 
than the mean depth of the lake, while, on the other hand, the amount 
assigned to the deeper water is smaller than it should be. For ex- 
ample, the mean depth of Lake Mendota is 12.14 meters, so that the 
average quantities give results that are too high for areas with a 


TOTAL PLANKTON OF LAKE MENDOTA 103 


smaller depth than this and too low for the areas with a greater depth 
of water. This method of stating the results would show the actual 
conditions only in a lake with straight sides and a uniform depth. 
Taking the variations in depth into account would complicate the prob- 
lem too much and the discussion is confined, therefore, to the averages 
for the total area of the lake. 

In computing the results for the entire area of the lake, the monthly 
averages per cubic meter of water were ascertained from table 24 and 
these averages were then multiplied by the mean depth of the lake 
(volume—area—mean depth). The results of these computations are 
shown in the second part of table 26 in which the amounts of organic 
matter are indicated in kilograms per hectare and in pounds per acre. 
The quantity per unit of area, when the total surface of the lake is 
considered, is a little more than 55.0 per cent of that found in the deep 
water as shown in the first part of this table; that is, by this method 
of calculating the results, approximately 45.0 per cent of the material 
noted in the deep water is attributed to the shallower water. On this 
basis the amount of dry organic matter in the total plankton varies from 
a minimum of 141.4 kilograms per hectare (126 pounds per acre) in 
February to a maximum of 287.5 kilograms (256 pounds per acre) in 
December, or slightly more than a twofold variation. The living ma- 
terial, on the other hand, would weigh at least ten times as much as 
this. 


104 PLANKTON OF WISCONSIN LAKES 


CHAPTER V 
THE PLANKTON OF LAKE MONONA 


Samples of net plankton were obtained from Lake Monona each year 
from 1911 to 1916, inclusive, with the exception of 1914. This ma- 
terial was secured chiefly for the purpose of making qualitative and 
quantitative comparisons with the results obtained on Lake Mendota. 
No winter observations were made on Lake Monona and it was not 
practicable to begin the work very promptly in the spring nor to con- 
tinue it very late in the autumn, so that the catches cover only the 
period from late May to late October or early November. 

The area, volume, and mean depth of Lake Monona are much smaller 
than those of Lake Mendota, but the maximum depth of the former 
is only about three meters less than that of the latter. (Table 1, p. 181). 
Lake Monona receives the water from the outlet of Lake Mendota as 
well as that from two other streams. The water of Lake Monona is 
subject to considerable pollution, which is a matter of much impor- 
tance both from a biological and from a chemical standpoint. It re- 
ceives (a) the effluent of the sewage disposal plant of Madison, (b) 
the greater part of the storm water drainage of this city, (c) the 
effluent from a small private sewage disposal plant at South Madison, 
and (d) some trade wastes consisting principally of the effluent from 
a large beet sugar factory. Just what effect this pollution has upon 
the plankton crop of the lake is not definitely known, but these polluted 
waters carry an abundance of material which may be utilized by vari- 
ous plankton organisms. 


THE Net PLANKTON 


Table 2 (p. 181) shows the number of samples of net plankton pro- 
cured each year, as well as the volume of water strained and the total 
quantity of dry plankton. Only four samples were taken in 1911 
and but six in 1912. In 1918 and in 1916 catches were obtained at 
more frequent intervals so that the data are sufficient to construct 
curves for the results of these two years. In general the net plankton 
of Lake Monona contained a larger proportion of algal material than 
that of Lake Mendota. 

Organic matter. The organic matter constituted from 68.9 per cent 
to 97.7 per cent of the total dry weight of the net plankton, with an 


PLANKTON OF LAKE MONONA 105 


average of 85.4 per cent for the whole series of 47 samples. The 
variations in the amount are summarized in table 29, in which the 
quantities are given in milligrams per cubic meter of water. Details 
for the organic matter and the chemical analyses are given in the gen- 
eral table, No. 45, p. 210. The maximum quantities in the different 
years ranged from 519.3 milligrams of organic matter in 1911 to 3,303.6 
milligrams per cubic meter of water in 1912. The smallest minimum 
was found in 1916 and the largest in 1915. The smallest mean was 
noted in 1911 and the largest in 1915. The average amount of organic 
matter yielded by the 47 samples of net plankton was 850.2 milligrams 
per cubic meter of water as compared with 332.5 milligrams in Lake 
Mendota. 

Curves indicating the number of milligrams of dry organic matter 
per cubic meter of water are shown in figure 37 for the summer and 
autumn of 1913 and 1916. The curve marked A represents the results 
for the net plankton in 1913 and the one marked B those of 1916. In 
the former there was a comparatively small rise in the quantity of or- 
ganic matter during the month of June but this was followed by a 
marked rise in July. The amount remained high during the early 
part of August so that the curve shows a conspicuous peak covering 
this period of time. This was followed by a minimum during the 
early part of September; following this came the autumnal rise which 
reached its maximum point about the middle of October. 

In 1916 there was a slight decrease in the amount of organic matter 
in the net plankton between the middle of May and the middle of June, 
but there was a marked increase during the third week in June, fol- 
lowed by a gradual decline during the first half of July. This forms 
a& prominent peak in the curve extending from the middle of June to 
about the middle of the third week in July. The quantity then re- 
mained fairly uniform until the middle of August, after which time 
it declined to a minimum covering the latter part of August and the 
first half of September. There was a gradual rise after this time but 
there was no autumn maximum this year comparable to that of 1913. 

Nitrogen. Quantitative determinations of the nitrogen were made 
on 41 samples of the net plankton from Lake Monona; the results of 
these analyses are summarized in table 30. The highest percentage of 
nitrogen was noted in material obtained in 1915 and the lowest in 1911. 
The mean percentage was lowest in 1916, namely, 8.37 per cent, and 
highest in 1915, namely, 10.71 per cent, thus showing a difference of 
2.34 per cent. 

When stated in milligrams per cubic meter of water, the largest 
amount of nitrogen was found in 1912, namely, 302.2 milligrams, and 
the smallest in 1911, that is, 13.2 milligrams. The mean quantity was 
highest in 1915 and lowest in 1911. 


106 PLANKTON OF WISCONSIN LAKES 


AUGUST SEPTEMBER OCTOBER 


3200 : 

ia oe 
Ca a 
Ms ee 


1200 


Fig. 37.—The quantity of dry organic matter in the net plankton of Lake 
Monona in 1913 and 1916, and in the nannoplankton in 1916. Curve A 
represents the net plankton in 1913, curve B the net plankton in 1916 and 
curve C the nannoplankton in 1916. The amounts are indicated in milli- 
grams per cubic meter of water. See tables 45 and 46. 


The ratio of the organic matter to the total nitrogen in the net 
plankton of Lake Monona varied from 8.5 to 14.6. (See table 45.) 
In three-quarters of the samples upon which nitrogen determinations 
were made, however, the ratio fell between 9 and 12, thus showing a 
fairly constant proportion of nitrogen in the organic matter. The pro- 
portion was larger in the samples collected in 1915 than in those of any 
other year; in only one sample obtained in this year did the nitrogen 


PLANKTON OF LAKE MONONA 107 


fall below 10.0 per cent of the organic matter, while it reached or ex- 
ceeded this percentage in but one sample collected in the other years. 

Crude protein. The results for crude protein are shown in table 
31, p. 198. The highest percentages for the different years varied from 
60.4 per cent in 1912 to 73.2 per cent in 1915, a difference of approxi- 
mately 13.0 per cent; the minima ranged frem 42.7 per cent in 1911 to 
62.6 per cent in 1915, a difference of almost 20.0 per cent. With the 
exception of 1915 the range of variation in the mean is less than 5.0 
per cent; the mean percentage of 1915, however, is from 9.5 per cent 
to 13.1 per cent higher than those of the other years. This is a smaller 
range of variation than in the net plankton from Lake Mendota where 
there is a difference of 15.1 per cent in the mean percentages of the 
different years. In general the net plankton of Lake Monona has a 
distinetly higher percentage of crude protein than that of Lake Men- 
dota. Only three of the annual means for Lake Mendota exceed the 
minimum for Lake Monona. (See table 7, p. 187.) The mean per- 
centage of crude protein in the 41 samples from Lake Monona, on which 
nitrogen determinations were made, is 56.4 per cent, while that for 166 
samples from Lake Mendota is 52.4 per cent, just 4.0 per cent smaller. 

The largest amount of crude protein was found in 1912, namely, 
1,888.7 milligrams per cubic meter of water and the smallest amount 
in 1911, or 112.5 milligrams. The latter year also yielded the smallest 
mean of the five years in which observations were made on Lake 
Monona. 


Ether extract. The percentage of ether extract in the net plankton 
ranged from a minimum of 2.79 per cent in sample No. 330, July 30, 
1913, to a maximum of 11.16 per cent of the organic matter in sample 
No. 659, June 23, 1916. (Table 45.) The mean percentages for the 
different years fall between 4.77 per cent and 9.62 per cent. The mean 
for the 35 determinations is 7.23 per cent. This percentage is much 
lower than that of the net plankton of Lake Mendota; the highest mean 
of Lake Monona, 9.62 per cent, is distinctly lower than the lowest mean 
for the various years on Lake Mendota, namely, 11.27 per cent. (See 
table 32, p. 199.) 

The amount of ether extract per cubic meter of water varied from 
a minimum of 12.6 milligrams in sample No. 114, July 27, 1911, to a 
maximum of 171.8 milligrams in sample No. 5167, October 15, 1915. 
The mean quantities for the different years ranged from 27.5 milli- 
erams in 1911 to 105.6 milligrams in 1916. Only two determinations 
were made on the samples collected in 1916 and they were made on 
samples which contained a large amount of organic matter, so that they 
cannot be regarded as representative of the whole series of net catches 
obtained in 1916. The mean quantity for the 35 determinations on 


108 PLANKTON OF WISCONSIN LAKES 


Lake Monona material is 61.5 milligrams per cubic meter of water. 
This average is about one and a half times as large as that of the net 
plankton of Lake Mendota; while the percentage of ether extract aver- 
ages lower in the material from the former than in that from the 
latter lake, the amount per cubic meter of water is greater in Lake 
Monona because the quantity of organic matter is much larger than 
in Lake Mendota. 

Pentosans. The pentosans were determined for only 20 samples of 
net plankton from Lake Monona and 13 of these were on material col- 
lected in 1913; therefore, no comparisons of the different years can 
be made. (See general table No. 45). In the 20 samples on which 
determinations were made the percentage of the pentosans varied from | 
a minimum of 3.07 per cent to a maximum of 10.6 per cent. The latter 
was found in sample No. 345, August 27, 1918, which contained a large 
amount of Microcystis; this form, in fact, constituted by far the greater 
part of the net material. The average percentage for the 20 samples 
on which determinations were made, amounted to 8.23 per cent of the 
organic matter as compared with an average of 3.5 per cent for 92 
analyses of material from Lake Mendota. 

The quantity of pentosans varied from a minimum of 11.2 milligrams 
per cubic meter of water to a maximum of 228.3 milligrams. The 
latter was found in sample No. 238, October 15, 1912; this sample 
contained a larger amount of organic matter than usual and it con- 
tained very large numbers of Microcystis and Melosira. The 20 
analyses of material from Lake Monona yielded an average of 70.0 
milligrams per cubic meter of water, while the material from Lake 
Mendota gave an average of 11.7 milligrams for 92 samples, a sixfold 
difference. 

Crude Fiber. The crude fiber was determined for 26 samples of net 
plankton from Lake Monona; 16 of the catches belonged to the series 
obtained in 1913. Jn general the percentage of crude fiber was com- 
paratively low; it ranged from a minimum of 1.56 per cent of the 
organic matter in sample No. 340, August 20, 1913, to a maximum of 
9.74 per cent in No. 308, May 30 and June 4, 19138. The average for 
all of the analyses was 5.0 per cent as compared with an average of 6.82 
per cent for the net plankton of Lake Mendota. 

The quantity of crude fiber in the net plankton of Lake Monona 
varied from 7.2 milligrams per cubic meter of water to 92.5 milligrams. 
The 26 analyses yielded an average of 42.6 milligrams, or almost twice 
as much as the net plankton of Lake Mendota; while the percentage 
was lower in the former lake, the total yield was larger because the 
quantity of organic matter was larger than in Lake Mendota. 


PLANKTON OF LAKE MONONA 109 


Nitrogen Free Extract. The crude protein, ether extract, crude 
fiber, and ash were all determined on 27 of the 41 samples of net 
plankton from Lake Monona so that the nitrogen free extract can be 
eomputed for this number of catches. Together these four items con- 
stituted a minimum of only about 51.0 per cent of the dry material in . 
sample No. 345, thus leaving 49.0 per cent for the nitrogen free extract 
consisting of the carbohydrates that were not included in the crude 
fiber. 

The maximum percentage of crude protein, ether extract, crude 
fiber, and ash was found in sample No. 5167 in which they constituted 
almost 85.0 per cent of the dry weight of the sample, so that substan- 
tially only 15.0 per cent consisted of nitrogen free extract. These 
results show that the nitrogen free extract in the net plankton of Lake 
Monona ranges from a minimum of about 15.0 per cent of the dry 
material to a - maximum of 49.0 per cent, or somewhat more than a 
threefold variation. 


Ash. The lowest percentage of ash found in the 47 samples of net 
plankton from Lake Monona amounted to 2.28 per cent of the dry 
weight of the material in sample No. 345, August 27, 1913, and the 
highest was 33.19 per cent in sample No. 6111, August 15, 1916; thus, 
there was almost a fifteenfold difference between the maximum and the 
minimum. Table 33 (p. 199) gives a summary of the ash determina- 
tions and the complete data are given in the general table, No. 45. The 
lowest percentages were found from late July to early September, 1913, 
and the highest ones in the year 1916. Of the 13 samples obtained in 
1916 the percentage of ash amounted to 20.0 per cent or more in 8 of 
them, while in the other 34 samples of the series only 5 contained 20.0 
per cent of ash or more. In general, the net plankton of Lake Monona 
contained a distinctly smaller percentage of ash than that of Lake 
Mendota. 3s 

The net plankton of Lake Monona showed a considerable variation in 
the maximum and minimum percentages of ash from year to year, but 
the general result was a marked increase in the mean percentage of ash 
from 1911 to 1916; each year showed an appreciable increase over the 
previous one. The mean for 1916, for example, is more than two and 
a half times as much as that of 1911, the difference amounting to 13.51 
per cent. A similar rise in the mean percentage of ash in the net 
plankton of Lake Mendota was noted between 1911 and 1915, with the 
exception of 1914; the mean for 1915 was 11.44 per cent higher than 
that of 1911. (See table 14, p. 190). 

Silica. The percentage of silica was determined for all but two of 
the net catches from Lake Monona; in these two instances the material 
was accidentally lost before the determination was completed. The 


110 PLANKTON OF WISCONSIN LAKES 


results are summarized in the second part of table 33. Silica is present 
in largest amounts when diatoms are most abundant, usually in spring 
and in autumn. A high percentage of silica was found on August 15, 
1916, which was due to a large crop of diatoms at that time. 

The silica varied from a minimum of 0.12 per cent of the dry weight 
of the sample in one catch obtained in 1918 to a maximum of 27.05 per 
cent in one of the catches of 1916. The mean percentage was lowest 
in 1911 and highest in 1916, but there was no regular increase between 
these dates as already indicated for the ash. In general the mean 
percentage of silica was much lower in the net plankton of Lake Monona 
than in that for the corresponding years from Lake Mendota. 

The difference between the mean percentage of ash and the mean per- 
centage of silica of the same year represents the average of the other 
inorganic constituents of the ash. This difference is shown in the last 
column of table 33. The smallest difference is indicated for 1912 and 
the largest for 1916, the latter being more than twice as large as the 
former. There is the same marked increase in this difference in the 
later years over the earlier ones corresponding more or less closely to 
the increase in ash. In 1913 and in 1916 the differences were substan- 
tially the same for Lake Monona as for Lake Mendota, but in the other 
years they were lower in the former lake. 

The ash of two samples of net plankton from Lake Monona was sub- 
jected to a further analysis and the results are shown in table 15, p. 190. 
Both of these samples were collected in 1913, one on July 30 (No. 330) 
and the other cn September 24 (No. 354). The ash amounted to 6.20 
per cent of the dry sample in the former catch and to 12.16 per cent in 
the latter. The marked difference was due mainly to a difference in the 
amount of silica in the two samples. The latter sample also contained 
a distinetly larger proportion of Fe,O, and Al,O, than the former. De- 
ducting the percentage of silica and the pereentage of iron and alumina 
from the percentage of ash in each sample reduces the difference be- 
tween the two to only 0.71 per cent. Of the remaining constituents, 
P,O, and CaO are the most important. It will be noted, also, that the 
percentage of MgO in sample No. 330 is four times as large as that of 
No. 354. The variations in the percentages of the different constituents 
of these two ashes, however, come within the range of those noted in 
the ash analyses of the net plankton of Lake Mendota. 


ORGANISMS OF THE Net PLANKTON 


The crustacea were well represented in the net plankton of Lake 
Monona. The copepods included species of Diaptomus and Cyclops to- 
gether with their nauplii; there were five forms of Cladocera, namely, 
Daphma pulex, D. hyalina, D. retrocurva, Chydorus, and Leptodora. 


PLANKTON OF LAKE MONONA TM 


Half a dozen kinds of rotifers were noted, but only one, Anuraea coch- 
learis, appeared regularly in the catches. The flagellates were repre- 
sented by Ceratium and Volvox. 

The green and blue-green algae consisted of representatives of Micro- 
eystis, Coelosphaerium, Aphanizomenon, Anabaena, Lyngbya, Staura- 
strum, and Pediastrum, while the diatoms included forms belonging to 
Melosira, Tabellaria, Fragilaria, Asterionella, Synedra, and Stephano- 
discus. 

The largest net catch of the entire series of observations was obtained 
on October 15, 1912. (See sample No. 238, table 45). The chief consti- 
tuents of this material were two algal forms, namely, Microcystis and 
Melosira. The former yielded nearly three million colonies per cubic 
meter of water and the latter two and a half million filaments per cubic 
meter. Fragilaria was also fairly abundant at this time, the number 
being approximately three-quarters of a million per cubic meter. 

The samples collected in 1913 yielded two maxima of organic matter, 
one on July 24 (sample No. 326) and another on October 18 (No. 360). 
The former was due mainly to a rise in the amount of Lyngbya and 
Microcystis; a cladoceran, Chydorus, also showed a sixfold increase in 
number between July 15 and July 24. The October maximum was 
produced by a very large crop of the diatom Melosira. (See figure 37, 
curve A.) 

Two maxima of organic matter were also found in 1915, one on June 
23 (sample No. 554, table 45) and another on October 15 (sample No. 
5167). The catch obtained on June 28 consisted mainly of Aphanizo- 
menon, an enormous crop of which was present in the upper water at 
this time. The chief constituents of the net plankton during the Octo- 
ber maximum were Microcystis and Melosira. 

In 1916 there was almost a fivefold increase in the organic matter 
of the net plankton between June 14 and June 23 (sample No. 649 
and No. 659); this was due to a marked increase in the number of 
Daphnia pulex. By far the greater portion of the material in sample 
No. 659 consisted cf this cladoceran, while only a minor part was fur- 
nished by the algae; this is the only instance in any of the lakes 
wherein a marked rise of the organic matter of the net plankton could 
be attributed directly and solely to any organism other than an alga. 
The scarcity of the algae in the net plankton at this time was due, un- 
doubtedly, to the fact that the large Daphnia population was feeding 
extensively on them. (See figure 37, curve B.) 

Following the Daphnia maximum a very large crop of Volvox was 
found in the upper water of Lake Monona on July 6, 1916. Colonies 
of this flagellate were present in enormous numbers at a depth of three 
meters on this date so that a substantially pure catch, amounting to a 


1 PLANKTON OF WISCONSIN LAKES 


little more than 22 grams of dry material, was obtained for a chemical 
analysis. The results of this analysis are given in table 49, p. 215. 
This material was obtained about 10 a. m. on a bright, calm day and 
the colonies of Volvox were concentrated in a relatively narrow stratum 
about three meters below the surface of the water, thus showing a 
definite negative reaction to the sunlight. Their subsequent behavior 
was studied by Smith’ and the results of his investigation have already 
been published. 

The autumn maximum of organic matter in the net plankton of 1916 
was noted on November 9 (sample No. 6183) and it was due chiefly to a 
large crop of Melosira. Stephanodiscus also showed a fairly large 
increase in numbers at this time. 


Toe NANNOPLANKTON 


Quantitative studies of the nannoplankton of Lake Monona were 
made in 1915 and in 1916; 8 samples were obtained in the former year 
between June 23 and October 29, and 13 samples in the latter year be- 
tween May 12 and November 9. The amount of water centrifuged in 
1915 was 9,777 liters and 12,510 liters in 1916, making a total of 22,287 
liters for the two years. (See table 3, p. 182.) 

Organic Matter. The results of the determinations of the organic 
matter in the nannoplankton material from Lake Monona are shown in 
table 29, p. 198. In 1915 the largest amount of organic matter was 
found in sample No. 5166, October 15, and the smallest amount in No. 
063, July 13; the former sample yielded 3,746.5 milligrams per cubic 
meter of water and the latter 723.0 milligrams, a little more than a 
fivefold difference. In 1916 the maximum amount was noted in sample 
No. 6158 on October 12, namely, 5,696.2 milligrams and the minimum 
amount in No. 648 on June 14, namely, 672.1 milligrams; the former 
quantity is approximately eight and a half times as much as the latter. 
(Table 46.) While the maximum was very much higher in 1916 than — 
in 1915, the mean of the two years was substantially the same, namely, 
2,355.6 milligrams and 2,339.5 milligrams, respectively, the average 
amount for 1915 being only 16.1 milligrams smaller than that for 1916. 

The maximum amounts of dry organic matter in the nannoplankton 
of Lake Monona were appreciably higher than those noted for Lake 
Mendota; in the latter lake, for example, the maximum for 1915 was 
2,776.3 milligrams per cubic meter of water, 3,151.5 milligrams in 1916, 
and 2,603.4 milligrams in 1917. In 1915 a summer minimum of 944.2 
milligrams was noted in August in Lake Mendota, while in the summer 
of 1916 the smallest amount of organic matter, 1,053.5 milligrams per 


*Amer. Jour. Botany, Vol. 5, 1917, pp. 178-185. 


PLANKTON OF LAKE MONONA 113 


cubic meter, was found about the middle of June. Thus, the summer 
minima were much higher in Lake Mendota in both years than in Lake 
Monona; the minimum in the former lake which came nearest to the 
summer minima of the latter was the one obtained on March 9, 1917, 
when the nannoplankton of Lake Mendota yielded only 795.2 milli- 
grams of dry organic matter per cubic meter of water. The range of 
variation in the amount of organic matter in the nannoplankton of Lake 
Monona is much greater, therefore, than in that of Lake Mendota. 

The results for organic matter in the nannoplankton of Lake Monona 
ean not be indicated by a diagram for the year 1915 because the ob- 
servaticns were not made frequently enough in that year for this pur- 
pose, but the collections were made with sufficient regularity in 1916 
for the construction of a curve showing the results. The curve marked 
C in figure 37 indicates the number of milligrams of dry organic matter 
per cubic meter of water in the nannoplankton samples obtained in 
1916. This diagram shows that there was a decline in the organic 
matter between May 12 and June 14, with only a slight rise between the 
latter date and July 6. By July 18, however, there was an appreciable 
increase in the quantity and this rise continued thereafter until it 
reached a maximum point on October 12. A very marked decrease 
followed on October 29 and on November 9, so that the curve shows 
a Sharp peak covering the autumnal period. Attention should be called 
to the fact here that, in late June and early July, curve C falls below 
eurve B in this diagram; the former curve (C) represents the organic 
matter in the nannoplankton and the latter (B) that in the net plank- 
ton. That is, during this period of time the nannoplankton yielded a 
smaller amount of organic matter per cubic meter than the net plank- 
ton. This phenomenon was noted in two samples obtained in 1915 and 
in two collected in 1916. 

Aecording to curve C the story of the organic matter in the nanno- 
plankton of Lake Monona was comparatively simple in 1916. The first 
observation of that year served to show that there was a more or less 
marked vernal maximum; this was followed by a decline to a summer 
minimum in late June and early July. Thereafter the organic matter 
steadily increased in amount until an autumnal maximum was reached 
on October 12; a marked decrease was noted during the following month 
and this decline continued, doubtless, until a winter minimum was at- 
tained. The annual cycle, therefore, consisted of four phases, namely, 
summer and winter minima separated by spring and autumn maxima, 
just as has been noted for Lake Mendota. A comparison of curve C 
of this diagram with curve B in figure 35 shows, though, that the vari- 
ous changes were much less complex in Lake Monona than in Lake Men- 
dota during the same period of time in 1916, so that the curve is much 
simpler for the former lake than for the latter. 


114 PLANKTON OF WISCONSIN LAKES 


Nitrogen. The nitrogen content of all of the nannoplankton samples 
cebtained on Lake Monona in 1915 and in 1916 was determined. A sum- 
mary of the results is given in table 384. On an ash free basis the nitro- 
gen ranged from a maximum of 11.28 per cent to a minimum of 5.92 
per cent in the former year and from 10.08 per cent to 3.58 per cent 
in the latter year. The difference for 1915 was about twofold and for 
1916 almost threefold. Both maximum and minimum were higher in 
1915 than in 1916, and the mean percentage, likewise, was almost two 
per cent higher in the former year. 

The maximum percentage of nitrogen in the net plankton was some- 
what higher in 1915 than in the corresponding nannoplankton eatches, 
but it was lower in the net plankton in 1916 than in the nannoplankton. 
(Compare tables 30 and 34.) The minimum for nannoplankton was 
below that of the net plankton both in 1915 and in 1916; the mean 
percentages also show small differences in favor of the former. 

A rise in the percentage of nitrogen in the nannoplankton of Lake 
Monona accompanied the increase in the quantity of organic matter in 
1915 as well as in 1916. (See general table, No. 46.) In the former 
year the largest percentage was found in sample No. 5144, collected on 
September 24, in which the quantity of organie matter was within 15.0 
per cent of the maximum for the year, and in 1916 it was noted in sam- 
ple No. 6158, October 12, which yielded the largest quantity of organic 
matter in that year. In Lake Mendota, on the other hand, the maxi- 
mum percentage of nitrogen in the nannoplankton was found during 
the third week in June both in 1915 and in 1916 and at this time the 
quantity of organic matter was distinctly below the annual mean. In 
1917 the maximum was noted on May 28, which was about two weeks 
after the organic matter had reached its highest point; the quantity of 
crganic matter at this time was 80.0 per cent of the amount obtained 
two weeks earlier. 

When expressed in terms of milligrams per cubic meter of water the 
largest quantity of nitrogen was found in sample No. 6158 obtained 
from Lake Monona on October 12, 1916; but the minimum for 1916 as 
well as the mean for this year fell below those of the year 1915. 

The ratio of the organic matter to the total nitrogen in the nanno- 
plankton of Lake Monona varied from 8.1 to 27.9, more than a threefold 
difference. (See table 46.) This is a much greater variation than in 
the net plankton ; in the latter three quarters of the ratios fall between 
9 and 12, while in the nannoplankton material only half of them are 
within these limits. In eight of the nannoplankton samples, also, the 
proportion of nitrogen falls below the minimum of the net samples. 

Crude Protein. The nitrogen results have been expressed in terms 
of erude protein in table 35; the first part of the table shows the per- 


PLANKTON OF LAKE MONONA 115 


centage of crude protein in the organic matter and the second part the 
amount per cubic meter of water. As might be expected from the nitro- 
gen results the maximum and minimum percentages were higher in 1915 
than in 1916 and the mean percentage was nearly 12.0 per cent higher 
in the former year. In 1915 the crude protein fell below 50.0 per cent 
of the organic matter in only two of the eight samples, but in 1916 it 
fell below this figure in eight of the thirteen samples; in one instance, 
namely, sample No. 620, it fell as low as 22.4 per cent. The maximum 
was noted in one sample obtained in 1915 which contained 70.4 per cent 
of erude protein in the organic matter. 

The mean percentage of crude protein was higher in the nannoplank- 
ton material from Lake Monona than in that from Lake Mendota. (See 
table 19, p. 192.) The difference in favor of the former lake was 18.9 
per cent in 1915 and 12.7 per cent in 1916. The mean for the samples 
obtained from Lake Mendota in 1917 was substantially the same as that 
of Lake Monona in 1916; the samples collected from the former lake in 
1917, however, were all obtained by June 1 of that year and the ma- 
terial was, therefore, not representative of a complete year. As a whole 
the nannoplankton of Lake Mendota contained a smaller percentage of 
crude protein than that of Lake Monona. 

In 1915 the mean quantity of crude protein in the nannoplankton 
material from Lake Monona was a little more than twice as large as 
that in the nannoplankton from Lake Mendota, while in 1916 it was 
a little less than twice as large in the former lake. (Compare tables 19 
and 35.) 

Attention may also be called to the fact that the increase in the 
quantity of organic matter in the nannoplankton in Lake Monona was 
accompanied by a rise in the percentage of crude protein both in 1915 
and in 1916; in both years the maximum percentage was found when 
the organic matter had nearly or quite reached its maximum quantity. 
(See table 46.) 

The relation between the organic matter and the crude protein in 
the nannoplankton of Lake Monona for the year 1916 is shown in figure 
38, in which the amounts of each are given in milligrams per cubic 
meter of water. In this figure the space between the zero line and the 
curve marked B represents the crude protein, while the space between 
B and A, the latter being the curve for the organic matter, represents 
the other organic constituents of the nannoplankton. The smallest pro- 
portion of protein was found on May 12, that is, in the first sample 
obtained in this year, and the largest proportion was noted for the 
sample obtained on October 12, when the organic matter reached the 
highest point of the year. Between May 12 and June 14 the organic 
matter decreased in amount relatively more than the crude protein, 
but beyond the latter date the two curves are very similar in outline. 


116 PLANKTON OF WISCONSIN LAKES 


400 


oO 


Fig. 38.—The quantity of organic matter and of crude protein in the nanno- 
plankton of Lake Monona in 1916. Curve A represents the dry organic 
' matter and curve B the crude protein. The amounts are indicated in milli- 

. grams per cubic meter of water. 


“Ether Extract. Determinations of the ether extract were made on 
seven of the eight samples collected in 1915 and on nine of those ob- 
tained in 1916. In the former year the percentage of the extract varied 
from 3.34 per cent to 6.27 per cent of the organic matter, with a mean 
of 4.90 per cent. (See table 36, p. 200.) In 1916 there was a greater 
range of variation in percentage, namely, from 2.00 per cent to 9.50 per 
cent, but the mean was substantially the same as that in the previous 
year, that is, 4.75 per cent. 


PLANKTON OF LAKE MONONA 117 


When stated in terms of milligrams per cubic meter of water, the 
amount of the ether extract varied from 47.9 miligrams to 201.5 milli- 
grams in 1915, with a mean of 125.9 milligrams, and from 51.8 milli- 
grams to 154.2 milligrams in 1916, with a mean of 95.5 milligrams. 
Thus, the average amount was slightly more than 30.0 milligrams per 
cubic meter of water larger in 1915 than in 1916. 


Pentosans. Only four samples were analyzed for the pentosans, all 
collected in 1915. (See table 46, p. 212.) The average amount in these 
four samples was 140.4 milligrams per cubic meter of water, which was 
4.36 per cent of the organic matter in these samples. 


Crude Fiber. Quantitative determinations of the crude fiber were 
made on six samples collected in 1915 and on five obtained in 1916. In 
the former year the average amount was 57.2 milligrams per cubic 
meter of water, which was 2.23 per cent of the organic matter in these 
samples. (See table 46.) The five samples of 1916 yielded an average 
of 126.7 milligrams of crude fiber per cubic meter of water, or a little 
more than twice aS much as was found in 1915; this was 10.0 per cent 
of the organic matter in these five samples. 

Nitrogen Free Extract. The crude protein, ether extract, crude 
fiber, and ash were all determined for only 11 of the 21 samples of nan- 
noplankton from Lake Monona. In these samples the four items con- 
stituted from 77.0 per cent to almost 86.0 per cent of the dry weight of 
the material, leaving only 14.0 per cent to 23.0 per cent for the nitrogen 
free extract. This is a much smaller range of variation than was noted 
for the net plankton, in which the maximum percentage of nitrogen 
free extract was approximately 49.0 per cent, with a minimum of 
about 15.0 per cent. | 


Ash. In general the percentage of ash was not as high in the centri- 
fuge material from Lake Monona as in that from Lake Mendota. In 
the former lake the ash averaged about 49.0 per cent of the dry material 
which was obtained with the centrifuge, while in the latter lake the 
average was about 56.0 per cent. In Lake Monona the ash varied from 
a minimum of slightly less than 29.0 per cent to a maximum of almost 
67.0 per cent. (See table 46.) Both of these percentages are smaller 
than the maximum and minimum of Lake Mendota (table 44). The 
average percentage of ash was somewhat smaller in the catches obtained 
from Lake Monona in 1915 than in those collected in 1916, because the 
bowl water was added to the samples of the latter year. Thus, in the 
former year, the ash was derived from the plankton organisms and from 
the silt which was removed from the water by the centrifuge, while 
in the latter year there was an additional source, namely, the bowl 
water. 


118 PLANKTON OF WISCONSIN LAKES 


Allowing 10.0 per cent for the ash of the organisms when the diatoms 
were least abundant, the amount of ash coming from this source varied 
from 80.0 milligrams to 200.0 milligrams per cubie meter of water in 
1915 and from 83.0 milligrams to 228.0 milligrams in 1916. 

In 1915 the silt, as determined by difference (see p. 81), ranged 
from 251.0 milhigrams to 878.0 milligrams per cubic meter of water in 
June and July. This represents somewhat more than a threefold varia- 
tion ; the smallest amount was noted in sample No. 563 and the largest 
in sample No. 583. In estimating the silt in the 1916 catches, the ash of 
the bowl water has been regarded as 655.0 milligrams, just as in Lake 
Mendota (p. 81). On this basis the amount of silt ranged from about 
200.0 milligrams in sample No. 648 to 950.0 milligrams per cubie meter 
of water in sample No. 696. Thus, all of the samples which have been 
regarded as falling within the 10.0 per cent limit of ash in the organ- 
isms, show less than one part of silt per million parts of water. Both 
the minimum and maximum amounts of silt in Lake Monona are smaller 
than those of Lake Mendota. 


ORGANISMS OF THE NANNOPLANKTON 


The centrifuge material from Lake Monona was made up largely of 
the same kinds of organisms that were noted in Lake Mendota (p. 84). 
The ciliates were represented by Vorticella, Halteria, and some uniden- 
tified forms that were present in a few of the catches. The flagellates 
consisted of Chlorochromonas, Cryptomonas, Euglena, a disc-lke flagel- 
late which was not identified, and, rarely, Peridinium. The monads 
and the dise-shaped form were most abundant. The rhizopods were 
represented by Amoeba and two unidentified forms belonging to this 
group. 

Representatives of several genera of algae were noted, namely, An- 
kistrodesmus, Aphanocapsa, Arthrospira, Closterium, Chroococcus, 
Oocystis, Oscillatoria, Scenedesmus, Schroederia, and Sphaerocystis. 
Small colonies and fragments of the algae belonging regularly to the net 
plankton were more plentiful in this material than in the centrifuge 
samples from Lake Mendota. The smaller colonies of Microcystis were 
especially abundant in some of the samples obtained in 1915, while 
fragments of the chains of Anabaena were numerous in some of the 
eatches both in 1915 and in 1916. The diatoms consisted of Cocconeis, 
Navicula, Stephanodiscus, and a good many fragments of Melosira. 

In the centrifuge material secured in 1915, the most abundant forms 
were the monads, the dise-shaped flagellate, Aphanocapsa delicatissima, 
and Oscillatoria. The monads reached a maximum number of 819,200 
per liter on July 13 and 409,600 individuals per liter were also noted on 
August 31. The dise-shaped flagellate numbered 102,400 individuals 


PLANKTON OF LAKE MONONA 119 


per liter on July 30 and also on August 31. The largest numbers of 
Aphanocapsa were found on June 23 and on August 31, namely, 645,100 
and 716,800 individuals per liter, respectively. A minute Oscillatoria 
rose to a maximum of 409,600 filaments per liter on September 24. 
Fragments of the strands of Anabaena and filaments of Melosira were 
most abundant in September and October. 

The minimum amounts of material obtained in the two catches of 
July corresponded to relatively small numbers of the various forms 
with the exception of the monads. In addition to this a much smaller 
variety of organisms was noted in June and July than in the later 
months. The marked rise in the quantity of organic matter in August 
was due chiefly to an increase in the variety of organisms rather than 


to any unusual rise in the number of any particular form. On June 


23 but four forms of organisms were noted; this number rose to six 
on July 13, to nine on July 30, to seventeen on August 31, and to 
nineteen on September 15. The latter represented the greatest variety 
for this year. The small colonies of Coelosphaerium and the diatoms 
showed an increase in numbers corresponding to a rise of the organic 
matter to a maximum on October 15. (Table 46.) 

In 1916 Schroederia and Aphanocapsa were the most abundant forms 
in the material obtained on May 12; the former numbered 2,867,000 per 
liter and the latter 409,600. By June 14 these two forms had decreased 
very much in numbers, the latter, in fact, was not found after this date. 
The monads had increased somewhat in numbers but not sufficiently to 
counterbalance the decrease in the other forms, so that the minimum 
amount of organic matter obtained in 1916 was noted on June 14. 

In 1916, also, a smaller variety of organisms was found early in the 
season than later; the largest number of forms, namely, twenty-cne, 
was obtained on August 29, while up to the first of August the number 
ranged from seven to ten. The distinct rise in the organic matter in 
the latter half of July was correlated in time with an increase in the 
variety of the organisms. (See figure 38, curve A.) The maximum on 
October 12 corresponded to a rise in the monads, in Aphanocapsa and 
in the diatoms, as well as a larger number of fragments of Anabaena. 
After this date there was a decrease in the number of most forms and 
also a decrease in the number of forms present, corresponding to a 
marked decline in the amount of organic matter. 


Tue Total PLANKTON oF Lake Monona 


Sinee the ash of the centrifuge material contains inorganic substances 
derived from other sources than the nannoplankton organisms, it is best 
to consider the total plankton (the net plankton plus the nannoplank- 
ton) only on an ash free basis. In this connection also, it is necessary 


120 PLANKTON OF WISCONSIN LAKES 


to confine the discussion of the net plankton to those samples which were 
obtained at the same time as the samples of nannoplankton; that is, 
only those collected in 1915 and in 1916 are considered here. 

In 1915 the quantity of dry organic matter in the net plankton 
varied from a minimum of 404.0 milligrams per cubic meter of water in 
sample No. 564 to a maximum of 2,161.8 milligrams in sample No. 5167 
(table 45). In 1916 the minimum and maximum amounts were 109.3 
milligrams and 1,226.1 milligrams, respectively, in samples No. 6123 
and No. 659. In the former year the variation was somewhat more than 
fivefold, while in the latter year it was a little more than elevenfold. 

The dry organic matter in the nannoplankton of Lake Monona ranged 
from 723.0 milligrams in sample No. 563 to 3,746.5 milligrams per cubic 
meter of water in sample No. 5166 (table 46). In 1916 the minimum 
amount was 672.1 milligrams, noted in sample No. 648, and the maxi- 
mum was 5,696.2 milligrams per cubic meter of water in sample No. 
6158. In the former year the maximum was just a little more than five 
times as large as the minimum, while in the latter year it was about 
eight and a half times as large. 3 

A comparison of the organic matter in the net plankton with that in 
the corresponding samples of nannoplankton shows that the latter was 
smaller in amount than the former in four samples. These nanno- 
plankton samples are Nos. 5438, 583, 658, and 672 and the corresponding 
net samples are Nos. 544, 584, 659, and 673; in the other seventeen sam- 
ples of nannoplankton the organic matter was larger in amount than in 
the corresponding net samples. (Tables 45 and 46.) The minimum 
ratio is shown by samples No. 658 and No. 659 in which the quantity 
of organic matter in the nannoplankton is only two-thirds as large as 
that of the net plankton, the ratio being 1 : 0.66; in the other three of 
these samples the nannoplankton yielded somewhat more than four- 
fifths as much organic matter as the corresponding net catches. In sev- 
enteen of the samples the quantity of organic matter was larger in the 
nannoplankton catches than in the corresponding net plankton catches; 
in four of these samples the nannoplankton yielded somewhat less than 
twice as much organic matter as the net plankton, while sample No. 
6134 contained approximately twenty-eight times as much organic mat- 
ter as the corresponding net sample, No. 6135, and sample No. 6122 
almost twenty-three times as much as No. 6123. These results differ 
from those obtained on Lake Mendota in two respects, namely, all of the 
nannoplankton samples from Lake Mendota yielded a larger amount 
of organic matter than the corresponding catches of net plankton, but 
the maximum excess of the former over the latter was not quite as large, 
being only about twenty-five times as large instead of about twenty- 
eight times. 


PLANKTON OF LAKE MONONA 121 


In 1915 the organic matter of the total plankton of Lake Monona 
ranged from a minimum of 1,127.0 milligrams per cubic meter of water 
in samples No. 563 and No. 564 to a maximum of 5,908.3 milligrams in 
samples No. 5166 and No. 5167. (Tables 45 and 46.) In 1916 the mini- 
mum amount of organic matter in the total plankton was 926.2 milli- 
grams per cubic meter of water in samples No. 648 and No. 649 and the 
maximum quantity was 6,088.8 milligrams in samples No. 6158 and No. 
6159. In the former year the variation in quantity was somewhat more 
than fivefold and it was almost sevenfold in the latter year. The 
minimum amount of organic matter in the total plankton was much 
smaller in 1916 than in 1915, but the maximum quantity was somewhat 
larger in the former year. 


The above quantities indicate only the dry wee of the organic mat- 
ter; the live weight would be approximately ten times as much. Thus, 
the live weight of the organic matter in the total plankton obtained 
on October 12, 1916, samples No. 6158 and No. 6159, for example, was 
approximately 61 grams per cubic meter of water. The total live 
weight of these organisms would include the ash also. 


Table 25 shows the mean quantity and the chemical composition of 
the net plankton and the nannoplankton of Lake Monona. The 47 sam- 
ples of net plankton yielded an average of 850.2 milligrams of dry or- 
ganic matter per cubic meter of water. This material contained 497.5 
miligrams of crude protein, 51.2 milligrams of ether extract, 48.7 
milligrams of pentosans, and 30.8 milligrams of crude fiber. The crude 
protein, ether extract, and crude fiber made up 579.5 milligrams, or 
68.2 per cent, of the organic matter. The remainder of the organic 
matter, 31.8 per cent, constituted the nitrogen free extract. 

The 21 samples of net plankton obtained in 1915 and in 1916, which 
correspond to the nannoplankton catches, yielded a smaller mean quan- 
tity of dry organic matter than the whole series of samples, namely, 
only 813.8 milligrams per cubic meter. The proportion of crude pro- 
tein, ether extract, and pentosans was somewhat larger in these 21 
samples than in the complete series; the crude protein, ether extract, 
and crude fiber amounted to 594.0 milligrams in the former, or sub- 
stantially 73.0 per cent of the mean quantity of organic matter in these 
catches. These three items, therefore, constituted 4.8 per cent more of 
the organic matter in the 21 samples of net plankton than they did in 
the 47 samples. Thus, the nitrogen free extract in the former amounted 
to 27.0 per cent of the organic matter as compared with 31.8 per cent 
in the latter. 

The nannoplankton material gave an average of 2,350.0 milligrams 
of dry organic matter per cubic meter of water and this contained 
1,310.0 milligrams of crude protein, 113.3 milligrams of ether extract, 


122 PLANKTON OF WISCONSIN LAKES 


102.5 milligrams of pentosans, and 111.8 milligrams of crude fiber. To- 
gether the crude protein, ether extract, and crude fiber make up 1,535.1 
milligrams, or 65.3 per cent of the organic matter in the nannoplankton 
material. This leaves 34.7 per cent of the organic matter to be classed 
as nitrogen free extract, which is a larger percentage than that noted 
in the net plankton. 

The total plankton yielded 3,163.8 milligrams of dry organic matter 
per cubic meter of water in Lake Monona. This material contained an 
average of 1,821.2 milligrams of crude protein, 170.3 milligrams of 
ether extract, 150.1 milligrams of pentosans, and 137.6 milligrams of 
erude fiber. The crude protein, ether extract, and crude fiber made up 
2,129.1 milligrams of the organic matter, or 67.3 per cent, thus leaving 
32.7 per cent of the organic matter as nitrogen free extract. 


ORGANIC MatTrer PER Unit oF ARBA 


The plankton material from Lake Monona was obtained regularly 
from the deeper portion of the lake, that is, where the water reaches a 
depth of 20 meters to 22.5 meters, the latter being the maximum depth, 
and the samples of water from which the material was secured covered 
the lake down to a depth of 18 meters to 20 meters each time. This 
plankton material, then, represents the deeper part of the lake and the 
results may, therefore, be compared with similar ones obtained on Lake 
Mendota. 

The 20 meter contour line of Lake Monona bounds an area of 56,700 
square meters and the volume of water down to this depth is 1,134,000 
cubic meters; between 20 meters and 22.5 meters the volume is 59,000 
cubic meters, thus making a total of 1,193,000 cubic meters. From this 
volume and the monthly averages of the organic matter in the total 
plankton for the two years 1915 and 1916 combined, the results shown 
in the first part of table 37 (p. 200) have been obtained. The results 
given for the months of May and November are based on a single set of 
observations in each of these months, but the other months indicated in 
the table are represented by three or four sets of observations. 

This table shows a smaller quantity of organic matter in the total 
plankton in May than in June, while the smallest average was found in 
July. The August and September averages show a marked increase 
leading up to the maximum quantity in October. The single set of 
observations taken in November yielded only a little more than half 
as much organic matter as the average for October. The October maxi- 
mum is almost four times as large as the July minimum. 

The amount of organic matter in the total plankton of Lake Monona 
was smaller in the months of May and July than in the corresponding 
months in Lake Mendota, but it was larger in the other months. The 


PLANKTON OF LAKE MONONA 123 


maximum difference appears in October when the amount in Lake 
Monona is two and a half times as large as that of Lake Mendota. (See 
table 26.) 

The second part of table 37 shows the quantity of dry organic matter 
in the total plankton per unit of area of Lake Monona when the entire 
body of water is taken into account. The mean depth of the lake is 
8.43 meters. As noted in the discussion of the results on Lake Mendota 
(p. 102), the amount of material attributed to areas shallower than the 
mean depth is larger than is actually present there, and that assigned 
to areas that are deeper than the mean is too small when computations 
are made on this basis. On the other hand, such results give some idea 
of the general abundance of the plankton material when the whole lake 
is taken into account, and for this reason they are well worth con- 
sidering. 

Taking the entire body of water into account, the amount of dry or- 
ganic matter in the total plankton of Lake Monona varied from a 
minimum of 124 kilograms per hectare (111 pounds per acre) in July 
to a maximum of 478 kilograms per hectare (426 pounds per acre) 
in October. The minimum of Lake Monona is below that of Lake Men- 
dota (table 26), but the maximum of the former lake is more than one 
and a half times as large as that of the latter. 

The average amount for the whole of Lake Monona is a little more 
than 40.0 per cent of the quantity noted for the deep water in the first 
part of table 37; this is due to the fact that the mean depth of the whole 
lake is only slightly more than 40.0 per cent of the mean depth of the 
area bounded by the 20 meter contour line. In Mendota the mean 
depth of the entire lake is about 55.0 per cent of the mean depth within 
the 20 meter contour line, so that the average quantity of organic mat- 
ter in the total plankton when the whole body of water is taken into 
account is about 55.0 per cent of the amount found in the deep water. 

It should be noted that the results given for Lake Monona in table 
37 are stated in terms of dry organic matter; the weight of the living 
organic matter would be about ten times as large as these amounts. 
Thus, within the 20 meter contour line, the live weight of the organic 
matter in the total plankton would range from a minimum of 3,100 
kilograms per hectare of surface (2,760 pounds per acre) in July to a 
maximum of 11,920 kilograms per hectare (10,630 pounds per acre) in 
October. When the whole body of water is taken into account, the 
amounts vary from 1,240 kilograms per hectare (1,110 pounds per acre) 
in July to 4,780 kilograms per hectare (4,260 pounds per acre) in 
October. To obtain the total live weight of these organisms, it would 
be necessary to add to these amounts from 10 per cent to perhaps 15 
per cent of the dry weight of the organic matter for the inorganic mat- 
ter or ash that they contain. 


124 PLANKTON OF WISCONSIN LAKES 


CHAPTER VI 


THE PLANKTON OF LAKES WAUBESA AND 
KEGONSA 


LAKE WAUBESA 


Net PLANKTON 


Material for a quantitative study of the net plankton of Lake Wau- 
besa was obtained in 1918, in 1915, and in 1916; the number of sam- 
ples each year was two, four, and twelve, respectively. The quantity 
of water strained for each catch and the amount of dry plankton ob- 
tained therefrom are given in table 2, p. 181. No samples were taken 
during the winter season and the catches for the first two years were 
made at irregular intervals in the summer and autumn; they were se- 
cured at fairly regular intervals in 1916, being taken at approximately 
two week periods from May 24 to October 30. 

The largest quantity of net plankton per cubic meter of water was 
found in 1915, while the catches taken in 1916 yielded the smallest 
amount of dry material; the average for the former year was a little 
more than three times as large as that of the latter year. The mean 
quantity in the two samples of 1913 was a little larger than the average 
for the entire series of samples of net plankton from this lake. 

Organic Matter. The results for organic matter are summarized in 
table 29, p. 198. The quantity varied from a minimum of 471.1 milli- 
grams per cubic meter of water in one sample collected in 1916 to a 
maximum of 4,232.5 milligrams in one secured in 1915. The two catches 
made in 1913 yielded about the same amount of material per cubie 
meter, but there was nearly a threefold difference between the maxi- 
mum and minimum amounts of organic matter in the 1915 samples and 
more than a fivefold difference in those of 1916. The mean quantity of 
organic matter in the four samples of 1915 was about three times as 
large as the mean for 1916 and almost twice as large as that for 1913. 
The eighteen samples yielded an average of 1,665.8 milligrams of or- 
eanic matter per cubic meter of water. This quantity is nearly twice 
as large as the organic matter in the net plankton of Lake Monona and 
approximately five times as large as that of Lake Mendota. The per- 
centage of organic matter in the net plankton of Lake Waubesa varied 
from a minimum of 69.3 per cent to a maximum of 93.3 per cent of the 


PLANKTON OF LAKES WAUBESA AND KEGONSA 125 


JUNE JULY AUGUST SEPTEMBER | OCTOBER 


bar 

fe[+fels]«[+lelale[-felole|sjelel<| [2] ole) 
6000 
5600 
5200 
4800 
4400 
#4000 
3600 


9200 


2400 
2o00 
1600 
1200 F 
600 


#00 


Oo 
Fig. 39.—The quantity of dry organic matter in the net plankton, in the nanno- 
plankton and in the total plankton of Lake Waubesa in 1916. Curve A 
represents the net plankton, curve B the nannoplankton and curve C the 
total plankton. The amounts are indicated in milligrams per cubic meter 

of water. See tables 47 and 48. 


126 PLANKTON OF WISCONSIN LAKES 


dry material, with an average of 89.2 per cent for the eighteen samples. 

The results for organic matter in 1916 are shown in figure 39, in 
which the curve marked A indicates the quantity per cubic meter of 
water in the various samples. This curve shows that there was a slight 
decrease in the organic matter between May 24 and June 15; the 
amount noted on the latter date was the smallest obtained in the various 
samples from this lake. A small peak in July and another in August 
represent moderate increases at these times, but the large increase came 
during the latter part of September. The maximum point was reached 
in the sample collected on October 2. The two samples collected later in 
October showed a marked decline in the quantity of organic matter so 
that the autumnal peak is sharp and prominent; the amount was only 
about 12.0 per cent larger on October 30 than on September 7. 


Nitrogen. The quantity of nitrogen was determined in all of the 
samples of net plankton from Lake Waubesa. The percentage varied 
from a minimum of 5.82 per cent in one sample collected in 1916 to a 
maximum of 8.57 per cent in a catch obtained in 1913. The mean 
percentages were substantially the same in 1915 and 1916, but that of 
1913 was somewhat higher than the other two. (See table 38, p. 200.) 

A maximum of 343.7 milligrams of nitrogen per cubic meter of water 
was noted in one of the 1915 samples and a minimum of 36.1 milligrams 
in a 1916 catch. The smallest mean was found in the material collected 
in 1916 and the largest in that of 1915, the latter being almost three 
times as large as the former. 

The ratio of the quantity of organic matter to the quantity of total 
nitrogen in the net plankton of Lake Waubesa ranged from 10.2 to 16.3, 
these extremes being found in two samples collected in 1916, namely, 
No. 6141 and No. 6117. (Table 47, p. 213.) These results show that 
the net plankton of Lake Waubesa contained a smaller proportion of 
nitrogen than that of Lake Monona, in which the ratio ranged from 8.5 
to 14.6; in the net plankton of Lake Mendota the ratios showed a wider 
variation, ranging from 8.7 to 17.4. 

Crude Protein. The nitrogen results are given in terms of crude 
protein in table 39, p. 201; the first part of this table shows the varia- 
tions in the percentage of crude protein in the organic matter, while 
the second part gives the maximum and minimum quantities. The 
maximum percentage, namely, 53.6 per cent of the organic matter, was 
found in one of the samples collected in 1913, while the minimum was 
noted in a sample collected on October 23, 1915, which contained 32.1 
per cent of crude protein. 

The quantity of crude protein ranged from a minimum of 225.6 
milligrams per cubic meter of water to a maximum of 2,148.1 milli- 
grams; the former was found in 1916 in sample No. 663 and the latter 


PLANKTON OF LAKES WAUBESA AND KEGONSA 137 


in 1915 in sample No. 5129. The latter is almost ten times as large as 
the former. The mean quantity for the entire series of eighteen sam- 
ples is 785.6 milligrams, which is 47.1 per cent of the average amount of 
organic matter. This percentage of crude protein is much lower than 
the means for the net plankton of Lakes Mendota and Monona; the 
mean for the former is 52.3 per cent and for the latter 58.5 per cent. 
(See table 25.) This indicates that the net plankton of Lake Waubesa 
contains a smaller proportion of nitrogenous material than either of the 
other lakes. Both the maximum and the minimum percentages of crude 
protein are smaller in Waubesa also than in the other two lakes. 
(Tables 8 and 31.) 


Ether Extract. The ether extract was determined in twelve of the 
net plankton samples from Lake Waubesa, namely, the two obtained 
in 1913, four in 1915, and six in 1916. (See table 47.) The extract 
varied from a minimum of 2.81 per cent of the organic matter to a 
maximum of 8.82 per cent, with a mean of 4.64 per cent. The quantity 
of the extract ranged from 50.0 milligrams to 295.2 milligrams per cubic 
meter of water, almost a sixfold difference. The average for the twelve 
determinations is 115.8 milligrams per cubic meter of water. 


Pentosans. The pentosans were determined in six samples of the net 
plankton from Lake Waubesa. The percentage ranged from 4.05 per 
cent to 6.77 per cent of the organic matter in these samples; the mean 
is 5.90 per cent. The quantity varied from 75.9 milligrams to 264.5 
milligrams per cubic meter of water, representing more than a threefold 
variation. The mean quantity for the six determinations is 163.5 milli- 
grams. + hl A 


Crude Fiber. The amount of crude fiber was ascertained in ten 
samples. In these catches the crude fiber varied from a minimum of 
1.82 per cent to a maximum of 10.18 per cent of the organic matter, the 
mean being 4.70 per cent. The quantity ranged from 17.9 milligrams 
to 189.6 milligrams per cubic meter of water, thus showing more than 
a tenfold difference. 


Nitrogen Free Extract. The crude protein, ether extract, crude 
fiber, and ash were all determined for ten of the eighteen samples of 
net plankton from Lake Waubesa and together these four items consti- 
tuted from 53.8 per cent of the dry material in sample No. 691 to 73.5 
per cent in sample No. 6175. Deducting these percentages from 100 
leaves a minimum of 26.5 per cent and a maximum of 46.2 per cent for 
the nitrogen free extract; this is less than a twofold variation in the 
percentage of the nitrogen free extract. The range of variation in the 
nitrogen free extract in the net plankton of Lake Monona was much 
greater, ranging from 15.0 per cent to 49.0 per cent. 


128 PLANKTON OF WISCONSIN LAKES 


Ash. The ash constituted from 6.68 per cent to 30.73 per cent of the 
material in the various samples, with a mean of 10.86 per cent. The 
percentage was highest in spring and in autumn when the diatoms were 
most abundant and lowest in the summer when the green and blue- 
green algae predominated. (Table 47.) 

In most of the catches silica was the most important constituent of 
the ash, varying in amount from 1.04 per cent to 23.52 per cent of the 
dry material. The silica was notably high in those samples containing 
a large percentage of ash, thus indicating the presence of diatoms in 
such samples. pe eg Pe Rag 

Deducting the percentage of silica from the percentage of ash leaves 
from 4.85 per cent to 14.87 per cent of the dry sample for the other 
inorganic constituents of the ash. The samples obtained in 1918 and in 
1915 yielded a smaller amount of such constituents than those collected 
in 1916. The first sample taken in 1916, No. 633, gave the maximum 
difference, 14.87 per cent; the two samples following this one showed a 
slight decline in the percentage of the other inorganic constituents. 
The most marked decrease was noted between samples No. 667 and No. 
679, taken on June 29 and July 11, in which the difference fell from 
12.15 per cent in the former to 7.91 per cent in the latter. Thereafter 
the decline continued until the difference between the percentage of ash 
and the percentage of silica amounted to only 5.21 per cent on August 
23, or but little more than a third as much as the maximum of May 24. 
In September and October of this year the difference ranged from 7.21 
per cent to 9.60 per cent. 

The ash of one sample, No. 352, was subjected to a further analysis 
and the following results are stated in terms of percentages of the dry 
plankton material: ash 22.78 per cent, silica 16.87, Fe,O, and AI,O, 
1.35, P,O; 0.67, CaO 2.20, and MgO 1.28 per cent. This sample was 
obtained on September 19, 1913, when the diatoms were fairly abundant 
and the silica derived from their shells constituted more than two-thirds 
of the ash, leaving only 6.41 per cent for the other inorganic consti- 
tuents. 


ORGANISMS OF THE Net PLANKTON 


The net plankton of Lake Waubesa was made up of crustacea, a very 
few rotifers, some protozoa, and various forms of algae. The crustacea 
were represented by species of Diaptomus, Cyclops, Daphnia, and 
Chydorus. The rotifers included a few individuals belonging to the 
genera Asplanchna and Triarthra, and two species of Anuraea. The 
flagellate Ceratium was noted in all of the material. 

The algae were represented by a rather large variety of forms. 
Among the blue-greens the most important forms were species of Ana- 


PLANKTON OF LAKES WAUBESA AND KEGONSA 129 


baena, Aphanizomenon, Lyngbya, and Microcystis, while the most im- 
portant diatoms were Melosira, Fragilaria, and Stephanodiscus. 

The largest quantity of organic matter noted in the net plankton of 
1915 was found in the catch taken on October 2 (No. 5153). In this 
maximum catch, only two crustacea appeared in considerable numbers, 
namely, 17,100 Cyclops per cubic meter of water and 44,000 Chydorus 
per cubic meter. The rotifer Anuraea cochlearis averaged 26,200 indi- 
viduals per cubic meter, while Ceratium numbered 905,000. There 
were 614,000 colonies of Microcystis per cubic meter of water in this 
eatch and slightly more than a million masses of Anabaena per cubic 
meter. The most abundant forms at this time, however, were the 
diatoms Melosira and Stephanodiscus, the numbers being, respectively, 
4,123,000 and 4,500,000 per cubic meter. 

In 1916 the crustacea showed a June maximum and an October maxi- 
mum, with a more or less pronounced minimum during the intervening 
months. Diaptomus, for example, numbered 3,500 individuals per cubic 
meter of water on June 15 and 4,200 on October 2, with a minimum of 
800 on August 8. Cyclops gave a maximum of 8,000 per cubic meter on 
June 29 and another of 18,100 on October 2, with a minimum of 1,500 
on July 25. The copepod nauplii ranged from a minimum of 4,300 per 
eubic meter of water on July 25 to a maximum of 19,500 on October 
30; a secondary maximum of 18,500 was noted on July 11. Daphma 
pulex was most abundant on June 29, namely, 3,000 per cubic meter 
and its numbers were negligible in the catches taken after July 11. 
Daphnia hyalina yielded a maximum of 18,700 individuals per cubic 
meter on June 29 and one of 11,200 on October 2, with an intervening 
minimum of 900 on August 8. Chydorus, on the other hand, showed 
no distinct maximum during the early part of the season, but the num- 
ber rose to 40,500 individuals per cubic meter on September 19 and 
again to 52,000 on October 30. 

Anuraea cochlearis was the only rotifer found in considerable num- 
bers; 223,200 individuals per cubic meter were obtained in the catch 
secured on October 2, 1916. Ceratium attained a maximum of 2,200,000 
per cubic meter on August 23 and it stood a little above two million on 
October 2. 

Microcystis was found in greatest abundance on September 19 when 
2,734,000 colonies per cubic meter of water were obtained. The number 
of masses of Anabaena rose slightly above one million per cubic meter 
on September 7, while Lyngbya was rather scarce during the entire 
season, the largest number being 490,000 filaments per cubic meter on 
July 25. The diatom Melosira rose to a maximum of 3,286,000 filaments 
per cubic meter on October 18, while Stephanodiscus reached a maxi- 
mum of 2,914,000 individuals per cubic meter on the same date. 


130 PLANKTON OF WISCONSIN LAKES 


NANNOPLANKTON 


Four samples of centrifuge material were obtained from Lake Wau- 
besa in 1915 and a dozen samples in 1916. In the former year the 
catches were made between August 11 and October 23, inclusive, but 
in the latter year samples were taken about every two weeks from May 
24 to October 30. The amount of water centrifuged in 1915 was 5,732 
liters and 11,641 liters in 1916, making a total of 17,373 liters for the 
two years. (Table 2.) 


Organic Matter. The results obtained for the organic matter in the 
sixteen samples of nannoplankton are summarized in table 29, p. 198, 
and the analyses are given in detail in table 48, p. 214. In 1915 the 
minimum quantity of dry organic matter was 2,871.2 milligrams per 
cubic meter of water and the maximum was 6,143.2 milligrams, the 
latter being more than twice as large as the former; the mean quantity 
for the four catches of this year was 4,260.2 milligrams. In 1916 the 
amount was much smaller, the minimum falling to 801.0 milligrams 
and the maximum reaching only 4,830.0 milligrams, a sixfold difference. 
The mean for 1916 was also much smaller than that of 1915, being only 
2,977.5 milligrams. In both years, however, the amount of organic 
matter in the nannoplankton of Lake Waubesa was larger than in Lake 
Mendota or in Lake Monona. 

The curve marked B in figure 39 gives the story of the organic mat- 
ter in the nannoplankton of Lake Waubesa in 1916. There was a de- 
cline in the amount in late May and early June reaching a summer 
minimum on June 15. This period was succeeded by one in which there 
was a regular increase lasting from early July to September 7, the 
highest point in the curve being attained on the latter date. A period 
of decrease was noted for the rest of September, but there was a mod- 
erate increase during the first half of October followed by a further 
decline during the remainder of this month. 

A comparison of this curve with the one marked C in figure 37 shows 
that the story of the nannoplankton of Lake Waubesa in 1916 was sub- 
stantially the same as that of the nannoplankton of Lake Monona in 
this year. There are slight differences, however; the rise in Lake Wau- 
besa was more rapid in the earlier stages and the decline more gradual 
so that the autumn peak of the curve has a broader aspect than that of 
Lake Monona. Another difference is that the small secondary peak in 
October shown in the curve for Lake Waubesa is not represented in 
the curve for Lake Monona. In general, though, the curves for the 
organic matter of the nannoplankton of these two lakes are very similar 
in outline and they both differ markedly from that of Lake Mendota for 
this year as shown in curve A, figure 31. 


PLANKTON OF LAKES WAUBESA AND KEGONSA 131 


Curves A and B in figure 39 represent the organic matter in the net 
plankton and in the nannoplankton, respectively, of Lake Waubesa; 
they show that the former was always smaller than the latter in 1916. 
In 1915, on the other hand, the net plankton gave a larger yield of dry 
organic matter than the corresponding nannoplankton catches in two 
instances. (See samples No. 597-598 and No. 5128-5129, tables 47 and 
48.) Thus, in half a dozen catches out of 37 from Lakes Monona and 
Waubesa, the net plankton yielded a larger amount of organic matter 
than the nannoplankton, but in all of the samples from Lake Mendota 
the net plankton gave a smaller amount of organic matter than the 
nannoplankton. 

These two curves (A and B in figure 39) do not indicate any definite 
eorrelation between the quantity of organic matter in the net plankton 
and that in the nannoplankton. There was a moderate rise in the 
former in late June and early July corresponding to a rise in the 
_ latter, but beyond this point the net plankton changed independently 
of the nannoplankton. In fact, the former showed a decline at the time 
that the latter reached its maximum point and the maximum of the net 
plankton was not reached for fully three weeks after the nannoplankton 
had passed its maximum. 

Curve C in figure 39 shows the amount of dry organic matter in the 
total plankton of Lake Waubesa, that is, the net plankton plus the 
nannoplankton. The autumnal peak of the total organic matter pre- 
sents a different outline from that of either curve A or curve B. The 
maximum point in the total organic matter is correlated in time, how- 
ever, with the maximum of the net plankton instead of the nannoplank- 
ton and the small secondary peak is found on the September side of the 
main peak as in the net plankton instead of on the October side as in 
the nannoplankton. These two facts show clearly that the quantity of 
net plankton in this lake is large enough to affect materially the form 
of the curve representing the organic matter of the total plankton. 
The dry organic matter in the total plankton amounted to 6,378.3 milli- 
grams per cubic meter on October 2, 1916, as compared with 1,271.7 
milligrams on June 15; the latter was the minimum for the year and 
the October maximum was five times as large as this minimum. 


Nitrogen. Quantitative determinations of the nitrogen were made on 
all of the samples of nannoplankton taken from Lake Waubesa. A sum- 
mary of these results is given in table 40, p. 201, and all of the de- 
terminations are shown in table 48. The material obtained in 1915 
contained a higher percentage of nitrogen than that secured in 1916. 
The mean for the former year was 9.03 per cent, while that of the latter 
was 7.41 per cent. Both the maximum and the minimum were higher 
in 1915 than in 1916. The difference between the maximum and the 


132 PLANKTON OF WISCONSIN LAKES 


minimum in the former year was 2.90 per cent and in the latter year 
3.91 per cent, so that there was not as large a variation in the percent- 
age of nitrogen in the centrifuge material from Lake Waubesa as in 
that from Lakes Mendota and Monona. (See table 18, p. 192 and table 
34, p. 199.) 

When expressed in terms of milligrams per cubic meter of water, the 
quantities of nitrogen were much higher in 1915 than in 1916; the mean 
for the former year was more than one and a half times as large as that 
for the latter. In 1915 the maximum quantity of nitrogen was two 
and a half times as large as the minimum, but in 1916 there was an 
eightfold difference between the maximum and the minimum amounts. 

The nannoplankton of Lake Waubesa showed almost a twofold varia- 
tion in the ratio of the organic matter to the total nitrogen, ranging 
from 9.9 to 18.7; this range was larger than that noted for the net 
plankton. (See table 47.) It is a smaller difference than that noted 
for the nannoplankton of Lake Monona in which the limits were 8.1 
and 27.9; it is smaller, also, than in the nannoplankton material from 
Lake Mendota which varied from 9.1 to 28.5. These results also show 
that the proportion of nitrogen in the organic matter had the widest 
range of variation in the net plankton of Lake Mendota, while the ma- 
terial from Lakes Monona and Waubesa possessed the same range, with 
a larger proportion of nitrogen in the former lake. In the nanno- 
plankton, however, the proportion was muck more constant in the ma- 
terial from Lake Waubesa than in that from the other two lakes; the 
largest variation was noted in the nannoplankton of Lake Monona. 


Crude Protein. The nitrogen results are summarized in terms of 
crude protein in table 41; the first part of this table shows the varia- 
tions in the percentage and the second part the changes in the quantity. 
There was a much larger proportion of crude protein in the material 
collected in 1915 than in that secured in 1916; in the former year the 
percentage of crude protein in the nannoplankton ranged from a mini- 
mum of 44.4 per cent of the organic matter to a maximum of 62.5 
per cent, with a mean of 56.4 per cent for the four catches obtained in 
this year. In 1916 the extremes were 33.5 per cent and 57.9 per cent, 
with a mean of 46.3 per cent for the twelve samples of this year; thus, 
the mean for 1916 was 10.0 per cent below that of 1915. The crude pro- 
tein fell below 50.0 per cent of the organic matter in only one of the 
four samples of 1915, but it reached or exceeded this amount. in only 
one of the dozen catches made in 1916. In two samples secured in 1916 
the crude protein fell below 40.0 per cent of the dry organic matter. 

The quantity of crude protein varied from a minimum of 1,361.9 mil- 
ligrams per cubic meter of water to a maximum of 3,471.9 milligrams 
in the 1915 samples of nannoplankton from Lake Waubesa, while in 


PLANKTON OF LAKES WAUBESA AND KEGONSA 133 


may] wy, ne | JULY AUGUST SEPTEMBER OCTOBER 
eal) [2 sils les |2 | 2 | se 7 | 2) 24 


Fig. 40.—The quantity of organic matter and of crude protein in the nanno- 
plankton of Lake Waubesa in 1916. Curve A represents the dry organic 
matter and curve B the crude protein. The amounts are indicated in milli- 
grams per cubic meter of water. See table 48. 


1916 the extremes were 341.2 milligrams and 2,800.0 milligrams. In the 
former year the mean quantity was 2,404.4 milligrams per cubic meter 
of water and in the latter year 1,378.1 milligrams, so that the mean for 
1916 was only a little more than half as large as that of 1915. 

Figure 40 shows graphically the relation of the crude protein to the 
organic matter in the nannoplankton material collected from Lake 
Waubesa in 1916. The curve marked A represents the quantity of 


134 PLANKTON OF WISCONSIN LAKES 


organic matter in milligrams per cubic meter of water in the various 
samples and eurve B indicates the amount of crude protein in this 
material. That is, the space between the zero line and curve B shows 
the proportion of crude protein, while the space between B and A rep- 
resents the other constituents of the organic matter. With one excep- 
tion the two curves possess a similar outline; during the second and 
third weeks in July there is a decrease in the crude protein which is 
correlated in time with a moderate increase in the organic matter. 


Ether Extract. The quantity of ether extract was determined in 
twelve of the sixteen samples of nannoplankton from Lake Waubesa. 
The percentage of extract varied from a minimum of 1.48 per cent to 
a maximum of 8.13 per cent of the dry organic matter, almost a sixfold 
difference. Both of these percentages were found in material collected 
in 1916. (Table 42.) A much smaller range of variation was found in 
the 1915 samples, the difference between maximum and minimum being 
less than two per cent. The mean for the twelve samples on which de- 
terminations were made, amounts to substantially four per cent of the 
organic matter in these samples. 

The quantity of ether extract varied from 61.5 ‘illierame to 308.0 
milligrams per cubic meter of water, with a mean of 121.0 milligrams 
for the twelve samples. The mean quantity in the 1915 catches was 
more than twice as large as that in the 1916 material. 


Pentosans. The pentosans were determined in the four samples col- 
lected in 1915, but not in any of the catches made in 1916. In these 
four samples the pentosans varied from 4.55 per cent to 6.72 per cent of 
the dry organic matter. (See table 48, p. 214.) The quantity of pen- 
tosans in this material ranged from a minimum of 193.0 milligrams to 
a maximum of 346.0 milligrams per cubic meter of water; the mean for 
the four samples was 237.0 milligrams. 

Crude Fober. Twelve samples of nannoplankton were analyzed for 
the crude fiber. The percentage of this material in these samples varied 
from a minimum of 0.83 per cent to a maximum of 10.43 per cent of 
the organic matter; the mean for all determinations was 4.45 per cent. 
(Tables 25 and 48.) 

Quantitatively the amount ranged from 49.7 milligrams to 332.6 mil- 
ligrams per cubic meter of water, with a mean of 135.5 milligrams. 
The smallest quantity was found in sample No. 597 and the largest in 
No. 6150. 

Nitrogen Free Extract. The chemical analyses were complete enough 
to ascertain the percentage of nitrogen free extract in a dozen samples 
of nannoplankton from Lake Waubesa. In these catches it ranged 
from a minimum of 15.0 per cent in sample No. 666 to a maximum of 
29.0 per cent of the dry weight of the material in sample No. 597, 


PLANKTON OF LAKES WAUBESA AND KEGONSA 135 


-representing almost a twofold variation. The percentage was some- 
what larger in the nannoplankton material from Lake Waubesa than in 
that from Lake Monona where the minimum was 14.0 per cent and the 
maximum 23.0 per cent. 

On an ash free basis the nitrogen free extract constituted 42.8 per 
cent of the dry organic matter in sample No. 666 and 50.7 per cent in 
sample No. 597. From 30.7 per cent to 52.7 per cent of the organic 
matter in the twelve samples from Lake Waubesa consisted of nitrogen 
free extract. The minimum was found in samples No. 5128 and No. 
5174 and the maximum in sample No. 690. 

Ash. The ash in the centrifuge catches from Lake Waubesa ranged 
from a minimum of 33.44 per cent to a maximum of 65.35 per cent of 
the dry material. (Table 48.) The average for the entire series 
amounted to 49.38 per cent, or substantially the same as that noted for 
the centrifuge samples from Lake Monona. 

The diatoms were fairly abundant in all of the nannoplankton sam- 
ples from Lake Waubesa except four and these four were probably the 
only ones in which the ash of the organisms fell within the 10.0 per 
cent limit; two of these samples were obtained in June, 1916, No. 650 
and No. 666, and two in July, 1916, No. 678 and No. 690. Allowing 
10.0 per cent for the ash, the organisms in these four samples contrib- 
uted from 89.0 milligrams to 253.0 milligrams per cubic meter of 
water to the total ash of the samples. When the ash derived from the 
organisms and that derived from the bowl water are deducted from the 
total ash, the remainder, which may be attributed to silt, varies from 
368.0 milligrams in sample No. 650 to 1,185.0 milligrams per cubie 
meter of water in sample No. 678; this means from 0.37 to 1.13 parts 
of silt per million parts of water. These results show that silt is a little 
more abundant in the water of Lake Waubesa than in that of Lake 
Monona, but it is substantially the same as that in Lake Mendota. 


ORGANISMS OF THE NANNOPLANKTON 


The protozoan population in the nannoplankton of Lake Waubesa 
consisted of essentially the same forms that were found in Lake Mo- 
nona. There was a somewhat greater variety of algal forms, however, 
than in Lakes Mendota and Monona. The more abundant algae were 
Aphanocapsa delicatissima, Oocystis, and the young colonies and frag- 
ments of Microcystis and Anabaena; several other forms were obtained 
from time to time in the various catches, but they were never found 
in any considerable numbers. The chief diatoms were Melosira and 
Stephanodiscus. 

Chlorochromonas was abundant in the four catches which were ob- 
tained in 1915; it ranged from a minimum of 135,000 per liter of 


136 PLANKTON OF WISCONSIN LAKES 


water on August 11, to a maximum of 768,000 per liter on October 23. 
Cryptomonas reached a maximum of about 65,000 per liter on the latter 
date, while the same maximum was noted for the disc-shaped flagellate 
on August 11. Aphanocapsa delicatissima yielded a maximum of 444,- 
000 colonies per liter on August 11, while another blue-green alga, 
Oscillatoria, rose to 477,500 per liter on October 23. In fifteen other 
forms of algae, exclusive of the diatoms, the numbers varied from 2,000 
to 75,000 per liter of water. The diatom Melosira numbered 307,000 
filaments per liter on October 2, while Stephanodiscus rose to 20,000 
per liter on this date. 


In the series of catches obtained in 1916, Chlorochromonas showed 
two maxima; the first one was noted on June 25 when the number rose 
to 665,500 per liter and a second maximum of 512,000 was found on 
September 7. The minimum number for the entire series of catches 
collected in this year was found on July 25, namely, 10,000 per liter. 
The maximum for the disc-shaped flagellate was 204,000 per liter on 
September 7. Seven other forms of protozoa were noted from time to 
time, but none of them appeared regularly in the various catches; the 
most abundant one was Cryptomonas, of which 105,000 individuals per 
liter were found on October 2. 


Aphanocapsa delicatissima gave a maximum of 512,000 per liter on 
October 2, while the maximum for Oscillatoria was 768,000 per liter on 
August 23. It may be noted here that Oscillatoria was found in con- 
siderable abundance both in Lake Monona and in Lake Waubesa, but 
it was not present in any of the centrifuge material from Lake Mendota. 
Small colonies of Microcystis were present in all of the nannoplankton 
collected in Lake Waubesa after the middle of July; they numbered 
205,000 per liter in both of the August samples in 1916. With the ex- 
ception of the first sample (No. 632) portions of the filaments of Ana- 
baena were more or less abundant in all of the catches of this year; 
the number ranged from a minimum of 3,400 per liter to a maximum of 
410,000 per liter, the latter number being noted on September 7. Four- 
teen other forms of algae, exclusive of the diatoms, were found in the 
various catches, but all of them were irregular in their appearance; 
their numbers varied from about 1,000 to 102,000 per liter of water. 

The most important diatom was Melosira; it was present in all of 
the catches obtained after the middle of July. The largest number, 
375,000 per liter, was noted on September 19. Half a dozen other dia- 
toms were found in the material but they did not appear regularly in 
the various catches; of these forms Stephanodiscus, Coscinodiscus, and 
Cocconeis were obtained most frequently. 

Only one form, Aphanocapsa, gave a maximum number on October 2 
when the largest quantity of organic matter was obtained in the cen- 


PLANKTON OF LAKES WAUBESA AND KEGONSA 137 


trifuge catch; several other forms, however, showed a marked increase 
in number at this time and these increases together with that of Aphano- 
capsa were sufficient to yield a maximum of organic matter. 

The number of forms represented in the centrifuge material from 
Lake Waubesa in 1916 increased as the season advanced, just as noted 
in Lake Monona. On May 24, for example, ten different forms were 
present in the centrifuge catch, and the number varied from seven to 
ten until July 25, when it rose to fifteen. In the catch taken on Sep- 
tember 19, the various forms number twenty and a maximum of twenty- 
five was found on October 30. 


Toran PLANKTON OF LAKE WAUBESA 


In the 18 samples of net plankton from Lake Waubesa, the dry or- 
ganic matter varied from a minimum of 471.1 milligrams to a maximum 
of 4,232.5 milligrams per cubic meter of water, almost a tenfold varia- 
tion in quantity. The whole series of net catches yielded an average of 
1,665.8 milligrams of organic matter per cubic meter of water, of which 
785.0 milligrams consisted of crude protein, 115.8 milligrams of ether 
extract, 98.3 milligrams of pentosans, and 78.3 milligrams of crude fiber. 
(Table 25.) The crude protein, ether extract, and crude fiber amounted 
to 979.1 milligrams per cubic meter of water, or 58.7 per cent of the 
organic matter. This leaves an average of 41.3 per cent of the organic 
matter as nitrogen free extract. 

In the 16 samples of net plankton corresponding to the same number 
of samples of nannoplankton, the mean quantity of organic matter was 
somewhat smaller than in the entire series of net catches, namely 1,639.2 
milligrams per cubic meter of water. In this material the crude pro- 
tein, ether extract, and crude fiber made up 56.9 per cent of the organic 
matter, leaving 48.1 per cent of nitrogen free extract. The latter is 
nearly two per cent larger in these samples than in the complete series. 

The organic matter of the nannoplankton varied from a minimum 
of 801.0 milligrams to a maximum of 6,143.2 milligrams per cubic meter 
of water. The mean quantity for the 16 samples was 3,299.1 milli- 
grams, of which 1,635.0 consisted of crude protein, 132.0 milligrams of 
ether extract, 183.1 milligrams of pentosans, and 146.8 milligrams of 
erude fiber. The crude protein, ether extract, and crude fiber accounted 
for 1,913.8 milligrams, or 58.0 per cent of the organic matter; this 
leaves 42.0 per cent of the organic matter in the nannoplankton of Lake 
Waubesa as nitrogen free extract. The percentage of extract is a little 
larger in the net plankton than in the nannoplankton. 

The quantity of dry organic matter in two net samples from Lake 
Waubesa, No. 598 and No. 5129, was larger than that in the correspond- 
ing samples of nannoplankton, No. 597 and No. 5128. (Tables 47 and 


138 PLANKTON OF WISCONSIN LAKES 


48.) In net sample No. 598 the organic matter was about 17.0 per cent 
larger than in nannoplankton sample No. 597, while nannoplankton 
sample No. 5129 yielded only about two-thirds as much organic matter 
as net sample No. 5128. Four net samples from Lake Monona also 
yielded a larger amount of organic matter than the corresponding nan- 
noplankton samples, which makes a total of six sets of catches showing 
this phenomenon. All of the nannoplankton samples from Lake Men- 
dota yielded a larger amount of organic matter than the corresponding 
net samples. 

In the four sets of samples that were obtained in Lake Waubesa in 
1915, the dry organic matter of the total plankton ranged between a 
minimum of 6,538.0 milligrams in samples No. 5174 and No. 5175 and a 
maximum of 9,489.7 milligrams in samples No. 5152 and No. 5153. 
(Tables 47 and 48.) In 1916 the quantity varied from 1.272.1 milli- 
grams in samples No. 650 and No. 651 to 6,378.3 milligrams per cubic 
meter of water in samples No. 6150 and No. 6151; this represents a five- 
fold variation in the quantity of organic matter obtained in the sam- 
ples of this year. The maximum amount was found on the same date in 
both years, October 2, but the maximum for 1915 was almost one and 
a half times as large as that of 1916. The dry organic matter con- 
stitutes only about one-tenth of the weight of the living organic matter 
because most of the organisms in the plankton contain at least 90.0 per 
cent of water in the living state; the 1915 maximum, therefore, repre- 
sents approximately 95 grams of living organic material per cubic meter 
of water. 

The Waubesa maximum of dry organic matter in the total plankton 
noted in 1915 was not only the largest for this lake, but it was also 
much larger than the maxima for Lakes Mendota and Monona. It was 
a little more than one and a half times the maximum of Lake Monona, 
6,088.8 milligrams per cubic meter of water, and almost three times as 
large as that of Lake Mendota, which was 3,327.3 milligrams per cubic 
meter. The minimum amount of organic matter in the total plankton 
of Lake Waubesa, 1,272.1 milligrams per cubic meter of water, was 
about one-third larger than the minimum of Lake Mendota, 906.0 milli- 
grams, and also about a third larger than the minimum of Lake Monona, 
926.2 milligrams. 

The mean quantity of organic matter in the total plankton of Lake 
Waubesa amounted to 4,938.3 milligrams per cubic meter of water. 
(Table 25.) This material contained 2,401.2 milligrams of erude pro- 
tein, 227.5 milligrams of ether extract, 284.7 milligrams of pentosans, 
and 218.1 milligrams of crude fiber. The crude protein, ether extract, 
and crude fiber amounted to 2,846.8 milligrams, or 57.6 per cent of the 
organic matter ; this leaves 42.4 per cent of the organic matter as nitro- 
cen free extract. 


PLANKTON OF LAKES WAUBESA AND KEGONSA 139 


Total Organc Matter per Umt Area. The plankton material was ob- 
tained in the deepest portion of Lake Waubesa, that is, in the region 
where the water reaches a maximum depth of a little more than 11 
meters. The 10 meter contour line bounds an area of 101.2 hectares and 
the volume of water within this boundary line is 10,306,000 cubic 
meters. In this region of the lake, then, the maximum crop of total 
plankton in 1915, namely, 9,489.7 milligrams per cubic meter of water, 
amounted to 966 kilograms of dry organic matter per hectare, or about 
862 pounds per acre. The living organic matter would represent about 
9,660 kilograms per hectare, or 8,620 pounds per acre. 

The mean quantity of organic matter for the 16 samples of total 
plankton is 4,938.3 milligrams per cubic meter of water, which repre- 
sents 499 kilograms of dry organic matter per hectare of surface, or 
445 pounds per acre. 

For the entire lake the maximum crop of total plankton in 1915 
represented 465 kilograms of dry organic matter per hectare, or 415 
pounds per acre, and the mean of the 16 samples amounted to 242 
kilograms per hectare, or 216 pounds per acre. The quantity per unit 
area for the entire lake was somewhat less than half as large as that 
for the deep water. 

LAKE KEGONSA 


The material from Lake Kegonsa consisted of a single sample of net 
plankton which was obtained on July 29, 1913. This catch gave the 
largest yield of net plankton per cubic meter of water that was secured 
during the entire investigation (see table 2, p. 181); it was almost 
one-third larger than its nearest competitor. Owing to the high per- 
centage of ash in this material, namely, 38.85 per cent of the dry sample, 
with the silica amounting to 32.38 per cent of the dry material, the 
quantity of organic matter per cubic meter fell below that of a net 
sample taken in Lake Waubesa on September 10, 1915. (See table 47, 
p. 213, sample No. 5129 and table 25, p. 196.) In this net catch from 
Lake Kegonsa the phytoplankton was much more abundant than the 
zooplankton and the chief constituent of the former was Melosira. The 
ash content of this diatom was high, as indicated by the high percentage 
of silica, and this served to reduce the proportion of organic material 
in the sample. 

The percentage of nitrogen in the organic matter of the net catch 
from Lake Kegonsa was somewhat lower than the average for the net: 
material from the other lakes, namely, 6.88 per cent as compared with 
7.54 per cent for Lake Waubesa and 9.36 per cent for Lake Monona. 
(Table 25.) Thus the crude protein constituted a proportionally smaller 
percentage of the organic matter.. The large quantity of organic matter 
per cubic meter of water, on the other hand, gives a larger yield of ni- 


140 PLANKTON OF WISCONSIN LAKES 


trogen, and consequently of crude protein, than the averages for the 
other lakes when the results are stated in terms of milligrams per cubie 
meter of water. It will be noted that the amount of nitrogen is almost 
ten times as large in this net sample from Lake Kegonsa as the average 
for the various net catches of Lake Mendota and somewhat more than 
twice as large as the average of Lake Waubesa. 

The pereentage of ether extract is larger than the average for the 
net material of Lake Waubesa, but lower than the averages of Lakes 
Mendota and Monona. The percentage of pentosans is higher than the 
average for Lake Mendota, but lower than the averages of the other 
two lakes, while the order is reversed for the crude fiber. Owing to the 
fact that the organic matter in the net catch from Lake Kegonsa is 
much larger than the averages for the other lakes, the ether extract, 
pentosans, and erude fiber are distinetly larger in this sample than the 
averages for the other lakes when the results are given in terms of 
milligrams per cubic meter of water. 


SUMMARY AND DISCUSSION 141 


CHAPTER VII 
GENERAL SUMMARY AND DISCUSSION 


In previous chapters attention has been called to the fact that there 
are marked variations in the quantity of net plankton and of nanno- 
plankton during the course of the year. This fact is shown graphically 
in figures 8, 9, 28, and 36. There are also more or less marked changes 
in the chemical composition of the plankton, which is illustrated by the 
variations in the percentages of nitrogen, ether extract, pentosans, and 
erude fiber. (Tables 43 to 48.) These variations in quantity and in 
chemical composition make it difficult to give a summary of the entire 
series of plankton samples. It is necessary to remember, therefore, in 
connection with the general data presented in table 25 (p. 196) that 
this plankton material is subject to such changes in order to guard 
against a wrong interpretation of these data. Furthermore, it should 
be noted that the data presented in this table do not represent the total 
amount of organic matter produced by the plankton; they show only 
the average standing crop maintained during the period of this investi- 
gation. 

The mean quantity of dry organic matter indicated for the different 
lakes in table 25 represents the average amount for all of the plankton 
samples from each lake; this mean also represents the average amount 
for all depths in the deeper part of each lake. The plankton was more 
abundant in the epilimnion than in the hypolimnion during the summer 
period of stratification (see p. 101), but the water from all depths 
was combined into one sample in this investigation. In addition to the 
average for the complete series of net catches, the averages for the net 
samples corresponding to the nannoplankton samples are indicated also. 
The mean percentages of nitrogen, ether extract, pentosans, and crude 
fiber were ascertained by dividing the mean quantities of these sub- 
stances in the various samples on which such determinations were made 
by the average amount of organic matter in the same samples. For 
example, the general table (No. 43) shows that nitrogen determinations 
were made on 166 samples of net plankton from Lake Mendota; these 
samples yielded an average of 28.6 milligrams of nitrogen per cubie 
meter of water, excluding the nitrogen of the crude fiber, and an aver- 
age of 341.8 milligrams of dry organic matter. The nitrogen in this 
material, therefore, amounted to 8.37 per cent of the organic matter. 


142 PLANKTON OF WISCONSIN LAKES 


Multiplying the average quantity of organic matter in the 184 samples 
of net plankton, namely 332.5 miligrams per cubic meter of water, by 
the above percentage gives an average of 27.8 milligrams of nitrogen 
per cubic meter of water for the entire series of net catches in Lake 
Mendota. The other mean percentages and mean quantities were ob- 
tained in asimilar manner. For the crude protein the results for nitro- 
gen were multiplied by the factor 6.25. 

In the following summary of the results, the fractions of milligrams 
have been omitted ; they have been used, however, in computing the data 
into larger units. These fractions of milligrams are shown in table 25 
and in tables 43 and 48. 


Net PLANKTON 


Lake Mendota. Table 25 shows that 184 samples of net plankton 
from Lake Mendota yielded an average of 332 milligrams of dry organie 
matter per cubic meter of water, while the 84 samples corresponding to 
the series of nannoplankton samples gave an average of 343 milligrams. 
In the former series the nitrogen made up 8.87 per cent of the organic 
matter (equivalent to 52.31 per cent of crude protein), the ether extract 
11.78 per cent, the pentosans 2.88 per cent, and the crude fiber 6.54 per 
cent. In other words the crude protein, the ether extract, the pento- 
sans, and the erude fiber constituted about 73.51 per cent of the 
average quantity of organic matter. The remaining portion consisted 
of nitrogen free extract or carbohydrates that are not included in the 
pentosans and the crude fiber. The percentage of nitrogen in the com- 
plete series of net catches is substantially the same as in the 84 samples 
corresponding to the nannoplankton samples, but the percentages of 
ether extract and of the pentosans are slightly higher and that for 
crude fiber is somewhat lower. 

In the whole series of net catches from Lake Mendota, the organic 
matter varied from a minimum of 42 milligrams to a maximum of 1,135 
milligrams per cubic meter of water. The regular station was located 
within the area bounded by the 20 meter contour line; when stated in 
terms of larger units for this area (see p. 98), the amount ranges 
from about 9 kilograms to 250 kilograms per hectare (8 pounds to 223 
pounds per acre) for this portion of the lake, with an average of a little 
more than 72 kilograms per hectare (65 pounds per acre). These 
figures represent only the dry weight of the organic matter; the living 
organic material would weigh approximately ten times as much. For 
the 84 catches corresponding to the nannoplankton samples the average 
is somewhat higher, or approximately 76 kilograms per hectare of sur- 
face (68 pounds per acre). 

Assuming that the net plankton is uniformly distributed over the 
whole lake, the mean quantity of organic matter (332 milligrams per 


SUMMARY AND DISCUSSION 143 


cubic meter of water) multiplied by the total volume of the lake (table 
1) gives an average standing crop of 159 metric tons of dry organic 
matter for the entire body of water. Since the lake has an area of 3,940 
hectares, this crop amounts to a little more than 40 kilograms per hec- 
tare, or 36 pounds per acre, when expressed in terms of a unit of sur- 
face. The 84 net catches yield an average of just a little less than 42 
kilograms of dry organic matter per hectare, or 37 pounds per acre. 

Lake Monona. The 47 catches of net plankton from Lake Monona 
gave an average of 850 milligrams of organic matter per cubic meter 
of water, or a little more than two and a half times as much as those of 
Lake Mendota. In Lake Monona the amount ranged from a minimum 
of 109 milligrams to a maximum of 3,306 milligrams per cubic meter. 
No winter catches are included in this series so that the general average 
may be somewhat higher than it would be if some winter catches had 
been taken. On the other hand, it may be said that 111 net samples 
from Lake Mendota taken during the same months of the same years as 
the catches from Lake Monona yielded a smaller average than the com- 
plete series of samples, or only 309 milligrams as compared with 332 
milligrams per cubic meter of water. 

The nitrogen amounted to 9.36 per cent of the dry organic matter in 
the net plankton of Lake Monona, the ether extract 6.02 per cent, the 
pentosans 5.73 per cent, and the crude fiber 3.62 per cent. The per- 
centages of nitrogen and pentosans are larger in the net material from 
Lake Monona than in that from Lake Mendota, but the reverse is true 
of the percentages of ether extract and crude fiber. The crude protein, 
ether extract, pentosans, and crude fiber constitute 73.87 per cent of the 
dry organic matter, leaving 26.13 per cent of undetermined nitrogen 
free extract. (Table 25.) 

The 21 samples of net plankton from Lake Monona corresponding to 
the same number of nannoplankton catches yielded a somewhat smaller 
average amount of dry organic matter than the complete series of net 
catches, namely, 813 milligrams per cubie meter of water. The per- 
centages of nitrogen, ether extract, and pentosans are larger in this 
material, but the percentage of crude fiber is smaller in the complete 
series of net samples. 

The catches were taken in the deeper water of Lake Monona so that 
the results may be stated in larger units for the area within the 20 
meter contour line (see p. 122). The average quantity of dry organic 
matter in the net plankton is 17.9 grams per square meter of surface for 
this part of the lake, or 179 kilograms per hectare (160 pounds per 
acre) for the whole series of net catches. The average is somewhat 
smaller for the 21 samples corresponding to the nannoplankton series, 
namely, 171 kilograms per hectare or 153 pounds per acre. The varia- 


144 PLANKTON OF WISCONSIN LAKES 


tions in the average for the complete series of net catches range from a 
minimum of 23 kilograms to a maximum of 695 kilograms per hectare 
(20 pounds to 620 pounds per acre) for the deep part of the lake. 


When the area and the volume of the entire lake are taken into ac- 
count, the amount of dry organic matter in the 47 net samples from 
Lake Monona varies from a minimum of 9 kilograms to a maximum 
of 278 kilograms per hectare of surface (8 pounds to 248 pounds per 
acre), with an average of about 72 kilograms per hectare (65 pounds 
per acre). For the whole lake the average standing crop of net plank- 
ton yields 101 metric tons of dry organic matter. The mean quantity 
of organic matter in the 21 samples of net plankton corresponding to 
the nannoplankton catches is a little less than 69 kilograms per hectare 
(61 pounds per acre). 

Lake Waubesa. The 18 samples of net plankton from Lake Waubesa 
yielded an average of 1,665 milligrams of dry organic matter per cubie 
meter of water, or almost twice as much as the average for Lake Monona 
and five times as much as the mean of Lake Mendota. (Table 25.) The 
nitrogen constituted an average of 7.54 per cent of the dry organic mat- 
ter, the ether extract 4.64 per cent, the pentosans 5.90 per cent, and the 
erude fiber 4.70 per cent. The percentages of nitrogen and of ether 
extract are appreciably below those of the net plankton from Lakes 
Monona and Mendota, but the percentage of the pentosans is higher. 
The percentage of crude fiber is higher than that noted in the material 
from Lake Monona, but lower than that of Lake Mendota. The average 
percentages of crude protein, ether extract, pentosans, and crude fiber 
account for 62.36 per cent of the organic matter, leaving 37.64 per cent 
of undetermined nitrogen free extract. In the net material from Lake 
Mendota the nitrogen free extract constituted an average of only 26.49 
per cent of the organic matter and in that from Lake Monona only 26.13 
per cent. 


The quantity of dry organic matter in the complete series of net 
catches from Lake Waubesa varied from a minimum of 471 milligrams 
to a maximum of 4,232 milligrams per cubic meter of water. In that 
part of the lake having a depth of 10 meters or more (p. 139) the 
amount varied from about 48 kilograms to 427 kilograms per hectare 
(42 pounds to 381 pounds per acre). The average standing crop of dry 
organic matter in this region of the lake was 170 kilograms per hectare 
(151 pounds per acre). For the 16 samples of net plankton correspond- 
ing to the same number of nannoplankton catches, the average is some- 
what smaller, namely, 1,639 milligrams per cubic meter of water, or 
167 kilograms per hectare (149 pounds per acre) for the deep part of 
the lake. 


SUMMARY AND DISCUSSION — 145 


Taking the entire lake into account the net plankton of Lake Waubesa 
gave an average of slightly more than 81 kilograms of dry organic 
matter per hectare of surface (72 pounds per acre) for the complete 
series of catches; the range is from a minimum of 23 kilograms to a 
maximum of 195 kilograms per hectare (20 pounds to 174 pounds per 
acre). The average for the 16 catches corresponding to the nanno- 
plankton is 80 kilograms per hectare, or a little more than 71 pounds 
per acre. 

A comparison of the results for the deep portions of the three lakes 
shows that the complete series of net catches from Lake Mendota yielded 
an average of somewhat more than 73 kilograms of dry organic matter 
per hectare, the series from Lake Monona a little more than 171 kilo- 
grams, and that from Lake Waubesa approximately 170 kilograms. 
Thus, the average amounts are substantially the same for Monona and 
Waubesa, with a slight advantage in favor of the former, while the 
average of Mendota is less than half as large as those of the other two 
lakes. Table 1 shows that the maximum depth of Waubesa is slightly 
less than half as much as the maxima of the other two lakes, yet it 
maintains substantially as large an average crop of net plankton in the 
deep water as Monona and a far larger amount than Mendota. Com- 
puted on the basis of the entire volume and area of each lake the aver- 
age amounts of dry organic matter in the standing crop of net plankton 
per unit of surface are as follows, Mendota 42 kilograms per hectare 
(87 pounds per acre), Monona 72 kilograms per hectare (64 pounds 
per acre), Waubesa 81 kilograms per hectare (72 pounds per acre). 
Thus, for the entire lake the standing crop of net plankton of Lake 
Waubesa yields an average of almost twice as much organic matter 
per unit of surface as that of Lake Mendota and 12.5 per cent more 
than that of Lake Monona. 

It should be noted again that the series of net catches from Lakes 
Monona and Waubesa cover only the period from late May to early 
November in the different years, so that the averages may be higher 
than they would be if catches had also been taken during the interval 
extending from the middle of November to the middle of May. On the 
other hand, it is shown on page 143 that the average for just those net 
samples from Mendota which correspond in time to those taken on 
Monona is about 8.0 per cent smaller than the average for the complete 
series of net catches from Mendota. 

Lake Kegonsa. Only one sample of net plankton was obtained from 
Lake Kegonsa and that contained 3,378 milligrams of dry organic mat- 
ter per cubic meter of water. (Table 25). For the entire lake this 
amount is equivalent to 174 kilograms per hectare (155 pounds per 
acre). The percentage of nitrogen in this material is smaller than the 


146 PLANKTON OF WISCONSIN LAKES 


averages for the other three lakes, but the percentage of ether extract 
is a little larger than the average of Lake Waubesa. The percentage of 
the pentosans falls below the averages for the net material from Monona 
and Waubesa, but it is larger than the average of Mendota; the per- 
centage of crude fiber, on the other hand, is larger than the averages 
of Monona and Waubesa, but smaller than the mean of Mendota. To- 
gether the crude protein, ether extract, and crude fiber constitute 53.36 
per cent of the organic matter in the net catch from Lake Kegonsa, 
leaving 46.64 per cent as nitrogen free extract. The pentosans make 
up 4.36 per cent of the nitrogen free extract which leaves 42.28 
per cent of the dry organic matter as undetermined carbohydrate 
material. 
THE NANNOPLANKTON 


Lake Mendota. The second part of table 25 gives a general summary 
of the nannoplankten for the three lakes from which such material was 
secured. A summary of the net samples corresponding to the nanno- 
plankton catches is given also in order to show figures that are directly 
comparable for the two series of samples. 

The dry organic matter of the nannoplankton of Lake Mendota varied 
in amount from a minimum of 795 milligrams to a maximum of 3,151 
milligrams per cubic meter of water. (Table 17.) The average amount 
for the entire series of nannoplankton catches is 1,630 milligrams per 
cubic meter. (Table 25.) Nitrogen constituted 6.84 per cent of the 
mean quantity of organic matter (equivalent to 42.75 per cent of crude 
protein), ether extract 6.55 per cent, the pentosans 4.82 per cent, and 
the crude fiber 5.19 per cent. The percentages of nitrogen, ether ex- 
tract, and crude fiber are smaller than those of the corresponding sam- 
ples of net plankton, but the percentage of pentosans is larger. The 
erude protein, ether extract and crude fiber make up 54.49 per cent 
of the organic matter in the nannoplankton of Lake Mendota and the 
remainder consists of the carbohydrates which constitute the nitrogen 
free extract. Of the latter; the pentosans make up 4.82 per cent. 

In the deep water of Lake Mendota the organic matter of the nanno- 
plankton ranged from 175 kilograms to 694 kilograms per hectare (155 
pounds to 619 pounds per acre), while the average quantity for the en- 
tire series of nannoplankton samples was 359 kilograms per hectare, or a 
little more than 320 pounds per acre. When the entire body of water is 
taken into account the mean quantity of dry organie matter becomes 198 
kilograms per hectare of surface, or about 177 pounds per acre. 

Lake Monona. The 21 samples of nannoplankton from Lake Monona 
gave an average of 2,350 milligrams of dry organic matter per cubic 
meter of water, with a minimum of 666 milligrams and a maximum of 
5,696 milligrams. (Tables 25 and 29.) Of the mean quantity of 


SUMMARY AND DISCUSSION 147 


organic matter 8.92 per cent consisted of nitrogen, 4.82 per cent of 
ether extract, 4.86 per cent of pentosans, and 4.76 per cent of crude 
fiber. With the exception of that for crude fiber, these percentages are 
smaller than those of the corresponding samples of net plankton. The 
crude protein, ether extract, and crude fiber constitute 65.33 per cent 
of the dry organic matter in the average crop of nannoplankton of 
Lake Monona; the remainder, 34.67 per cent, is represented by the 
nitrogen free extract, of which 4.36 per cent consists of pentosans. The 
percentage of nitrogen in the average standing crop of nannoplankton 
of Lake Monona is considerably larger than that in the nannoplankton 
of Lake Mendota, but the percentages of ether extract, pentosans, and 
crude fiber are larger in the material from the latter lake. 

For the area bounded by the 20 meter contour line in Lake Monona, 
the average quantity of organic matter in the nannoplankton amounts 
to 494 kilograms per hectare (441 pounds per acre), with a variation 
ranging from 140 kilograms to 1,198 kilograms per hectare (125 pounds 
to 1,069 pounds per acre). For the entire body of water the mean quan- 
tity of organic matter is 198 kilograms per hectare (177 pounds per 
acre), with a minimum of 56 kilograms and a maximum of 480 kilo- 
erams per hectare (50 pounds to 428 pounds per acre). 

Lake Waubesa. The average amount of dry organic matter in the 
16 samples of nannoplankton from Lake Waubesa is 3,299 milligrams 
per cubic meter of water, of which 7.92 per cent consists of nitrogen 
(equivalent to 49.50 per cent of crude protein), 4.00 per cent of ether 
extract, 5.55 per cent of pentosans, and 4.45 per cent of crude fiber. 
(Table 25.) The crude protein, ether extract, and crude fiber consti- 
tute 57.95 per cent of the organic matter in the nannoplankton mate- 
rial, leaving 42.05 per cent to be classed as nitrogen free extract, of 
which 5.55 per cent consists of pentosans. 

Unlike the material from the other two lakes the mean percentage 
of nitrogen is higher in the nannoplankton than in the net plankton of 
Lake Waubesa. The mean percentage of crude fiber is slightly larger 
in the nannoplankton, but the percentages of ether extract and of pen- 
tosans are larger in the net plankton. In comparison with the other two 
lakes the mean percentage of nitrogen in the nannoplankton material 
from Lake Waubesa is lower than that of Lake Monona and higher 
than that of Lake Mendota; the percentage of ether extract is lowest in 
the material from Waubesa, while the mean percentage of pentosans 
and of crude fiber is highest in the material from this lake. 

Within the area bounded by the 10 meter contour line (p. 139) the 
mean quantity of dry organic matter in the nannoplankton amounts to 
336 kilograms per hectare (300 pounds per acre). The range of varia- 
tion is from a minimum of 81 kilograms to a maximum of 625 kilograms 


148 PLANKTON OF WISCONSIN LAKES 


per hectare (73 pounds to 558 pounds per acre). For the entire lake 
the amount varies from about 39 kilograms to 800 kilograms per hectare 
(35 pounds to 268 pounds per acre), with an average of 161 kilograms 
per hectare of surface, or about 144 pounds per acre. 

The mean quantity of dry organic matter in the nannoplankton is 
largest for the deep water area in Lake Monona, namely 494 kilograms 
per hectare (441 pounds per acre) ; Lake Mendota is second with 359 
kilograms per hectare (820 pounds per acre), while Lake Waubesa 
comes last with 336 kilograms per hectare (300 pounds per acre). Com- 
puted on the basis of the entire body of water, the mean quantities are 
the same for Mendota and Monona, namely, 198 kilograms per hectare 
(177 pounds per acre), while the mean quantity for Waubesa is 161 
kilograms per hectare (144 pounds per acre). Thus the mean quantity 
per unit of surface in Lake Waubesa is less than 20.0 per cent below 
that of the other two lakes in spite of the fact that it is a much shal- 
lower body of water. 

The nannoplankton samples taken on Lake Mendota during the same 
intervals of time as those obtained from Lake Waubesa give a smaller 
mean quantity of organic matter than the complete series of catches 
from the former lake. That is 38 nannoplankton samples secured on 
Lake Mendota between August 10 and Octcber 31, 1915, and between 
May 22 and November 1, 1916, give an average of 1,548 milligrams of 
dry organic matter per cubic meter of water as compared with 1,630 
milligrams in the complete series of 87 samples. For the deep part of 
Lake Mendota this means a decrease to 340 kilograms per hectare (303 
pounds per acre), and for the entire lake a decrease to 87 kilograms per 
hectare (167 pounds per acre). On the seasonal basis, then, the area 
within the 10 meter contour line in Lake Waubesa maintains almost 
as large a standing crop of nannoplankton as the area bounded by the 
20 meter contour in Lake Mendota. When the entire lake is taken into 
account on this seasonal basis, the average quantity of dry organic mat- 
ter in the standing crop of nannoplankton in Lake Mendota is consider- 
ably smaller during the two periods of time indicated above than in 
the complete series, but this amount is still about 16.0 per cent larger 
than the average for Lake Waubesa. 


ToTAL PLANKTON 


Lake Mendota. The average yield of dry organic matter in the 
total plankton (net plankton plus nannoplankton) in the series of sam- 
ples from Lake Mendota is 1,974 milligrams per cubic meter of water. 
(Table 25.) The various chemical analyses show that nitrogen consti- 
tutes an average of 7.11 per cent of this organic matter (equivalent 
to 44.49 per cent of crude protein), the ether extract 7.53 per cent, the 


SUMMARY AND DISCUSSION 149 


pentosans 4.57 per cent, and the crude fiber 5.32 per cent. Using the 
erude protein value of the nitrogen these four substances account for 
61.9 per cent of the organic matter, leaving 38.1 per cent as undeter- 
mined nitrogen free extract. 

For the deep part of Lake Mendota the mean quantity of dry organic 
matter in the total plankton amounts to approximately 435 kilograms 
per hectare (338 pounds per acre) ; the monthly variations in the quan- 
tity of organic matter are shown graphically in figure 86. The amount 
varied from a minimum of 256 kilograms per hectare (228 pounds per 
acre) in February to a maximum of 522 kilograms per hectare (465 
pounds per acre) in December. 

When the volume and the area of the entire lake are taken into 
account the average amount of organic matter in the standing crop of 
total plankton is 240 kilograms per hectare (214 pounds per acre). 
Crude protein and ether extract make up a little more than 52.0 per 
cent of this organic material. 

Lake Monona. The mean quantity of organic matter in the total 
plankton of Lake Monona is 3,163 milligrams per cubic meter of 
water, of which 9.21 per cent consists of nitrogen (57.56 per cent of 
crude protein), 5.86 per cent of ether extract, 4.74 per cent of pento- 
sans, and 4.35 per cent of crude fiber. The crude protein and the three 
non-nitrogenous substances that were determined make up 72.01 per 
cent of the organic matter, while the remainder consists of undeter- 
mined nitrogen free extract. 

In the deep water area of the lake the average amount of dry 
organic matter in the standing crop of total plankton is a little more 
than 665 kilograms per hectare (594 pounds per acre), while the yield 
of the whole lake is about 267 kilograms per hectare of surface (288 
pounds per acre). 

Lake Waubesa. The 16 samples of total plankton from Lake Wau- 
besa gave an average of 4,398 milligrams of dry organic matter per 
cubic meter of water. The mean percentage of nitrogen in this ma- 
terial is 7.78 per cent, of ether extract 4.61 per cent, of pentosans 
5.76 per cent, and of crude fiber 4.41 per cent. (Table 25.) By con- 
verting the nitrogen into crude protein these four substances account 
for 63.4 per cent of the organic matter, leaving 36.6 per cent as un- 
determined nitrogen free extract. 

The average quantity of dry organic matter in the standing crop of 
total plankton is 503 kilograms per hectare (448 pounds per acre). for 
the area bounded by the 10 meter contour line, while the yield of the 
entire lake is 241 kilograms per hectare of surface (215 pounds per 
acre). 

In what has been designated as the deep water portions of the three 
lakes the total plankton of Monona gives the largest average amount of 


150 PLANKTON OF WISCONSIN LAKES 


dry organic matter per unit of area, namely, 665 kilograms per hectare 
(594 pounds per acre), while Waubesa is second in rank with 503 kilo- 
grams per hectare (448 pounds per acre) and Mendota is third with 
435 kilograms per hectare (888 pounds per acre). When the results 
are computed on the basis of the volume and area of the entire lake, 
Monona is again first in rank with 267 kilograms per hectare (238 
pounds per acre), Waubesa is second with 241 kilograms per hectare 
(215 pounds per acre), and Mendota is third with 240 kilograms per 
hectare (214 pounds per acre). 

The mean percentage of nitrogen is largest in the total plankton of 
Lake Monona and smallest in that of Lake Mendota. The percentage of 
ether extract, on the other hand, is largest in the material from Lake 
Mendota and smallest in that obtained from Lake Waubesa; the latter 
lake has the highest mean percentage of pentosans and Lake Mendota 
the largest mean percentage of crude fiber. 


Discussion OF RESULTS 


For the sake of emphasis it may be worth while to repeat the state- 
ment that the data pertaining to the quantity of material per unit of 
volume or of area refer to the dry weight of the organic matter and that 
the living organic matter would weigh about ten times as much. The 
total weight of the living material would also include the inorganic 
constituents of the ash. Just how much the ash of the nannoplankton 
organisms would add to the weight of the living organic material in 
these organisms can not be determined from the present data because 
the centrifuge catches contained a certain amount of inorganic material 
derived from the silt in addition to the ash of the organisms. Since the 
quantity of inorganic material yielded by the nannoplankton organisms 
is not known, the amount of ash in the total plankton can not be in- 
dicated. 

A number of determinations show that the ash content of the plank- 
ton algae ranges from about 3.0 per cent to almost 10.0 per cent of the 
dry weight in the green and blue-green forms, and from 40.0 per cent 
to more than 50.0 per cent in the diatoms. In Euglena and Volvox 
the ash constitutes from 4.0 per cent to 9.0 per cent of the dry weight. 
It seems probable, therefore, that the ash of most of the nannoplankton 
organisms does not constitute more than 10.0 per cent of the dry 
weight, but the diatoms yield a much larger percentage. When diatoms 
are abundant in the nannoplankton in the spring and in the autumn, 
the percentage of ash will be relatively high but when the diatoms are 
at a minimum in summer and in winter it will be comparatively low. 

The mean quantity of ash in the 184 samples of net plankton from 
Lake Mendota amounts to 23.5 per cent of the dry weight of the ma- 


SUMMARY AND DISCUSSION 151 


terial. This gives an average yield of 102 milligrams of ash per cubic 
meter of water for the entire series of samples. The 47 net samples 
from Lake Monona contain an average of 14.6 per cent of ash, or 145 
milligrams per cubic meter of water. In the 18 net samples from Lake 
Waubesa the ash content averages 16.4 per cent of the dry weight of 
the material, which gives a yield of 331 milligrams per cubie meter of 
water. 

The quantitative data presented in the various tables do not represent 
the total amount of plankton produced during any given period of time, 
but they show simply the standing crop of this material that was pres- 
ent at the time the observations were made. Neither do the mean quan- 
tities shown in table 25 refer to the problem of production, but they 
indicate the average amount of organic matter in the standing crop of 
plankton when the whole series of samples from each lake is considered 
asa unit. The seasonal or annual production of this material involves 
the question of the rate of the turnover in the plankton crop and this 
is a very complex problem. 

The plankton of a lake may be regarded as analogous to a pool in 
a stream, with a current of water constantly flowing in on one side and 
a regular outflow on the other. The pool itself represents the standing 
crop of plankton, while the inflowing stream is analogous to the process 
of plankton production and the outflowing one typifies the losses of this 
material from various causes. The stream of water that is continually 
passing through the pool closely resembles the constant stream of 
plankton life which exists in a body of water. Since the standing crop 
of plankton shows marked variations in quantity during the year, it is 
necessary to regard the pool and stream as variable in size, expanding 
to several times their normal size in periods of flood and falling con- 
siderably below normal in periods of drought. In spite of these seasonal 
variations in quantity, however, a surprisingly close correlation in the 
size of the standing crop of plankton is found at corresponding periods 
of the different years. (Figures 34 and 35.) 

In the foregoing illustration it would be a relatively simple problem 
to ascertain, with some degree of accuracy, the amount of water that 
passes through the pool annually, and this may be thought of as corre- 
sponding to the annual production of plankton. It is also a compara- 
tively simple problem to ascertain the annual productivity of a given 
area of land because the crop can be limited to a single kind of grain or 
hay and because there are definite seasons for planting and harvesting 
the crop. But the problem of ascertaining the plankton productivity 
of a body of water is far more complex than the determination of the 
amount of water that passes through the pool in the foregoing illustra- 
tion, or than determining the productivity of a piece of land. 


152 PLANKTON OF WISCONSIN LAKES 


One of the chief complexities involved in the question of plankton 
production is the variety of the organisms constituting the crop; that 
is, the standing crop of plankton is always made up of a number of 
different kinds of organisms. At certain times of the year perhaps 
not more than twelve or fifteen genera of organisms may be repre- 
sented in the total plankton, while at other seasons the number may 
increase to thirty or forty genera, possibly more; the number depends 
partly upon the character of the lake and partly upon the season of the 
year. The various forms differ widely in character, also, ranging from 
one-celled plants and animals to organisms as complex in structure as 
the erustacea and the insect larvae. 

There is no period of time during the year when one crop of plankton 
ceases and another begins, so that there is no definite starting point 
for the estimation of the annual crop of plankton, such as one finds 
for a land crop, for example. Neither is there any exact date of ma- 
turity, or harvest season, for the plankton crop as there is for the land 
erop. The crop of plankton, therefore, represents a continuous stream 
of life which flourishes at all seasons of the year and which passes on 
from year to year as long as favorable conditions obtain in a body of 
water. 

Some of the plankton forms are present in varying degrees of abun- 
dance at all seasons of the year ; others make their appearance from time 
to time when conditions are favorable for them, rise to a maximum, and 
then decline in numbers. The decline of one form may be accompanied 
by the rise of another, so that the developmental cycles of the two over- 
lap in time, or two or more forms may rise to their maxima simultane- 
ously ; therefore, this overlapping may produce a great variety of com- 
plexities. These complications usually result, however, in a series of 
waves or pulses of plankton in the course of the year, with the largest 
crops generally coming in the spring and in the autumn. 

In comparison with land productivity another marked difference is 
shown by the lake in that plankton production takes place at all depths 
in a body of water ; the plankton soil of a lake, therefore, is coextensive 
with the depth of the water. 

The various plankton organisms possess very different rates of re- 
production. Under favorable conditions the aquatic bacteria may pass 
through several generations in the course of a single day, while the 
algae and protozoa may divide only once or perhaps twice in this in- 
terval of time and the plankton ecrustacea may require two weeks or 
longer to pass from one generation to the next. Temperature is a very 
important factor in determining the rate of reproduction in the various 
forms; that is, the reproductive process is most vigorous at the higher 
temperatures which prevail during the summer months and it is least 


SUMMARY AND DISCUSSION 153 


vigorous during the winter period of low temperatures. A decrease of 
a few degrees in temperature is sufficient to cause a marked decline in 
the rate of reproduction ; thus even in summer this process will proceed 
more vigorously in the warm water of the epilimnion than in the cooler 
water of the hypolimnion. 

Another factor which helps to complicate the problem of determining 
the plankton productivity of a body of water is the great variation in 
the length of life of the different forms. In the unicellular plankton 
organisms, which multiply chiefly by fission, the lifetime of an indi- 
vidual covers the period from one e¢ell division to the next; the lifetime 
of a bacterium, therefore, may be less than an hour under favorable 
temperature and food conditions, or it may be greatly prolonged, per- 
haps to several days, by a low temperature of the water. Among the 
algae and protozoa the span of hfe may be less than a day at summer 
temperatures or it may be prolonged to several days at lower tem- 
peratures. 

In the higher plankton forms reproduction takes place by means of 
germ cells and the lifetime of the individual is much longer. Some of 
the ecrustacea, for example, may live for several weeks, or even for 
several months when the temperature of the water is low. The lifetime 
of the various planktonts, therefore, shows a great variation, ranging 
from a minimum of less than an hour in some forms to a maximum of 
several weeks or even months in others. Thus, the determination of the 
rate of reproduction and of the length of life of individuals belonging 
to the various forms in their natural environment will be a very im- 
portant advance toward the solution of the problem in plankton pro- 
ductivity. In fact, very little progress can be made in answering this 
question until such data are obtained. 

There is also a great diversity in the size of different planktonts. 
The coceus forms of bacteria, for example, are only 0.22» to 0.75u in 
diameter, while Leptodora among the plankton crustacea may reach a 
length of 18 millimeters. 

The question of plankton productivity is complicated still further by 
the fact that the losses as well as the gains of material must be taken 
into account. These losses are sustained in two ways, namely, (1) 
through the consumption of some of the planktonts as food by various 
organisms, (2) through the death of some of the material. The losses, 
like the gains, continue throughout the year so that production and de- 
struction are simultaneous processes and the quantity of plankton that 
is present at any given time during the year is the resultant or the 
algebraic sum of these two processes; this resultant constitutes what 
has been termed the standing crop of plankton. Whenever production 
takes place at a faster rate than destruction, there is an increase in the 


154 PLANKTON OF WISCONSIN LAKES 


standing crop of plankton and the degree of increase depends upon the 
excess of the former over the latter. On the other hand, when the rate 
of destruction exceeds that of production there is a decrease in the 
standing crop of planktcn which corresponds to the excess of the former 
over the latter ; when these two processes just about balance each other 
the quantity remains fairly uniform. The variations in the quantity of 
organic matter in the standing crop of plankton in Lake Mendota are 
well illustrated in figures 34 and 35. 

The present data do not enable one to make a definite assessment of 
the value of the plankton crop in the biologic economy of the lake, 
nevertheless it is well worth while to consider certain phases of this 
question. The larger zooplankton forms, such as the crustacea and the 
rotifers, and even the minute forms such as algae and protozoa, are 
eaten more or less extensively by fishes. The plankton crustacea are 
of special economic importance in this respect since most freshwater 
fishes at some period in their lives feed chiefly or exclusively upon these 
small organisms. Some fishes, in fact, are plankton feeders during the 
whole period of their existence. Among the crustacea the Cladocera are 
more important than the Copepoda because they are used for food more 
extensively by fishes; hundreds or even thousands of Daphnias may be 
found in a single fish stomach, while the smaller Cladocera may be 
eaten in much larger numbers. Some of the insect larvae prey upon the 
Cladocera and they, in turn, are fed upon by the fishes: Also midge 
larvae feed upon plankton algae and they, too, constitute an item in the 
menu of fishes. 

The bivalve mollusks depend chiefly upon the bacteria, algae, and 
protozoa of the plankton for their food. Part of the plankton sinks to 
the bottom of the lake and this constitutes a source of food for the insect 
larvae, mollusks and worms which dwell upon the bottom; this material 
is especially important for the bottom dwellers which are found in the 
deeper portions of a lake. 

In the assemblage of plankton organisms themselves those forms 
which do not bear chlorophyl are dependent, either directly or indi- 
rectly, upon those members which do possess this substance for their 
food. The crustacea are rather voracious feeders and their food con- 
sists chiefly of algae and protozoa; when the former are abundant, 
therefore, they consume large quantities of the latter organisms and at 
such times they are very important agents in reducing the stock of the 
organisms on which they feed. The rotifers also feed upon the algae 
and protozoa, while the protozoa that do not possess chlorophyl, in 
turn, feed upon the algae and the bacteria. 

Just how much food a rotifer or a crustacean consumes each day is 
not known, but the following figures show how much water would have 


SUMMARY AND DISCUSSION 155 


to be depleted of its population in order to furnish some of these forms 
with their own weight of organic matter for food. The average dry 
weight of some of the constituents of the plankton of Lake Mendota has 
been determined and the results are as follows: (1) A large Asplanchna 
weighs 0.000834 milligram, (2) a mature Cyclops 0.0041 milligram, (3) 
a Diaptomus 0.00858 milligram, (4) an adult Daphnia longispina hy- 
alina 0.02172 milligram. Taking the average quantity of organic mat- 
ter in the nannoplankton of Lake Mendota as a basis for the calculation, 
namely, 1,630 milligrams per cubic meter of water (table 25), each of 
these animals would have to remove all of the nannoplankton from the 
following quantity of water in order to obtain its own weight of dry 
organic matter for food; (1) An Asplanchna 0.5 cubic centimeter, (2) 
a Cyclops 2.5 cubic centimeters, (3) a Diaptomus 5.2 cubic centimeters, 
and (4) an adult Daphnia hyalina 13.3 cubic centimeters. These quan- 
tities of water seem very small, but when compared with the size of the 
organisms concerned they are very large. Disregarding temperature 
and assuming that one cubic centimeter of water weighs one gram, the 
above organisms would have to filter about 600,000 times their own 
dry weight of water in order to secure their own weight of dry organic 
matter in the form of nannoplankton. These animals may also feed 
upon some of the organisms in the net plankton and thereby reduce 
the above quantities of water proportionately. 

Computations based on the numerical data indicate that the crus- 
tacea and the rotifers contribute from 25.0 per cent to 75.0 per cent of 
the organic matter in the net plankton; the maximum percentage is 
found in late winter and in early spring when the algae and protozoa 
reach their lowest points in the net material; the minimum percentage 
is found in the early summer and in the autumn when the protista 
flourish most abundantly. Since the maximum percentage of crustacea 
and rotifers is correlated in time with one of the minimum periods of 
the net plankton, it seems probable that these two groups or organisms 
furnish something like 30.0 per cent to 40.0 per cent, or about one-third, 
of the mean quantity of organic matter in the net plankton; that is, an 
average of about 115 milligrams out of 3438 milligrams per cubic meter 
of water (table 25). If the protista of the net plankton are included 
in the computation, therefore, the quantity of water that a rotifer or a 
erustacean would have to strain to obtain its own weight of organic 
matter would be reduced about 12.0 per cent. 

These quantities of water are based on the mean quantity of organic 
matter in the nannoplankton and in the protista of the net plankton. 
Whenever the quantity of organic matter in these two groups of organ- 
isms is above the mean, the quantities of water would be smaller than 
the amounts indicated and whenever it is below the mean, these amounts 
of water would be larger than indicated above. 


156 PLANKTON OF WISCONSIN LAKES 


Owing to the absence of free oxygen in the hypolimnion of Lake Men- 
dota in July and August, the rotifers and crustacea are limited to the 
epilimnion and the mesolimnion during this time and the food supply 
in these two strata is about 12.5 per cent larger than the mean on which 
these computations are based. 

The data presented in this report do not indicate the quantity of 
plankton material produced annually, but by far the greater part of the 
standing crop of plankton is contributed by organisms that multiply 
rather rapidly under favorable conditions of temperature and food; 
thus the turnover in this stock of material is proportionally rapid. As 
indicated in a previous paragraph only about 115 milligrams of organie 
matter out of a mean of 1,974 milligrams (table 25) per cubic meter of 
water in the total. plankton is derived from the rotifers and crustacea, 
and these two forms have the slowest rate of reproduction among the 
various fresh-water planktonts. Substantially all of the other material 
is derived from organisms that reproduce much more rapidly; that is, 
from unicellular forms or from colonies consisting of groups of eells. 
The single cells of the various forms are capable of dividing once a day 
or oftener when the temperature of the water is 20° C. or higher; the 
bacteria, in fact, may divide oftener than once an hour. At a rate of 
one division per day the possible progeny of a single cell would amount 
to more than one billion in a period of thirty days, while one division 
every three days would result in the production of 1,024 descendants in 
the same length of time. 


The epilimnion of Lake Mendota has a temperature of 20° or higher 
from about the middle of June until the middle of September and dur- 
ing this period the turnover in the stock of plankton will be rapid in 
this stratum. It will not be so rapid in the mesolimnion and in the 
hypolimnion because the temperature of the water is lower in these 
strata. The cooling of the water in the autumn and early winter will 
tend to decrease the rate of production. The temperature of the water 
rises slowly during the winter, but it does not reach a mean of 4° until 
after the ice disappears, or about the middle of April. This is followed 
by a period of vigorous production which culminates in the vernal 
maximum. 

The foregoing discussion clearly brings out the fact that the problem 
of ascertaining the quantity of plankton that a body of water produces 
annually is a very complex one and its solution will require very exten- 
sive and detailed investigations; the present data show only the stand- 
ing crop of plankton and its quantitative variations. A more complete 
knowledge of (1) the rate of reproduction of the various planktonts 
under natural conditions of light, temperature, and food, (2) the 
length of their life in the natural environment, and (3) the average 


SUMMARY AND DISCUSSION 157 


weight of the different kinds of organisms per individual or colony, is 
of prime importance in the solution of this complex problem. Data 
regarding the first two questions will enable one to estimate, with some 
degree of accuracy, the annual turnover in the stock of plankton ma- 
terial, and therefore the annual production of this material, while data 
pertaining to the third question will make it possible to evaluate the 
relative importance of the different forms in the plankton complex of 
the lake. 

While these data do not indicate the rate of turnover in the stock of 
plankton, yet some computations based wholly on estimates may be 
worthy of consideration at this point. The mean quantity of dry or- 
ganic matter in the standing crop of total plankton of Lake Mendota 
amounted to approximately 945 metric tons during the period covered 
by these observations, or an average of 240 kilograms per hectare of 
surface (214 pounds per acre), when the entire lake is taken into ac- 
count. A turnover in this mean quantity of plankton organic matter 
once a month would give an annual production of 2,880 kilograms per 
hectare, or 2,568 pounds per acre; a turnover twice a month would 
double this amount. 

Since the planktonts which contribute by far the greater part of the 
organic matter multiply rapidly under favorable conditions, once a day 
or oftener, the turnover in this stock of plankton material may take 
place more frequently, perhaps as often as once a week, on an average. 
With a turnover of fifty times per year, the annual production would 
amount to 12,000 kilograms of dry organic matter per hectare of sur- 
face, or 10,700 pounds per acre. Which of the above amounts ap- 
proaches the annual production most closely can not be determined with 
any degree of accuracy until more data are available relating to the 
rate of reproduction of the various plankton organisms in their natu- 
ral environment. It seems most probable, however, that the time inter- 
val for the turnover in the mean quantity of plankton will be found 
to fall somewhere between one and two weeks during the greater part 
of the year. 


158 PLANKTON OF WISCONSIN LAKES 


CHAPTER VIII 


CHEMICAL ANALYSES OF VARIOUS ORGANISMS 


The purpose of this investigation was not only to obtain a general 
idea of the quantity and of the chemical composition of the plankton 
as a whole, but also to ascertain the chemical composition of the differ- 
ent constituents of the plankton whenever they could be secured in a 
pure or substantially pure state. Several plankton forms were obtained 
in sufficient abundance and purity to warrant analyses of them and the 
results of the analyses of these forms are shown in table 49 (p. 215). 
Since the general problem of the total productivity of a lake was kept 
in mind during this study, samples of the larger forms were secured 
from time to time and the results of the analyses of these samples are 
also given in table 49. 

The analyses of fifty-two samples are shown in this table, of which 
thirty-four are results obtained on plankton organisms and eighteen 
on non-plankton forms. Nineteen of the fifty-two samples represent 
plant material and thirty-three animal material. 

Among the plants the Myxophyceae or blue-green algae are repre- 
sented by Microcystis, Anabaena, Coelosphaerium, Aphanizomenon, and 
Lyngbya; the Chlorophyceae or green algae include Ankistrodesmus, 
Volvox, Spirogyra, and Cladophora, while the diatoms or Bacillariaceae 
are represented by a sample containing both Fragilaria and Tabellaria. 
All of these algae except Spirogyra and Cladophora are regular plank- 
ton forms; very rarely Spirogyra and Cladophora may be found in the 
plankton but they are only accidental constituents. Samples of three 
of the large aquatic plants which grow in the shallow water and which 
represent the phanerogams, namely, Potamogeton, Vallisneria, and My: 
riophyllum, have been analyzed. 

Of the thirty-three samples of animal material, nineteen represent 
plankton crustacea belonging to six genera; the Copepoda include three 
genera, namely, Diaptomus, Cylops, and Limnocalanus, and the Clado- 
cera are represented by three species of Daphnia, by Holopedium, and 
Leptodora. Besides the plankton forms of crustacea two of the larger 
forms of this group are represented, namely, a crayfish belonging to the 
genus Cambarus and the amphipod Hyalella. The Oligochaeta are 
worms belonging to the genera Limnodrilus and Tubifex, while the 
Hirudinea are represented by a sample containing two or three species 


Pes 


ANALYSIS OF ORGANISMS 159 


of leeches. The last ten items in the table represent aquatic insects, all 
except the last two being larvae; the sample of Gyrinids consisted of 
adults, while that of the Hemiptera contained both young and adults. 
The Corethra larvae are limnetie in habit and they are the only insect 
constituents of the plankton. 

_ In part I of table 49 the results of the analyses are stated in percent- 
ages of the dry weight of the sample when the ash is included; in part 
II the percentages are given on an ash free basis. The percentage of 
ash varied so much in the different samples that it seemed worth while 
to state the results on an ash free basis also, so that the different com- 
ponents of the organic matter in the various samples might be more 
readily compared. 

The chemical study of these samples included the usual determina- 
tions that are made in a food analysis, namely, a quantitative deter- 
mination of the nitrogen, the ether extract, the crude fiber, and the 
ash. The nitrogen multiplied by the factor 6.25 gives the erude pro- 
tein. These items do not account for all of the organic matter in a 
sample and the remainder is usually designated as the nitrogen free 
extract; that is, the sum of the percentages of crude protein, ether ex- 
tract, crude fiber, and ash deducted from 100 gives the percentage of 
nitrogen free extract. This extract consists principally, if not entirely, 
of carbohydrate material, but in some instances small amounts of other 
substances may be present; it may contain, for example, a certain 
amount of fats which are not completely removed by ether from some 
of the compounds in which they occur. The pentosans are the only 
carbohydrates which have been studied in these samples and they 
were determined quantitatively in thirty-one samples as indicated in 
table 49. The other carbohydrate constituents of these samples remain 
as a problem for future chemical investigation. 


PLANTS 


Myxophyceae. Ten samples containing material derived from four 
genera of blue-green algae, namely, Microcystis, Anabaena, Aphanizo- 
menon, and Lyngbya, are shown in table 49. With the exception of one 
sample they yielded a high percentage of nitrogen; a sample of Micro- 
eystis gave only 6.32 per cent of nitrogen when the ash is included in 
the dry weight of the sample, but in the other nine the nitrogen ranged 
from 8.21 per cent to 9.94 per cent. This high percentage of nitrogen 
represents a correspondingly large amount of crude protein; the latter, 
in fact, falls below 50.0 per cent of the dry weight in only one sample, 
while it exceeds 60.0 per cent in one instance. This large percentage 
of crude protein is not surprising in view of the fact that these forms 
contain only a relatively small amount of material besides the proto- 


160 PLANKTON OF WISCONSIN LAKES 


plasmic content of the cell; the cell walls are fairly delicate and the 
gelatinous covering yields but a small amount of dry matter. On an 
ash free basis, the crude protein makes up from 54.12 per cent to 66.45 
per cent of the dry weight of the organic matter, if the one sample with 
a minimum of 41.60 per cent is omitted. 

The ether extract constituted a relatively small percentage of the 
dry material in these ten samples of blue-green algae, ranging from a 
minimum of 1.11 per cent to a maximum of 5.02 per cent. On an ash 
free basis these percentages are somewhat higher. The green color of 
the ether extract indicated that it contained a certain amount of 
chlorophyl. 

The percentage of crude fiber was small, with the exception of one 
sample of Lyngbya in which it amounted to 7.39 per cent of the dry 
weight of the sample; it was less than one per cent in four of the nine 
samples on which determinations were made. 

Including the ash the nitrogen free extract ranged from a minimum 
of 25.72 per cent of the dry weight to a maximum of 52.09 per cent; it 
exceeded 40.0 per cent in only one of the nine samples of blue-green 
algae shown in table 49. No crude fiber determination was made on one 
of the samples of these algae so that the nitrogen free extract can not 
be indicated for this sample. Pentosan determinations were made on 
eight of these samples and they show percentages ranging from 2.04 
per cent to 7.80 per cent of the dry weight of the sample. 

On an ash free basis the nitrogen free extract constituted from 27.62 
per cent to 54.82 per cent of the organic matter in the samples of blue- 
green algae, and the pentosans varied from 2.20 per cent to 8.46 per 
cent. The former is substantially a twofold variation and the latter 
almost fourfold. 

The ash varied from a minimum of 4.31 per cent to a maximum of 
7.81 per cent of the dry weight, so that it may be regarded as relatively 
small in amount. Quantitative determinations of the silica were made 
on nine samples; in seven instances the quantity of this substance was 
less than one per cent of the dry weight of the sample, while two were 
higher, one yielding a maximum of 1.62 per cent. 

The various samples show not only that the different kinds of blue- 
ereen algae differ somewhat in their chemical composition, but also 
that the same form is subject to more or less marked variations in this 
respect. The specimens of Microcystis, for example, show considerable 
differences in their nitrogen content and smaller but distinct differences 
in the percentages of ether extract, crude fiber and ash. The two sam- 
ples of Lyngbya from Lake Monona also differ appreciably in their 
organic constituents although the second one was collected only four 
days later than the first one; the ash, however, is about the same in 


ANALYSIS OF ORGANISMS 161 


both samples. These variations in the chemical composition of the same 
form are due, doubtless, to the fact that the samples represent different 
stages in the life cycle of the alga. 

Hyams and Richar’s' found, for example, that young, green fila- 
ments of the blue-green alga Oscillatoria prolifica contained 9.0 per cent 
of nitrogen, while mature, brown colored specimens yielded only 7.9 
per cent; the ash in the former amounted to only 4.5 per cent, while 
in the latter it ranged from 6.1 per cent to 6.7 per cent. (See table 51.) 
Most of the difference in the ash was due to a greater abundance of 
silica in the mature material; the silica in the young amounted to 1.46 
per cent and in the mature to 2.90 per cent. This evidence seems to 
indicate that an alga collected during a period of rapid growth and 
vigorous reproduction differs somewhat in chemical composition from a 
sample of the same alga which is collected after the form has become 
fully mature. 

In other analyses Hyams and Richards obtained 11.0 per cent of 
nitrogen in one sample of Oscillatoria prolifica and 10.3 per cent in 
another; both of these percentages are higher than the maximum of 
nitrogen in the samples of blue-green algae shown in table 49. 

Turner? also analyzed samples of Oscillatoria prolifica and found 
that the air dried material contained 9.7 per cent of moisture, 7.4 per 
cent of nitrogen, 2.2 per cent of ether extract, and 6.4 per cent of ash. 
When recalculated on an oven dry basis, the nitrogen equals 8.2 per 
cent, the ether extract 2.4 per cent, and the ash 7.1 per cent. (Table 
51.) His results for nitrogen, therefore, are substantially the same as 
those found in four samples of blue-green algae in this series of analy- 
ses, and his percentages of ether extract and ash are also similar to 
some of the results obtained on this Wisconsin material. 

Whipple and Jackson® found 9.6 per cent of nitrogen in Anabaena 
and 8.3 per cent in Microcystis (Clathrocystis). (Table 51.) The for- 
mer is higher than the percentage of nitrogen in the Anabaena material 
obtained from Lake Mendota on September 19, 1914, while the latter is 
substantially the same as the average of the four samples of Microcystis 
shown in table 49. 

Chlorophyceae. The sample of Ankistrodesmus was grown in a cul- 
ture containing Knop’s solution; this nutrient solution yielded a pre- 
cipitate which was removed from the water by the centrifuge along 
with the alga. This made the ash content of the sample too high, 
namely 41.61 per cent, so that the results for Ankistrodesmus are given 
only on an ash free basis. The percentage of nitrogen in this alga is a 


1Technology Quarterly, Vol. 15, 1902, pp. 308-315. 
* Jour. Amer. Chem. Soe., Vol. 38, 1916, p. 1402. 
* Jour. N. E. Waterworks Assoc., Vol. 14, 1899, pp. 1-25. 


162 PLANKTON OF WISCONSIN LAKES 


little below the mean of the blue-green algae, but the percentages of 
ether extract and of crude fiber are appreciably higher than in the 
latter. Only one sample of the blue-greens has a smaller percentage of 
nitrogen free extract than Ankistrodesmus and only one has a smaller 
percentage of pentosans. 

On July 6, 1916, an abundant growth of the flagellate Volvox was 
found in Lake Monona and enough of this material was secured for a 
chemical analysis. This flagellate yielded a smaller amount of nitrogen 
than the blue-green algae; still the percentage of nitrogen is large 
enough to show that almost half of the dry material consists of crude 
protein. The percentages of ether extract and of crude fiber are some- 
what larger in this Volvox material than the average for the blue-green 
algae, with the exception of the crude fiber in Lyngbya. The pentosans 
are smaller in Volvox than in the blue-greens, while the nitrogen free 
extract is about the same as the average for the latter group. 

All of the percentages for Volvox are somewhat smaller than those 
for Ankistrodesmus except that of nitrogen free extract. (Table 49, 
p. 215.) 

Brandt‘ records the results of a chemical analysis of some marine 
flagellates; his sample consisted of peridinians, chiefly the flagellate 
Ceratium. The erude protein in this material amounted to 12.68 per 
cent of the dry weight of the sample, the fat or ether extract 1.3 per 
cent, the crude fiber 41.5 per cent, the nitrogen free extract 39.0 per 
cent, and the ash 5.2 per cent. (Table 51.) A comparison shows that 
the percentage of crude protein in the sample of Volvox from Lake 
Monona is more than three times as large as that in the above sample of 
marine peridinians, while the percentage of ether extract is more than 
four times as large in the former as in the latter. The crude fiber in | 
the peridinians is more than six times as large as that in Volvox and the 
nitrogen free extract is somewhat larger in the former; the percentage 
of ash is a little larger in Volvox than in the peridinians. 

The two samples of green algae which do not belong to the regular 
plankton forms, namely Spirogyra and Cladophora, yielded different 
results from those obtained on the blue-green algae and also from 
those of the two green algae noted above. The percentage of nitrogen 
is less than half as large as the mean of the blue-greens and also less 
than half as large as the percentages in Ankistrodesmus and Volvox; 
this means correspondingly low percentages of crude protein in the 
two filamentous algae. Whipple and Jackson> found 4.5 per cent of 
nitrogen in the sample of Spirogyra which they analyzed. (Table 51.) 


*'Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, p. 88. 
* Jour. N. E. Waterworks Assoc., Vol. 14, 1899, pp. 1-25. 


ANALYSIS OF ORGANISMS 163 


The ether extract in Spirogyra and Cladophora is substantially the 
same as the mean of the blue-green algae, but it is smaller than in 
Ankistrodesmus and in Volvox. The percentage of pentosans is larger 
in Spirogyra and Cladophora than in any of the other algae included in 
this series of samples; on an ash free basis the percentages are sub- 
stantially the same in these two filamentous algae, but they are more 
than five times as large as the minimum in the blue-green algae and 
more than eleven times as large as the percentage of the pentosans in 
Volvox. Cladophora yielded a large percentage of crude fiber, this item 
amounting to more than a quarter of the dry organic matter, but only 
a relatively small percentage was found in the sample of Spirogyra. In 
this respect the latter compares very favorably with about half of the 
samples of blue-green algae, but it is much smaller than the percentages 
in Ankistrodesmus and Volvox. The percentage of crude fiber in this 
sample of Cladophora is almost three times as large as the maximum 
of the other algae given in this table; in fact, it is the maximum ob- 
tained in this series of samples. 

The sample of Cladophora yielded a large percentage of ash and sil- 
ica; the material was obtained from rocks along the edge of the lake 
and was carefully washed when it was collected, but the large percent- 
ages of ash and silica seem to indicate that the sample contained some 
sand. The percentage of ash in Spirogyra is only about one-third as 
large as that in the Cladophora material, still it is appreciably larger 
than the percentages of ash in the samples of blue-green algae. 

Part II of table 49 shows that nearly three-quarters of the organic 
matter of Spirogyra consisted of nitrogen free extract, the percentage 
being larger than in any other form given in the table. With the excep- 
tion of one sample of Microcystis, the nitrogen free extract in the 
sample of Cladophora is larger than in all of the samples of -blue-green 
aleae listed in table 49. 

Bacillariaceae. The sample of diatoms obtained from Lake Mendota 
contained several forms, but the most abundant one was Fragilaria, 
with Tabellaria ranking second in importance. This material yielded a 
large percentage of ash which consisted chiefly of silica derived from 
the silicious shells of these organisms. As a result of the presence of 
such a large amount of inorganic material, the percentages of the 
organic constituents are relatively small when the ash is included. The 
percentage of nitrogen in the diatoms, for example, is less than half as 
large as that in the various samples of blue-green algae, with the excep- 
tion of one sample of Microcystis. On an ash free basis, however, the 
comparison is more favorable to the diatoms, but even then the per- 
centage of nitrogen falls below the minimum of the blue-greens; it is 
only two-thirds as large in the diatom sample as in the samples of An- 
kistrodesmus and Volvox. 


164 PLANKTON OF WISCONSIN LAKES 


The diatom sample contained a larger percentage of ether extract 
than any other sample of plant material. The percentage of this ex- 
tract in the diatoms is more than two and a half times as large as the 
maximum in the blue-green algae with the ash included, while it is 
more than four times as large as the latter on an ash free basis. The 
percentage of ether extract in the organic matter of the diatoms, in fact, 
is exceeded by that in only five samples of animal material as shown in 
part II of table 49. The large percentage of this extract in the diatoms 
is due to the fact that the reserve material in these forms consists of 
drops of oil instead of carbohydrate compounds. 

The percentages of crude fiber and of pentosans are relatively small 
in this sample of diatoms, and the nitrogen free extract, on an ash free 
basis, 1s substantially the same as the mean for the nine samples of blue- 
green algae. 

Whipple and Jackson® found 2.2 per cent of nitrogen in the diatom 
Asterionella, while the ash constituted 57.52 per cent of the dry ma- 
terial. The percentage of nitrogen in Asterionella is smaller than in 
the diatom material from Lake Mendota, while the ash is much larger; 
on an ash free basis the percentage of nitrogen in the former is 5.18 
per cent as against 6.05 in the latter. (See table 51.) The ash of the 
sample containing Asterionella contained 49.48 per cent of silica, leav- 
ing only 8.04 per cent for the other constituents; deducting the silica 
from the ash of the diatoms secured in Lake Mendota leaves 8.72 per 
cent. 

For marine diatoms Brandt’ gives the following results on an ash 
free basis: crude protein 28.7 per cent, fat 9.0 per cent, and carbohy- 
drate 63.2 per cent. (See table 51.) He regards the crude protein 
in his material as rather high, but it is more than 10.0 per cent below 
that found in the material from Lake Mendota; the percentage of fat 
in the marine material is only a little more than one-third as large as 
the ether extract in this sample of freshwater diatoms. In the marine 
diatoms the erude protein and the fat constitute only 36.7 per cent of 
the dry organic matter, while in the freshwater forms from Lake Men- 
dota these two items make up 60.29 per cent of the organic matter. 

On an ash free basis the nitrogen found in Asterionella by Whipple 
and Jackson is equivalent to 32.4 per cent of crude protein as compared 
with 28.7 per cent in the marine diatoms indicated by Brandt. 

Phanerogams. Two samples of Potamogeton were analyzed; one 
contained mature plants whose leaves were dead and the other consisted 
of plants whose leaves were still bright green. The stems as well as the 
leaves of the plants were used for both samples, but not the roots. The 


® Jour. N. E. Waterworks Assoe., Vol. 14, 1899, pp. 1-25. 
7 Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, p. 89. 


ANALYSIS OF ORGANISMS 165 


sample consisting of green plants yielded a larger percentage of nitro- 
gen and of ether extract than the one containing the mature plants, but 
the reverse was true of the pentosans, the crude fiber, and the ash. 
Small differences in the ash are not very significant, however, because 
the leaves are covered with a deposit of lime which comes off very read- 
ily when the plants are dry; it is very difficult to prevent a slight loss 
of this lime while the material is being dried and prepared for the 
analysis. 

The percentage of nitrogen is smaller in these two samples of Potamo- 
geton than in the other two samples of phanerogams, namely Vallisneria 
and Myriophyllum. In Vallisneria the percentage of nitrogen is sub- 
stantially the same as it is in the sample of Cladophora, but it is smaller 
than that in the sample of Spirogyra. On an ash free basis the per- 
centage of nitrogen is smaller in the sample of Vallisneria than in those 
of Cladophora, Spirogyra, or Myriophyllum; of these four samples the 
last one has the largest percentage of nitrogen, but even this maximum 
is much smaller than the percentages shown for the various samples of 
blue-green algae, or than those of Ankistrodesmus and Volvox. 

The two samples of Potamogeton yielded a smaller amount of ether 
extract than those of Vallisneria and Myriophyllum; the latter rank 
with Spirogyra and Cladophora in this respect. The sample of mature 
Potamogeton gave a smaller percentage of ether extract than the one 
containing green plants. 

On an ash free basis the crude fiber constituted from 16.52 per cent 
to 21.16 per cent of the dry organic matter in these four samples of 
phanerogamie plants. The maximum here is exceeded only by the per- 
centage found in Cladophora. The percentages of pentosans are about 
the same in the samples of Potamogeton as in those of Spirogyra and 
Cladophora, but they are smaller in Vallisneria and Myriophyllum. 

The percentage of ash is fairly large in the four samples of phanero- 
gamic plants, being exceeded among the plants only by the sample of 
diatoms and that of Cladophora. | 

The nitrogen free extract is rather large in these four samples of 
the large aquatic plants also, ranging from 51.20 per cent of the dry 
organic matter in Myriophyllum to 65.55 per cent in the mature Pota- 
mogeton. The maximum percentage is exceeded by only one other sam- 
ple of plant material, namely, that of Spirogyra. These percentages of 
nitrogen free extract are comparable to those that have been obtained 
for such land crops as alfalfa and timothy hay. 


ANIMALS 


Samples of the more common forms of the plankton crustacea were 
secured for analysis. The Copepoda are represented by Diaptomus, 


166 PLANKTON OF WISCONSIN LAKES 
Cyclops, and Limnocalanus, while the Cladocera comprise Holopedium, 
Leptodora, and three species of Daphnia. 

The crustacea and the insects possess chitinous coverings which ¢on- 
tain a certain amount of nitrogen. Since chitin is non-protein in char- 
acter it is necessary to make a correction for this part of the total nitro- 
gen in order to ascertain the amount of nitrogen that belongs to the 
crude protein. In making this correction the crude fiber has been 
regarded as chitin and the percentage of nitrogen in the erude fiber 
has been determined in most of these samples. The percentage of nitro- 
gen in the fiber varied from a little less than 6.0 per cent to a little 
more than 7.0 per cent of the dry weight of the crude fiber; thus for 
the samples on which no determinations were made, an average of 6.5 
per cent of the crude fiber has been deducted from the total nitrogen as 
a correction for the nitrogen of the chitin. In the samples on which 
determinations were made the actual amount of this nitrogen has been 
deducted from the total. The percentages of nitrogen shown in table 
49, therefore, represent the amount in the crude protein and they do not 
include the nitrogen in the chitin. 

Copepoda. The five samples of copepods yielded a high percentage 
of nitrogen, thus indicating a correspondingly large proportion of 
erude protein; the sample containing Limnocalanus gave the smallest 
percentage and that containing Diaptomus the largest. On an ash free 
basis only three other samples in this series yielded a larger percentage 
of nitrogen than Diaptomus and these three consisted of animal ma- 
terial; the maximum for the blue-green algae, however, is only four- 
tenths of one per cent below Diaptomus. 3 

All of the copepod samples contained a relatively large amount of 
ether extract or fat; Diaptomus gave the smallest percentage and Lim- 
nocalanus the largest. In the latter the ether extract amounted to 41.6 
per cent of the organic matter; this large yield is due to fairly large 
drops of oil which are present in the thoracic region of this animal. 
The crude protein and the ether extract combined constitute from 73.0 
per cent to almost 85.0 per cent of the dry weight of these samples of 
copepods; this means that they are a source of very nutritious food for 
the organisms which feed on them. 

The Cyclops material yielded the largest percentage of crude fiber 
or chitin and Limnocalanus the smallest. 

The nitrogen free extract in these five samples of copepods varies 
from a minimum of 4.89 per cent of the dry organic matter in Cyclops 
to a maximum of 14.45 per cent in one of the samples containing both 
Cyclops and Diaptomus. In other words the crude protein, the ether 
extract, and the crude fiber constitute from 85.0 per cent to more than 
95.0 per cent of the organic matter in these copepod samples. 


ANALYSIS OF ORGANISMS 167 


The five samples of copepods listed in table 49 contained a relatively 
small percentage of ash and similar results have been obtained in more 
than forty other ash determinations on small samples of Diaptomus, 
Cyclops, Epischura, and Limnocalanus. A sample from Spring Lake, 
Wisconsin, containing Diaptomus yielded 17.07 per cent of ash and 
another from Silver Lake containing Cyclops gave 12.04 per cent of ash. 
These two samples of copepods are the only ones obtained from Wis- 
consin lakes which have contained more than 9.0 per cent of ash and 
the majority of them have yielded less than 6.0 per cent. Specimens of 
Cyclops® from Lake Okoboji, Iowa, gave 10.0 per cent of ash, while 
samples of Cyclops and of Diaptomus from some of the Finger lakes 
and from Lake George, New York, contained from 11.49 per cent to 
15.38 per cent of ash. ; 

Volk® records an analysis of another copepod, namely, Eurytemora. 
He states that 78.48 per cent of the dry weight of the material consisted 
of muscle and other tissue, 6.2 per cent fat, 11.08 per cent chitin, and 
4.24 per cent ash. (See table 51.) Apparently he determined the 
- ether extract, crude fiber, and ash, and then called the remainder 
‘muscle and other tissue.’’ The percentage of ether extract or fat is 
much smaller in Kurytemora than in the five samples of Wisconsin cope- 
pods, but the percentage of crude fiber or chitin is larger in the former ; 
the percentage of ash in Eurytemora is substantially the same as that 
of Limnocalanus. 

Brandt?® gives an analysis of a sample of freshwater copepods; his 
material yielded 57.25 per cent of crude protein, 6.01 per cent of ether 
extract or fat, 4.54 per cent of crude fiber or chitin, and 9.21 per cent 
of ash. (See table 51.) A comparison of his results with those obtained 
for the samples of copepod material collected in Wisconsin lakes, shows 
that two of the latter yielded a smaller percentage of crude protein 
and three a larger percentage; four of the latter contained a larger 
percentage of chitin or crude fiber and all five gave a larger percentage 
of fat or ether extract. The percentage of ether extract shown for 
Limnocalanus is more than six times as large as that in Brandt’s ma- 
terial. The percentage of ash recorded by Brandt is larger than that 
in the five copepod samples shown in table 49. The nitrogen free ex- 
tract in his sample of freshwater copepods amounted to 22.99 per cent 
of the dry weight; this is nearly twice as large as the maximum per- 
centage of nitrogen free extract in the five samples of Wisconsin cope- 
pods and five times as large as the minimum. 


§Birge and Juday, Univ. of Iowa Studies in Nat. Hist., Vol. 9, 1920, pp. 1-56. 
°Verhand. d. Naturwis. Vereins in Hamburg, 3. Folge XV, 1907, p. 45. 
1 Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, p. 71. 


168 PLANKTON OF WISCONSIN LAKES 


Brandt also records an analysis of a sample of marine copepods which 
yielded substantially the same percentage of nitrogen as his sample of 
freshwater copepods, but the ether extract and ash were somewhat 
larger in the marine copepods. 

Cladocera. Hight samples of Daphnia pulex var. pulicaria were 
collected from three different Wisconsin lakes. Those from Lake Mo- 
nona were secured in different years, while different years as well as 
different seasons of the same year are represented in the material from 
Devils Lake. These samples thus afford a good opportunity to study the 
variations in the chemical composition of this form. (Table 49.) 

The nitrogen ranged from a minimum of 5.82 per cent to a maximum 
of 8.63 per cent of the dry weight of the material, the latter being 
almost one and a half times as large as the former. The largest per- 
centage was found in material from Devils Lake and the smallest in a 
sample from Lake Monona; the difference is nearly as great, however, 
if only the samples from the former lake are considered because a maxi- 
mum of 8.27 per cent is recorded for one of the Devils Lake samples. 
The mean percentage of nitrogen in the five samples from Devils Lake 
is 7.13 per cent of the dry material, which is more than one per cent 
below the mean of the two samples of Daphnia pulex from Lake Mo- 
nona, and it is smaller also than the percentage of nitrogen found in the 
sample from Lake Waubesa. 

On an ash free basis the nitrogen varied from a minimum of 7.22 per 
cent to a maximum of 10.98 per cent. With the exception of one sam- 
ple, considerably more than half of the organic matter in these eight 
catches of Daphma pulex consisted of crude protein. 

Similar variations were noted in the ether extract or fat, the range 
being from 2.82 per cent of the dry sample to 27.9 per cent. The sam- 
ple of Daphnia pulex obtained from Lake Waubesa yielded the maxi- 
mum percentage of extract and one of those from Devils Lake the mini- 
mum. On an ash free basis the maximum amount of ether extract con- 
stituted 31.75 per cent of the organic matter. The physical and chem- 
ical constants of the ether extract obtained from the Monona sample 
which was collected on April 4, 1914, were determined by Schuette’’. 
The index of refraction at 25° C. is 1.4810, the iodine number 172.88, 
the saponification number 208.56, the Reichert-Meisel number 0.94, and 
the Polenske number 1.22. The odor from the extract was decidedly 
like that of a fish oil and it was noted also that this extract solidified 
upon exposure to the air. 

The variations in the ether extract bear a fairly close relation to the 
age and the condition of the Daphnias. The developing embryos are 
well supplied with fat and the material will yield a high percentage of 


"Trans. Wis. Acad. Sci., Arts, and Let., Vol. 19, 1918, p. 599. 


ANALYSIS OF ORGANISMS 169 


ether extract if the sample consists largely of adult females with many 
embryos in their brood chambers. On the other hand, if the sample con- 
tains a large proportion of immature individuals, or if it is made up of 
females carrying relatively few embryos, the yield of ether extract will 
be much smaller. 

In these eight samples of Daphnia pulex the crude fiber or chitin 
varied from a minimum of 3.34 per cent to a maximum of 10.89 per cent 
of the dry weight of the material, a little more than a threefold differ- 
ence; on an ash free basis the percentages range from 3.62 per cent to 
13.08 per cent of the organic matter. The samples from Devils Lake 
show a larger percentage of crude fiber than those from Lakes Monona 
and Waubesa, the minimum of the former being larger than the maxi- 
mum of the latter. 

Three determinations showed that only small amounts of pentosans 
were present in the material, the maximum quantity being 1.92 per cent 
of the dry weight of the sample. 

The nitrogen free extract ranged from a minimum of 8.25 per cent 
to a maximum of 25.19 per cent of the dry sample; on an ash free 
basis this extract constituted from 9.4 per cent to 31.34 per cent of the 
organic matter. In other words the crude protein, the ether extract, 
and the crude fiber account for approximately 70.0 per cent to 90.0 per 
cent of the organic matter in these eight samples of Daphnia pulex. 

The percentage of ash also showed somewhat more than a threefold 
variation, ranging from 7.62 per cent of the dry material in one sample 
from Lake Monona to 25.85 per cent in one sample from Devils Lake. 
The minimum percentage of ash in the five samples from Devils Lake is 
16.87 per cent and this is exceeded by one of the samples from Lake 
Monona; the other sample from Lake Monona as well as that from 
Lake Waubesa gave a much smaller percentage of ash. An ash deter- 
mination on a small sample of Daphnia pulex obtained from Devils Lake 
on September 30, 1918, yielded 11.06 per cent; this is appreciably 
smaller than the minimum noted above. 

The percentage of ash appears to bear some relation to the age of 
the specimens in Daphnia pulex. By means of small platinum crucibles 
and a sensitive assayer’s balance the amounts of organic matter and of 
ash may be readily ascertained for small numbers of these organisms, 
from 100 to 500 individuals being used for such determinations. In a 
series of such samples taken from material collected in Lake Monona on 
May 16, 1918, the largest females, noted as adults carrying embryos, 
yielded 18.93 per cent of ash; medium sized individuals, estimated as 
one-fourth to one-third grown, contained 18.89 per cent of inorganic 
material, while the smallest size gave 23.6 per cent of ash. In two sam- 
ples collected at Devils Lake, September 30, 1918, the ash amounted to 


170 PLANKTON OF WISCONSIN LAKES 


11.06 per cent of the dry weight of full grown females with eggs and to 
14.15 per cent in medium sized individuals. Similar differences in ash 
corresponding to differences in age have been noted in specimens of 
Daphnia hyalina. 

The various analyses of blue-green algae and of Daphnia pulex show 
clearly the necessity of analyzing a number of samples of each form if 
one wishes to obtain a general idea of the chemical composition. The 
number of analyses may be reduced, however, by making a composite 
sample: that is, one which contains material that has been collected at 
different periods in the development of the form in question. 

The composite sample containing Daphnia pulex and D. hyalina eol- 
lected in Lake Monona on August 5, 1913, and that of D. hyalina and 
D. retrocurva secured on Lake Mendota on August 28, 1917, yielded 
results similar to some of the samples of pure D. pulex. (Table 49.) It 
seems probable, therefore, that a series of catches of these two forms 
would give substantially the same results as those obtained for the 
various samples of D. pulex. 

Another cladoceran, Holopedium gibberum, also has percentages 
which agree closely with some of those obtained for the Daphnia ma- 
terial, with the exception of the pentosans. The percentage of nitrogen, 
for example, is the same as that in the composite sample containing D. 
pulex and D. hyalina, and it is very close to that of three other samples 
of Daphnia. The ether extract, crude fiber, and ash are also within the 
range of the percentages noted for the Daphnia material; the pentosans 
constitute a much larger percentage of the dry weight of Holopedium 
than of any of the other animal material. 

Quantitative determinations of the pentosans were made on fourteen 
samples of animal material as shown in table 49. With one exception 
the percentages are comparatively small in such samples and this excep- 
tion is Holopedium. The latter sample is the only one of the fourteen 
which yielded more than two per cent of the dry weight in pentosans 
and eight of them contained less than one per cent, the smallest amount 
being found in the leeches. 

The first determination of the pentosans in Holopedium was made on 
a sample collected in Kawaguesaga Lake at Minocqua, Wisconsin, in 
1918; the pentosans in this material amounted to 6.17 per cent of the 
dry weight, which was regarded as surprisingly high. A second sample, 
therefore, was secured from this lake in 1917 and this gave a still larger — 
percentage of pentosans, namely 9.62 per cent. This percentage is not 
only much larger than that in any of the other animal material included 
in this series of samples, but it is exceeded by the percentages in only 
two samples of plant material. In 1918 a third sample of Holopedium 
was secured from a neighboring lake and it yielded 4.35 per cent of 


ANALYSIS OF ORGANISMS 171 


pentosans. These three analyses, therefore, seem to establish the fact 
that Holopedium regularly possesses a much larger percentage of pen- 
tosans than the other animal forms represented in table 49 ; the percent- 
age shown in the table is the mean of the three determinations. Holo- 
pedium differs from all of the other Cladocera in that its body is en- 
closed in a large transparent, gelatinous case and the large yield of 
pentosans suggests that this gelatinous covering may be the source 
of these carbohydrates. 

The three samples of Leptodora showed a variation of more than 
two per cent in their nitrogen content; on an ash free basis the range 
is from 8.35 per cent to 11.09 per cent of the organic matter. The 
latter is the maximum percentage of nitrogen in the various samples of 
Cladocera and it is also a little larger than the maximum in the Cope- 
poda. The percentages of ether extract, pentosans, nitrogen free ex- 
tract, crude fiber, and ash fall within the range of variations shown by 
the various samples of Daphnia. The mean percentage of nitrogen free 
extract and of ash is smaller in the Leptodora material, however, than 
in the Daphnias. 

The percentages obtained for the organic constituents of the fourteen 
samples of Cladocera are very similar to those noted in the five samples 
of Copepoda as shown in table 49; the most striking difference is the 
larger percentage of ether extract in Limnocalanus. As a whole the 
samples of Cladocera yielded a much larger proportion of ash than 
those of the Copepoda; the maximum percentage of ash in the former is 
more than four times as large as the maximum in the latter group of 
erustacea, while the minimum of the former is nearly one and a half 
times as large as the maximum percentage of ash in the latter. 

Volk’? records an analysis of another cladoceran, namely, Bosmina, in 
which he found that 77.83 per cent of the dry sample consisted of 
‘‘muscle and other tissue,’’ 10.66 per cent of fat or ether extract, 8.21 
per cent of crude fiber or chitin, and 3.3 per cent of ash. (See table 
51.) This result is similar to that shown by Volk for Eurytemora 
which is referred to on a previous page; that is, apparently he de- 
termined the ether extract, the crude fiber, and the ash, and then desig- 
nated the remainder as ‘‘muscle and other tissue.’’ The percentage of 
fat or ether extract in Bosmina is the same as that in Holopedium, while 
the percentage of crude fiber or chitin in the former is substantially the 
same as that noted in three of the Daphnia samples. The percentage of 
ash in Bosmina is much smaller than those noted for the various sam- 
ples of Cladocera in table 49. 

Cambarus. Forty-four specimens of crayfishes belonging to the 
genus Cambarus were collected and analyzed for the purpose of getting 


“ Verhand. d. Naturwis, Vereins in Hamburg, 3. Folge XV, 1907, p. 45. 


172 PLANKTON OF WISCONSIN LAKES 


data for comparison with the results obtained on the plankton crus- 
tacea. These crayfishes ranged in age from about six months to two 
years and the whole animal was used for the sample. The crayfish ma- 
terial yielded a smaller percentage of nitrogen and of ether extract 
than most of the samples of plankton crustacea; only two samples of 
Daphnia gave a lower percentage of nitrogen than the crayfishes and all 
of the copepod material contained a larger amount. On an ash free 
basis, however, the percentage of nitrogen in the ecrayfishes is larger 
than it is in two samples of Copepoda and also larger than in eleven of 
the fourteen samples of Cladocera. 

The ether extract in the crayfishes is smaller than in seven of the ten 
samples of Daphnia and very much smaller than in all of the copepod 
material; this statement holds true for the percentages which include 
the ash as well as for those which do not. 

The percentage of crude fiber is slightly larger in one sample of 
Daphnia than in Cambarus, while the sample of Cyclops shows nearly 
as large a percentage as the latter; in all of the other samples of Cope- 
poda and Cladocera the percentage of crude fiber or chitin is appre- 
ciably smaller than in the crayfish material with the ash included. On 
an ash free basis, the crude fiber is larger in Cambarus than in any of 
the samples of Copepoda or Cladocera in table 49. 

With the ash included, the nitrogen free extract is smaller in the 
crayfish material than in all except two of the fourteen samples of 
Cladocera and it is also smaller than two of the five samples of Cope- 
poda. On an ash free basis the percentage of nitrogen free extract in 
the crayfish sample is below those of nine samples of Cladocera, but it is 
larger than those in all of the copepod samples. 

The percentage of ash is much larger in the crayfishes than in the 
various samples of Cladocera and Copepoda, being one-third larger than 
the maximum for Daphnia and from five to eight times as large as the 
percentage of ash in the five copepod samples. 

Hyalella. Including the ash the sample composed cf the amphipod 
Hyalella knickerbockeri yielded a larger percentage of nitrogen than 
the crayfish material, but on an ash free basis the two are very nearly 
the same. (Table 49.) The percentages of ether extract and of nitro- 
gen free extract are larger in the former than in the latter sample, but 
the reverse is true of the crude fiber and of the ash. 

In comparison with the plankton crustacea this sample of Hyalella, 
with the ash included, yielded a somewhat larger percentage of nitrogen 
than the minimum of the copepod samples, namely, that in Limno- 
ealanus, but on an ash free basis the percentage in the former is much 
larger than that in the latter. In fact the organic matter of Hyalella 
contained a larger proportion of nitrogen than three of the copepod 


ANALYSIS OF ORGANISMS 173 


samples. The percentage of ether extract is smaller in Hyalella than in 
the five samples of copepods, while the crude fiber and the nitrogen free 
extract are larger in two of the latter samples. The percentage of ash 
in the Hyalella material is from four to seven times as large as that in 
these five samples of copepods. 

Including the ash, only two of the fourteen samples of Cladocera 
yielded a smaller percentage of nitrogen than this sample of Hyalella, 
but on an ash free basis only three of the former samples exceeded the 
percentage of nitrogen in the latter. Including the ash seven samples 
of Cladocera contained a larger percentage of ether extract than Hy- 
alella, but this number is reduced to five on an ash free basis. On the 
basis of dry material nine samples of Cladocera gave a larger percent- 
age of crude fiber and twelve a larger amount of nitrogen free extract 
than Hyalella, but both of these numbers are reduced to eight on an 
ash free basis. The sample of Hyalella yielded a larger percentage of 
ash than those of the Cladocera. 

Oligochaeta. The bottom mud in the deeper portions of Lake Men- 
dota is inhabited by worms belonging to the genera Limnodrilus and 
Tubifex. Enough specimens of these two forms were secured to permit 
the determination of the nitrogen and of the ash. The nitrogen in this 
sample of Oligochaeta amounted to 7.76 per cent of the dry weight, 
which is equivalent to 48.5 per cent of crude protein; on an ash free 
basis these amounts become 8.1 per cent and 50.62 per cent respectively. 
These percentages are substantially the same as those noted in Volvox 
and in about half a dozen samples of plankton crustacea. 

The percentage of ash is smaller in this sample of Oligochaeta than in 
the various samples of plankton crustacea with the exception of Limno- 
calanus. 

Hirudinea. A sample consisting of 286 leeches was collected on 
August 28, 1915, in Lake Mendota. The specimens varied in length 
from about six centimeters to ten centimeters and they represented 
three different species. These leeches yielded a larger percentage of 
nitrogen than any other form given in table 49. With the ash included, 
11.13 per cent of the dry weight of this sample consisted of nitrogen, or 
69.56 per cent of crude protein; on an ash free basis these amounts 
become 11.82 per cent and 73.88 per cent respectively. The ether 
extract amounted to 11.33 per cent of the organic matter, so that the 
erude protein and the ether extract together constituted 85.21 per cent 
of the organic matter in the sample; this large percentage is exceeded 
only by that of the crude protein and ether extract in Limnocalanus, 
which constituted 88.35 per cent of the organic matter in this copepod. 

The percentages of pentosans and crude fiber are very small in this 
sample of leeches; the ash is somewhat larger than that in the sample 


174 PLANKTON OF WISCONSIN LAKES 


of Oligochaeta, but it is about the same as the percentages in most of 
the copepod material and in some of the insect larvae. 

Insect Larvae. Eight samples of aquatic insect larvae, from Ephe- 
merida to Corethra plumicornis inclusive in table 49, were analyzed. 
The specimens of Sialis infumata consisted of larvae that had migrated 
to the shore for the purpose of pupating; this material, therefore, rep- 
resents the full grown larvae. The two samples of Corethra also were 
full grown larvae; the other five samples contained insect larvae of 
different sizes, thus representing different stages in the growth of these 
forms. 

In these eight samples of insect larvae the nitrogen varied from a 
minimum of 7.86 per cent in Chironomus tentans to a maximum of 
10.74 per cent of the dry weight of the material in Corethra punctipen- 
mis; on an ash free basis the range is from 7.76 per cent to 11.67 per 
cent of the organic matter in these two forms. The crude protein in 
these larvae, therefore, made up from 48.5 per cent to 72.94 per cent of 
the organic matter. The ether extract showed more than a twofold 
variation in percentage, ranging from a minimum of 8.0 per cent in 
Chironomus tentans to a maximum of 18.5 per cent of the dry sample 
in Sialis; on an ash free basis the former becomes 8.43 per cent and 
the latter 19.44 per cent of the organic matter. The Sialis larvae were 
provided with reserve food to earry them through the pupating period 
of two weeks or more and the high percentage of ether extract indicates 
that part of this reserve probably consisted of fat. 

The percentage of crude fiber or chitin showed a little more than a 
threefold variation in these samples of insect larvae, with a minimum in 
Sialis and a maximum in the Anisoptera. The nitrogen free extract 
reached a minimum in the Zygoptera larvae and a maximum in Chiro- 
nomus tentans. The latter, in fact, yielded a larger percentage of 
ritrogen free extract than any other sample of animal material. Small 
amounts of pentosans were found in three of the samples. 

The ash varied from a minimum of 4.76 per cent in Corethra plumi- 
corns to a maximum of 15.11 per cent of the dry weight in the Trichop- 
tera larvae. Seven of the samples yielded less than one per cent of 
silica, while the eighth, consisting of Ephemerid larvae, possessed 3.85 
per cent. 

On an ash free basis the percentages of nitrogen in the samples of 
insect larvae show about the same range of variation as the various 
samples of plankton crustacea; also some of the former are substan- 
tially the same as the percentages of nitrogen in the other four samples 
of animal material, namely, the crayfishes, the amphipods, the worms, 
and the leeches. The leeches yielded a slightly larger percentage of 
nitrogen than the maximum of the insect larvae. 


yy 


ANALYSIS OF ORGANISMS 1795 


The five copepod samples, in general, yielded a larger percentage of 
ether extract than the insect larvae, the amount in Limnocalanus being 
a little more than twice as large as the maximum in the latter samples. 
In seven samples of Cladocera the percentage of ether extract fell below 
the minimum in the insect larvae and three contained a larger percent- 
age than the maximum in the latter. The crayfish sample yielded a 
smaller amount of extract than the minimum of the insect larvae, 
while the amphipods and the leeches corresponded closely to the 
Anisoptera and Zygoptera respectively. 

The crude fiber and the nitrcgen free extract in the insect larvae 
came within the range of variation noted in the plankton crustacea, 
except that the percentage of nitrogen free extract in Chironomus ten- 
tans exceeded that in any of the other samples of animal material. 

Three of the samples of insect larvae yielded about the same per- 
centage of ash as the copepod samples, but the other five insect samples 
gave a larger percentage of ash than the copepod material. About 
half of the Daphnia samples, together with the crayfishes and the am- 
phipods, contained a larger amount of ash than the maximum in the 
insect larvae, while the worms and leeches yielded about the same per- 
centages of ash as those noted in Corethra plumicornis and Chironomus 
tentans. 

Gyrinidae. This sample consisted of adult whirligig beetles that 
were obtained from the surface of the water. This material contained a 
much smaller percentage of nitrogen than the insect larvae, yielding 
only a little more than half as much as the maximum found in Corethra 
punctipennis. The percentage of nitrogen in these beetles, in fact, is 
smaller than in any other sample of animal material. 

The percentage of ether extract is much larger in the Gyrinids than 
in the insect larvae, being from two to more than four times as large as 
the percentages in the latter; it is exceeded only by the large extract 
noted in Limnocalanus. 

Whirligig beetles have relatively large and thick wing covers of 
chitinous material; as a result the percentage of crude fiber or chitin 
is much higher in them than in most cf the insect larvae, that in Ani- 
soptera being nearest to it. On an ash free basis the percentage of crude 
fiber in the Gyrinids is next to the largest obtained for the animal 
samples, being exceeded only by Cambarus. 

The percentage of nitrogen free extract is relatively low in this sam- 
ple of Gyrinids; only two samples of insect larvae and four of plank- | 
ton crustacea have a lower percentage. 

This beetle material yielded an unusually small percentage of ash; 
it is the smallest noted in any of the samples recorded in table 49. It 
is only a little more than one-third as large as the minimum percentage 
in the insect larvae. 


176 PLANKTON OF WISCONSIN LAKES 


Hemiptera. The sample of Hemiptera contained representatives of 
several genera; nearly half of the material consisted of young Belosto- 
ma, while about a quarter of it was made up of Notonecta and Corixa. 
The remainder consisted of Nepa, Naucoris, and Ranatra, with a few 
individuals belonging to still other genera. This sample yielded a much 
larger percentage of nitrogen than the Gyrinids; with the ash in- 
cluded, only four samples of animal material gave a larger percentage 
of nitrogen than these Hemiptera, but on an ash free basis the percent- 
age of nitrogen in the latter is exceeded by that in seven other animal 
samples. 

This sample of Hemiptera yielded a relatively small percentage of 
fat or ether extract; only one other insect sample shows a smaller per- 
centage, namely, Chironomus tentans, while that of Sialis is more than 
twice as large and that of the Gyrinids is more than four times as large. 
The percentage of crude fiber is relatively high in the Hemiptera, being 
exceeded by only two other insect samples. The nitrogen free extract 
is smaller than in five of the other insect samples and larger than in 
three. 

The percentage of ash in the Hemiptera material is nearly four times 
as large as that in the Gyrinids, but it is fairly low in comparison with 
the other insect samples. 

McHargue™ analyzed samples of grasshoppers (Melanoplus) and 
June bugs (Lachnosterna). He states that 75.28 per cent of the dry 
weight of the grasshopper material consisted of crude protein and that 
the percentage was somewhat larger in the June bugs, no definite per- 
centage being given for the latter form. This author does not state, 
however, whether any correction was made in the total nitrogen for 
that part which enters into the composition of the chitinous coverings 
of these insects. If such a correction has not been made, the percentage 
of crude protein indicated by McHargue is larger than it should be; 
a little more than 6.0 per cent of the dry weight of chitin consists of 
nitrogen. 

The percentage of crude protein in the grasshopper material as given 
by McHargue is more than twice as large as that in the Gyrinids and it 
is 8.16 per cent larger than the maximum in the aquatic insect samples 
shown in table 49. The grasshoppers yielded only 7.21 per cent of ether 
extract, or less than the minimum of these aquatic insects; the former 
contained 5.61 per cent of ash and only three of the ten insect samples 
analyzed in this series fell below this percentage. The main constitu- 
ents of the grasshopper ash consisted of silica (0.6 per cent), potassium 
oxide (1.2 per cent), and phosphorus pentoxide (1.19 per cent). 


% Jour. Agric. Research, Vol. 10, 1917, pp. 633-637. 


ANALYSIS OF ORGANISMS 177 


McHargue made quantitative determinations of the various kinds of 
nitrogen in the grasshopper and June bug material along with those in 
beef roast and breast of turkey; he found that the different kinds of 
nitrogen in these two insects were similar in amount to those noted for 
the beef and the white meat of turkey. 

In the samples shown in table 49 the crude protein, the ether extract, 
the crude fiber, and the ash make up from 34.13 per cent in Spirogyra 
to 95.42 per cent of the dry weight of the material in Cyclops; in other 
words the nitrogen free extract ranges from a minimum of 4.58 per 
cent in Cyclops to a maximum of 65.88 per cent in Spirogyra. On an 
ash free basis from 4.89 per cent to 72.46 per cent of the organic matter 
in the various samples consisted of nitrogen free extract. 

The samples of plant material, in general, contained a larger per- 
eentage of nitrogen free extract than the animal samples. In the 
former the largest percentages were found in the non-plankton forms, 
that is, in Spirogyra, Cladophora, Potamogeton, Vallisneria, and Myrio- 
phyllum. With the ash included the smallest percentage of nitrogen 
free extract is recorded for the diatom sample, but on an ash free basis 
it is a sample containing Aphanizomenon and Anabaena. In the non- 
plankton plant material from 47.85 per cent to 72.46 per cent of the 
organic matter consisted of the undetermined carbohydrates which 
made up the nitrogen free extract, while in the plankton algae these 
earbohydrates constituted from 27.62 per cent to 54.82 per cent of the 
organic matter. 

In the thirty-three samples composed of animal material the sum of 
the percentages of crude protein, ether extract, crude fiber, and ash is 
smallest in Chironomus tentans (65.0 per cent) and largest in Cyclops 
(95.42 per cent), making the range of the nitrogen free extract in these 
samples from 4.58 per cent in Cyclops to 35.0 per cent in Chironomus 
tentans. With the exception of Chironomus tentans and one of the 
Daphnia samples, the nitrogen free extract constituted less than 25.0 
per cent of the dry weight of the material in the animal samples. 

No attempt has been made to ascertain the composition of the nitro- 
gen free extract in these samples of animal material, but the greater 
part undoubtedly consists of carbohydrates. Some fatty compounds 
are not entirely extracted with ether, so that, in some instances, small 
amounts of fats may be included in the nitrogen free extract, and pos- 
sibly other substances are present also. The nitrogen free extract 
amounts to more than 10.0 per cent of the dry material in twenty-four 
of these animal samples and to more than 20.0 per cent in eight of them; 
if this extract is regarded as chiefly carbohydrate in character, it means 
that these samples contain fairly large percentages of carbohydrates, 
especially the latter group. In various analyses carbohydrates are not 


178 PLANKTON OF WISCONSIN LAKES 


listed as being present in such food materials as beef, mutton, and fish, 
except in the heart and the liver, but they are found in some of the 
aquatic organisms that are used as human food in percentages that are 
comparable to the nitrogen free extract in these samples of animal ma- 
terial. In oysters, for example, the carbohydrates constitute 28.2 per 
cent of the dry weight of the animal, exclusive of the shell, while mus- 
sels contain 26.25 per cent and scallops 17.26 per cent; the edible por- 
tion of the crayfish yields 4.35 per cent of carbohydrates. 

Brandt** states that the carbohydrates may constitute from 20.0 per 
cent to 25.0 per cent of the dry weight of copepods, depending upon the 
amount of plant food that may be present in the alimentary canal of 
the specimens. The percentage of nitrogen free extract given for the 
various samples of plankton crustacea in table 49 exceeds Brandt’s min- 
imum in only five samples and only one exceeds his maximum. 

While part of the carbohydrate material in some of the animal sam- 
ples given in table 49 may be derived from plants that have been con- 
sumed as food, yet it does not seem probable that any large amount 
comes from this source because the food contained in the alimentary 
canal would constitute a relatively small part of the weight of the entire 
animal. In four of the Daphnia samples the nitrogen free extract con- 
stitutes from a quarter to nearly a third of the dry weight of the or- 
ganic matter and but little more than one-third of the organic matter in 
the plankton algae consists of nitrogen free extract or carbohydrates. 
The fact that the nitrogen free extract is nearly as large in the four 
Daphnia samples as in the plankton algae indicates that a large part of 
the carbohydrate material in this extract is derived from the animals 
themselves rather than from plant food in the alimentary canal. 

In the three samples of Leptodora, the nitrogen free extract consti- 
tutes from 9.25 per cent to 15.65 per cent of the organic matter and no 
part of the carbohydrate material in this extract comes from plants 
because this animal is predaceous, feeding upon other crustacea. In 
the leeches, also, which are not plant feeders, the nitrogen free extract 
constitutes 14.44 per cent of the organic matter and no part of the ear- 
bohydrates therein is of direct plant origin. 

The results obtained in these analyses show that most of the forms 
represented in table 49 are excellent sources of food for other organisms 
beause the major portion of most samples consisted of crude protein and 
ether extract. The smallest percentages of these two substances were 
found in the large aquatic plants. The plankton algae, on the other 
hand, yielded substantially as large a percentage of crude protein as 
the animal material, but the percentage of ether extract in the former 


1# 'Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, p. 77. 


ANALYSIS OF ORGANISMS 179 


was smaller than in the latter; the largest amount of ether extract 
obtained from plant material was noted in the sample of diatoms. 

Moore, Edie, Whitley, and Dakin’ analyzed several marine forms, 
but their results are not comparable with those given in table 49 because 
they removed the skeletons from the organisms which they anabzed 
and used only the soft parts. 


AsH ANALYSIS 


Some of the inorganic constituents of the ash were determined for 
a few of these samples as indicated in table 50. The results shown in 
this table are stated in percentages of the dry weight of the sample. 

Silica (SiO,). Quantitative determinations of the silica were made 
on the ash of most of these samples and the results are recorded in 
table 49. The diatoms yielded the largest percentage of silica, namely, 
30.78 per cent of the dry weight of the sample. Cladophora ranks 
second with 7.1 per cent and the Ephemerid larvae are third with 3.85 
per cent. One sample of Daphnia pulex contained 2.84 per cent of 
silica. Excluding these four samples, the silica in all of the other ma- 
terial amounted to less than two per cent of the dry weight, ranging 
from nothing in the Gyrinids to 1.96 per cent in Myriophyllum. 

Iron and Alumina (Fe,O, and Al,O,). The results for iron and alu- 
mina are shown in table 50. The sample of Myriophyllum contains the 
largest percentage of iron and alumina, namely, 4.35 per cent of the dry 
weight, while one of the samples of Daphnia pulex gives 2.94 per cent. 
In all of the other samples on which determinations were made, the 
percentage falls well below two per cent of the dry weight, varying 
from 0.1 per cent in Limnocalanus to 1.8 per cent in Cladophora. 

Phosphorus (P,0,;). The sample of Cladophora is the only one re- 
corded in table 50 in which phosphorus pentoxide falls below one per 
cent. In the other plant material the amount is distinctly larger than 
one per cent of the dry weight. The maximum percentages are found in 
the Cladocera, namely, the three samples of Daphnia pulex and Lepto- 
dora. All of the crustacea, in fact, show relatively large percentages of 
phosphorus pentoxide; the Cladocera rank highest, the crayfish (Cam- 
barus) and the amphipod (Hyalella) second, and the copepods (Cy- 
clops and Limnocalanus) third. The leeches (Hirudinea) and the in- 
sects average about the same as the copepods. 

Clarke and Salkover’® state that phosphorus is an important consti- 
tuent of the ash of two marine crustaceans; for the copepod Temora 
longicornis they record 2.77 per cent of the dry weight as calcium phos- 
phate (Ca,P,0,) and for the small shrimp Thysanoessa inermis 7.68 


4% Biochemical Journal, Vol. VI, 1912, p. 291. 
7° Jour. Washington Acad. Sci., Vol. 8, 1918, pp. 185-186. 


180 PLANKTON OF WISCONSIN LAKES 


per cent. In terms of phosphorus pentoxide these percentages are 1.27 
per cent and 3.52 per cent respectively. The marine copepod shows a 
somewhat smaller percentage of phosphorus pentoxide than the two 
freshwater copepods included in table 50, while the marine shrimp 
shows the same percentage as the freshwater Cladocera. 

Calcium (CaO). The largest percentage of calcium oxide or lime is 
recorded for the crayfish material, namely, 17.82 per cent of the dry 
weight, while the amphipod Hyalella comes second with 14.82 per cent 
and one sample of Daphnia pulex is third with 9.89 per cent. Calcium 
is a very important constituent of the shells of these crustacea. In two 
other samples of Daphnia pulex and in Leptodora, a much smaller 
amount of calcium oxide was found, ranging from 2.01 per cent to 3.22 
per cent. The percentage is still smaller in the copepods and leeches 
as well as in the insects. 

In the plant material the maximum percentage of calcium oxide is 
recorded for Myriophyllum, with Cladophora ranking second. The 
other plant samples show relatively small amounts. 

Magnesium (MgO). Only three of the eighteen samples on which 
determinations were made yielded more than one per cent of mag- 
nesium oxide. In the samples of plant material the maximum percent- 
ave was found in Cladophora, while the maximum for the animal sam- 
ples was found in the insect larva Sialis. These data indicate that 
magnesium is used only in rather small amounts by the various forms 
represented in table 50. 


APPENDIX 181 


APPENDIX 


STATISTICAL TABLES 
TaBLE 1. This table shows the maximum length and the width, the 


area, the depth, and the volume of the four lakes on which the plankton 
studies were made. 


Length Width Area in | Maximum| Mean Volume in 


Lake in Km. in Km. Sq. Km. | Depth in | Depth in | in Cubic 
Meters Meters Meters 
Mendota... 9.50 7.40 39.40 25.6 12.1 478,370,000 
Monona.... 6.70 3.85 14.10 22.5 8.4 118,887,000 
Waubesa... 6.75 2.25 8.24 11.1 4.9 40,252,000 
Kegonsa.... 4.83 3.62 12.70 9.6 4.6 59,060,000 


TasLE 2. The number of runs or catches made on the different lakes 
mm the various years, the volume of water strained, and the dry weight 
of the net plankton obtained are gwen in this table. 


Number Volume Net Plankton 
Lake Year of of Water 
Catches in Liters | Dry Weight | Milligrams 
in Grams per Cubic 


Meter of 
Water 
Mendota... ..s6.03.. 1911 38 164,740 42.535 258 
1912 58 369,900 222.502 601 
1913 91 623,800 355.165 569 
1914 37 260,100 76.937 296 
1915 115 441,460 220.181 498 
1916 65 74,960 35.497 473 
1917 11 12,070 4.233 350 
Total 415 1,947,030 957.050 491 
DIOMONB! 6. bck k es 1911 4 12,710 9.600 qo 
1912 6 43,140 69.899 1,620 
1913 16 99,510 144.864 1,556 
1915 8 9,776 19.221 1,966 
1916 13 12,510 7.698 615 
Total 47 177,646 251.282 1,414 
WV AUDESB' i. cc se 1913 2 11,800 26.729 2,265 
1915 4 aioe 22.426 3,912 
1916 12 11,641 14.499 1,245 
Total 18 29,173 63.654 2,182 
mevonsael..........| 1918 1 3,318 20.282 6,112 


Grand Total........ 481 2,157,167 | 1,292,268 


182 PLANKTON OF WISCONSIN LAKES 


TABLE 3. The number of centrifuge runs, the amount of water cen- 
trifuged wm the different years, and the quantity of dry material ob- 
tained are indicated in this table. 


Nannoplankton 
Lake Year Number Volume | |—— ,-——_ 
of Runs of Water | Dry Weight | Milligrams 
in Liters in Grams per Cubic 
meter of 

Water 
Mendota e..).3% 3.542% 1915 65 88,953 212.417 2,388 
1916 69 78,483 296 .430 Ote 
1917 11 12,670 45.858 3,619 
Total 145 179,506 554.705 3,090 
Monona etn wei 1915 8 9,777 43 .659 4,465 
1916 13 12,510 55.156 4,409 
Total 21 22,287 98.815 4,433 
Wiambesa. iiicsiene 1915 4 Doe 36 . 002 6,280 
1916 12 11,641 62.423 5,362 
Total 16 17,373 98 .425 5,665 


TaBLe 4. The quantity of water, in liters, used for the various sam- 
ples of net plankton and nannoplankton are gwen in this table as well 
as the dry weight of the plankton obtained therefrom. The quantita- 
tive results given in tables 43 to 48 are based upon the amounts indi- 
cated in this table; those samples secured in July, August, and Septem- 
ber were subject to correction as indicated on page 24. In the serves of 
sample numbers the first figure indicates the year in which the sample 
was taken; 102 for example, represents the second sample obtained in 
1911, 215 the fifteenth sample of 1912, and so on. 


Number| Quantity Dry Weight ||Number| Quantity Dry Weight 


of of Water, of Plankton, of of Water, of Plankton, 
Sample Liters Milligrams Sample Liters Milligrams 
102 1 ,925 520 125 10 ,500 4 ,450 
103 2 ,200 350 126 9 ,632 5 ,130 
104 3 ,060 1 ,000 128 3 ,132 1 ,550 
105 3 ,645 730 129 4 360 2 ,240 
106 2,570 1,940 130 4 ,080 2 ,505 
108 7 ,860 850 131 3 ,936 1 ,627 
109 6,777 540 202 8 ,350 850 
110 7,112 690 204 4 ,250 868 
111 8 ,400 770 205 11 ,928 5 ,914 
112 10 ,980 1,210 207 11 ,190 7,102 
113 10 ,314 1 ,000 208 14 ,842 5 ,632 
114 2 ,640 1 ,040 209 14 ,430 4 293 
115 12 ,457 1 ,660 212 11 ,680 1 ,672 
116 12 ,579 4 ,790 213 14 ,628 2 ,610 
117 12 ,432 4 ,840 214 10,100 4 ,236 
118 8 ,322 1,550 215 16 ,464 3 226 
119 10 ,500 2 ,090 218 15 ,358 4 ,661 
120 3,215 3,580 219 17 ,360 6 ,459 
123 10 ,535 2 443 220 14 ,712 10,258 


APPENDIX 183 
TABLE No. 4—ContTINUED 


Number} Quantity Dry Weight Number| Quantity Dry Weight 


of of Water, of Plankton, of of Water, of Plankton, 
Sample Liters Milligrams Sample Liters Milligrams 
222 14 ,094 5 ,568 355 16 ,260 22 ,036 
223 7 ,3840 6 ,220 356 5 ,820 7 483 
224 14 ,094 5 ,320 350 14 ,330 12 593 
225 13 ,030 7 ,066 359 15 ,630 14 ,229 
227 13 ,090 9 658 360 5 ,150 14 ,398 
228 13 ,028 6 ,121 361 15 ,840 14 ,027 
229 14 ,850 5 ,676 362 6 ,600 12 ,140 
230 14 ,540 10,711 363 16 ,000 11 ,120 
231 5 735 7,140 364 12 ,380 7 ,696 
232 13. ,045 11 ,576 365 26 ,400 12 ,792 
233 6 ,665 15 ,313 366 38 ,000 17 ,119 
236 13 ,950 17 ,087 367 18 ,530 12 ,017 
Ba 11 ,860 16 ,128 368 24 ,740 13 ,562 
238 7 ,060 27 ,004 369 22 ,540 18 ,259 
242 12 ,000 20 ,527 370 12 ,590 13 ,605 
243 11 ,220 12 535 371 8 ,760 6 ,949 
245 11 ,855 9 ,941 401 20 ,340 2 ,764 
246 6 ,240 9 ,986 402 26 ,250 4 ,242 
247 11 ,480 9 ,166 404 11 ,760 3 ,686 
248 10 ,680 8 ,403 405 14 ,600 5 ,491 
249 7 835 5 ,637 406 21 ,100 6 ,770 
301 12 ,500 2,573 407 16 ,510 7,618 
302 14 ,590 3 ,805 408 25 ,260 11 ,405 
303 13 ,580 6 225 409 15 ,590 6 ,680 
304 13 ,3800 7 ,182 410 13 ,900 5 ,203 
305 13 ,000 5 ,957 411 15 ,000 7,302 
306 12 ,400 6 ,712 412 20 ,650 7 ,000 
307 12 ,000 5 ,940 413 19 ,620 7,379 
308 14 ,300 5 353 414 18 ,800 4 ,503 
309 15 ,500 7 329 415 20 ,735 2 ,361 
310 15 ,096 4 503 500-1 2,220 9 ,232 
311 15 ,390 4 ,550 502 2 223 338 
312 17 ,340 3,295 503 14 ,400 2 ,096 
314 18 ,840 2,721 505 1,100 2 ,895 
315 5 ,800 5 ,480 506-7 15 ,032 2 ,291 
320 5 ,350 10 ,942 508-10 2 ,204 4,115 
322 6 ,330 19 346 509-11 2 ,204 660 
324 22 ,720 10 ,346 513-16 2 ,632 5 582 
325 17 ,700 15 ,288 514-17 2 ,632 1 ,076 
326 3,130 10,060 519 1,143 3 ,157 
328 17 ,310 19 ,501 520 1,143 763 
329 3 ,318 20 ,282 522 1,143 3,250 
330 5 ,400 13 ,112 523 1,143 808 
332 16 ,340 20 ,405 525-27 2,311 5 127 
333 3 ,020 8 ,340 526 1,183 1,150 
337 24 ,800 1 427 529-31 2 343 5 ,860 
338 5 ,430 11 ,089 530-32 2 ,343 1,914 
339 26 ,016 4,475 535-37 2 324 5 ,283 
340 8 ,300 8 ,891 536-38 2 324 1 ,657 
341 34 ,986 6 ,855 540 13 ,176 6 ,470 
345 5,472 6 ,529 541-45 2 328 3 ,361 
346 25 ,872 7 ,649 542-46 2 328 922 
347 9 588 5 ,213 543 869 L512 
349 8 ,520 4 ,726 544 869 2,109 
350 19 ,130 15 ,732 548 12 ,842 5 ,638 
351 13 ,970 11 ,499 549-51 3 ,009 4 ,902 
352 6 ,450 15 ,787 550-52 3 ,009 1 ,488 
353 16 ,250 HL. 12 553-57 3 ,042 5 654 


3504 7,000 6 ,557 554-58 3 ,042 1 ,202 


184 


Number 
of 
Sample 


SO eee | eee f 


561-65 
562-66 
563 
564 
567-75 
568-76 
579-81 
580-82 
583 
584 
5895 
586-90 
589 
595-100 
596-101 
597 
598 
5102-6 
5103-7 
5104 
5110-12 
5111-13 
5115 
5116-22 
5117-23 
5118 
5119 
5124-26 
5125-27 
5128 
5129 
5130 
5131 
5132-36 
5133-37 
5134 
5135 
5139-43 
5140-42 
5144 
0145 
5146-48 
5147-49 
5150 
5152 
5153 
5156-66 
5158 
5159 
5162-64 
5166 
5167 
5168-70 
5172 
5173-79 
5174 
5175 
5176-78 
5180 


PLANKTON OF WISCONSIN LAKES 


Quantity 
of Water, 
Liters 


2,979 
2,979 
865 
865 

3 022 
3,022 
3 ,004 
3,004 
1,091 
1,091 
1,540 
BZ 
1,577 
3 ,219 
3,219 
1,547 
1,547 
3 253 
3 253 
14 330 
3,139 
3,139 
14,165 
3,201 
3,201 
1,441 
1,441 
3.123 
3 1123 
1 525 
1,525 
11,790 
13 ,980 
3 079 
3 079 
1,472 
1,472 
14,850 
3 035 
1 523 
1,523 
3,135 
3135 
13 ,986 
1,474 
1,474 
3147 
10,780 
10 ,980 
2,985 
1,340 
1,340 
2.431 
7 ,360 
14,110 
1,186 
1,186 
2,470 
1,176 


TasLE No. 4—ConrTINUED 


Dry Weight 
of Plankton, 
Milligrams 


Number 
of 
Sample 


5181 
5182-84 
5183-85 
5187 
5188-90 
5192 
5194 
5195 
5196 
5198 
5200 
5202 
602 
603 
604 
605 
606 
607 
608 
609 
610 
611 
612 
613 
614 
615 
616-18 
617-19 
620 
621 
626-28 
627-29 
630-34 
631-35 
632 
633 
636-38 
637-39 
640-44 
641-45 
646-52 
647-53 
648 
649 
650 
651 
654-56 
657 
658 
659 
662-64 
663-65 
666 
667 
668-74 
669-75 
672 
673 
676-80 


Quantity 
of Water, 
Liters 


1,176 
2,238 
2, ,238 
4.980 
2, 294 
1,110 
5,480 
5 ,000 
1,120 
iL a 
1,190 
5,160 
1,109 
1,109 

515 

515 
1,068 
1,068 
1,100 
1,100 
1,091 
1,091 
1,108 
1,108 
1,090 
1,030 
2,306 
2,306 

966 

966 
2,210 
2.210 
2 372 
sie 
137 
tig 
2,270 
2,270 
2 341 
eal 
2,359 
2,359 
1,113 
1,113 
1,145 
1,145 
2,350 
1,182 
1.127 
1127 
2,360 
2,360 
1,143 
1,143 
2,365 
2 365 
1,122 
iio 
2 329 


Dry Weight 
of Plankton, 
Milligrams 


2,687 
7,446 
1,401 
3,982 
6,704 
4515 
4 383 
4 041 
5 240 
5 220 
4211 


re 


APPENDIX 185 
TABLE No. 4—ContTINUED 
Number} Quantity Dry Weight Number} Quantity Dry Weight 
of of Water, of Plankton, of of Water, of Plankton, 
Sample Liters Milligrams Sample Liters Milligrams 
677-81 2 3829 393 6156-60 2,352 8 ,595 
678 1,141 4 ,689 6157-61 2 352 1,374 
679 1,141 1,321 6158 741 7,303 
682-86 2,351 8 ,543 §159 741 402 
683-87 2,351 406 6162-66 2 350 8 ,297 
684 1,138 3,237 6163-67 2 ,350 1,733 
685 1,138 44] 6164 745 6 ,346 
688-92 2 357 8 ,495 6165 745 1,684 
689-93 2,057 324 6168 728 6 3836 
6$0 1 ,145 4 804 6169 728 773 
691 1,145 910 6170-72 2 3843 8 ,030 
694-98 2 ,356 7,155 6171-73 2 343 1,455 
695-S9 2 ,306 352 6174 744 4 ,828 
696 1,145 4,145 6175 744 S80 
697 1,145 439 6176-78 2 343 6,581 
6100-6 3 ,026 8 ,295 G@177-79 2 ,343 1,573 
610i 1,187 179 6180-84 DAVE 8 ,035 
6102 1,144 6 ,455 6181-85 2 2977 2,420 
6103 1,144 1 ,438 6182 726 3 ,289 
6108-12 2 316 6 ,433 6183 726 707 
6109-13 2 316 262 6186 ta 3,269 
6110 1,073 4 383 6187 ese 1 552 
6111 1 ,073 539 6188 1,183 3,320 
6114-18 2 ,353 5 ,966 6189 1,183 1 429 
6115-19 2 353 262 6190-92 2 132 7 234 
6116 ey 5 ,486 6191-93 2,132 3 217 
6117 747 1,068 6194 1 ,066 3 885 
6120-24 2 ,386 6 ,000 6195 1,086 1,355 
6121-25 2 ,386 328 6196 735 2 550 
6122 1,149 5 ,899 6197 73 1,249 
6123 1,149 211 702 860 27H 
6126-30 2 ,367 6 ,4382 703 860 1 ,036 
6127-31 2 ,367 343 704 904 2 ,147 
6128 1,091 8 ,290 705 904 231 
6129 1,091 1 ,035 706 909 1 ,765 
6132-36 2,361 7,941 707 909 146 
6133-37 2,361 561 703 1A GA 4 ,251 
6134 745 4 622 709 kai al 150 
6135 745 130 710 1,168 4,154 
6138-42 2 ,367 10,881 711 1,168 202 
6139-43 2,367 635 112 1,170 7,825 
6140 724 7 ,O86 713 se) 386 
6141 724 1 237 714 1,180 7,481 
6144 1,182 6 ,149 715 1,180 315 
6145-49 2,362 1,740 716 1.172 4 314 
6146 Tot 5,514 rays 1G lege 308 
6147 737 254 718 1,170 3 692 
6148 1,180 4 834 719 1,170 407 
6150 735 6 ,027 720 ies 4 ,200 
6151 735 2 ,361 tok 1,183 518 
6152 1,180 6 ,394 722 1188 3,611 
6153-55 2 355 1,719 723 1183 584. 
6154 1,175 5 247 


PLANKTON OF WISCONSIN LAKES 


186 


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APPENDIX waleg 


TABLE 6. The maximum, the minimum, and the mean amounts of 
orgamc matter in the net plankton of Lake Mendota for the different 
years are shown in this table. The results for 1914 and 1917 do not 
cover full years; in the former instance the observations extended only 
to the first of July and in the latter to the first of June. The results 
are stated wn milligrams of organic matter per cubic meter of water. 


Year Number of Maximum Minimum Mean 
Samples 
1911 Tepes 464 42 175 
1912 29 1 ,055 84 393 
1913 35 697 87 373 
1914 14 426 a9 289 
1915 34 810 113 367 
1915 39 1,185 Te 343 
1917 11 866 92 Zit 


TaBLE 7. The variations in the percentage of nitrogen in the net 
plankton and the mean percentage for the different years are indicated 
in this table. Nitrogen determinations were not made on all of the sam- 
ples collected in 1916 and 1917. The mean annual percentages are 
based on the mean quantity of nitrogen and the mean quantity of or- 
ganic matter in the samples on which mtrogen determmations were 
made. In the first part of the table the percentages of total nitrogen 
are indicated, but in the second part the percentages do not imclude 
the nitrogen of the chitinous shells of the crustacea. 


Number Per Cent of Sample Per Cent, Ash Fre? 
Year of — 

Analyses | Maximum | Minimum | Maximum | Minimum Mean 
1911 22 8.33 3.92 9.44 Dens 7.93 
1912 29 8.93 4.47 9255 5.61 8.11 
1913 35 9.22 4.16 1-20 6.26 8.14 
1914 14 9.96 6.60 11.10 6.95 8.65 
1915 34 Sit 4.59 10.72 ee 8.87 
1916 26 7.02 3.95 9.00 5.50 8.16 
1917 6 9.97 5.76 Us Re 8.43 10.21 


188 PLANKTON OF WISCONSIN LAKES 


TABLE 8. The maximum, minumum, and mean percentages and 
amounts of crude protein in the organic matter of the net plankton 
of Lake Mendota. This table 1s based on the second part of table 7. 
Crude protein equals N X 6.25. 


Number| Per Cent of Crude Protein | Crude Protein, Milligrams per 


Year id of in the Organic Matter Fé Cubic Meter of Water 
naly-_ |—A A _—~ s earjoeey oem crm 
ses |Maximum!] Minimum|] Mean |Maximum| Minimum) Mean |Maximum 
Minimum 
1911 22 59.0 Bffell 49.6 255.0 Deb 2 87.0 12.0 
1912 29 59.4 35.0 50.7 5 ee 48.1 {199.1 TICS 
1913 35 70.0 39.1 50.9 374.4 49.4 |189.8 76 
1914 1'. 69.3 43.4 5471 244.3 AS en Lop ae 5.6 
1915 34 67.0 ae BDL 449 4 582m \2ZO3k7 7.6 
1916 26 56.2 3405 51.0 581.3 325, 12-050 17.8 
1917 6 69.4 526 63.8 Le BDR 109.3 {159.9 2 i 


TABLE 9. The percentages of four forms of ntrogen found by Schu- 
ette in some net plankton from Lake Mendota. (Trans. Wis. Acad. Sci., 
Arts, and Let., Vol. XTX, 1918, p. 604.) 


Sample Total Nitrogen Di-amino Mono-amino “Humic” 
Nitrogen as NH Nitrogen Nitrogen Nitrogen 
A 6.15 0.70 E22 3.69 0.39 
B 5.58 0.70 0.96 BBs 0.38 
C 8.25 0.97 1.90 4.77 0.30 
D 8.55 0.76 2.14 5.16 0.32 
E 9.94 1.09 2.79 5.39 0.67 


TaBLE 10. The variations in the percentage and in the quantity of 
ether extract in the net plankton of Lake Mendota are shown in this 
table. 


Number Per Cent, Ash Free Milligrams per Cubic Meter 
Year of of Water 
Analyses. |—-——— ——, ),s Yt _._. -— _— 
Maximum} Minimum | Mean |Maximum| Minimum} Mean 
1911 22 26.58 sail LUssacays 52.4 6.6 21.9 
1912 28 24.3 5.93 io 135.8 jase one 
1913 35 20.16 312 10.3 102.7 4.0 AD 
1914 13 18.06 6.23 13.00 CORT 6.4 37.9 
1915 33 Zilles 6.07 12.42 tS ral 12.9 46.6 
1916 i! 14.48 (Oe TAT 60.5 16.9 Seagal! 


APPENDIX 189 


TaBLE 11. Some of the physical and chemical constants of the ether 
extract of three samples of net plankton as determined by Schuette. 
(Trans. Wis. Acad. Sci., Arts, and Let., Vol. XIX, 1918, p. 599.) 


Sample C D No. 403 
Percent of ether extract in dry sample.......... 8.01 13.47 21.25 
inde xonreirachion atizot Caso. ee ee ees 1.4777 1.4785 1.4810 
MOGIME EN UIMDEL ji. ick ee ee SR eet, 102.08 87.58 172.88 
Saponincation number... 2.0.0 sk cg ee os 248 .60 208 . 56 
iverehnert-leiss! number 6.2.1... k ik cde. 1.16 0.94 
Glemsixe MUIMDCK! oj )a5 be eee ie SL ee eee tok eich. 1.55 1.22 


TaBLE 12. The variations in the percentage and in the quantity of 
the pentosans in the net plankton of Lake Mendota are indicated im 
this table. 


Number Per Cent, Ash Free Milligrams per Cubic Meter 
Year of of Water 
Analyses_|—2————, — |] pA _ —“- 
Maximum] Minimum | Mean |Maximum| Minimum] Mean 
1912 il74 4.08 1.38 3.06 36.1 5.0 15e3 
1913 32 3.90 1.79 2.86 23.8 1.8 10.9 
1914 12 3.65 1.18 1 10.7 3.6 5.4 
1915 31 5.16 1.38 eed 33.6 3.0 12.9 


TABLE 18. The changes in the percentage of crude fiber in the or- 
ganic matter of the net plankton are shown for the different years in 
this table, as well as the variations m quantity per cubic meter of 
water. The mean percentage is based on the mean quantity of crude 
fiber and the mean quantity of organic matter in the samples on which 
crude fiber determinations were made. 


Number Per Cent, Ash’ Free Milligrams per Cubic Meter 
Year of of Water 
Analyses) |—--—————  -._ /, > _ Y  _ —- — 
Maximum] Minimum |} Mean |Maximum} Minimum | Mean 
1911 iyi 17.74 6.80 10.34 SA 2 Sy raters 
1912 Pa 20.21 3.54 6.86 67.0 10.8 30.4 
1913 35 ier 2.67 6.19 nee 5.8 Bork 
1914 13 TBAB 3.68 6.12 28.3 6.4 17.5 
1915 33 LO.32 4.15 6.00 37.9 5.9 22.0 


190 PLANKTON OF WISCONSIN LAKES 


TaBLE 14. A summary of the ash and silica determinations for the 
net plankton of Lake Mendota, 1911 t0 1917. The results are stated in | 
percentages of the dry weight of the net plankton. 


Difference 
Number Per Cent of Ash Per Cent of Silica between 
Year of a a ——_——_—_—__ ————__| Means of 
Samples| Maxi- | Mini- | Mean Mini- | Mean | Ash and 
mum mum mum Silica 


— | ff | | ES TS NL 


1911 22 40.67 7.54 | 17.65 1.97 9.22 8.43 
1912 29 38.30 5.62 | 19.47 0.76 | 12.37 7.10 
1913 35 43.78 | 10.08 | 22.70 0.50 | 14.45 8.25 
1914 14 13.63 9.13 | 11.56 1.34 3.84 Ae 
1915 34 48 .30 9.40 | 29.09 2.84 | 17.37 11.72 
1916 39 45.18 | 16.56 | 29.07 2.80 | 15.55 13.52 
LSE 11 46.66 | 13.40 | 27.44 3.79 | 10.04 17.40 


TABLE 15. Analyses of the ash of the net plankton. The results are 
stated in percentages of the dry weight of the net plankton. 


Number Fe.O3+ ; 
Lake of Ash SiO. Al.O3 Mn;0, P.O; CaO MgO 
Sample 

Mendota. 102 25 2.41 2.18 0.00 2.08 3.60 | 0.20 
108 19.87 gO iad oie Pemmenrtets hae, Lowman pen NLA Beare 8) 3. Sau, 1295 
112 12.53 2.90 QSOS her cee 212 6.18 | 0.62 
116 28 .38 21.50 2 0.00 1.438 2.97 1 O51 
117 40.67 32.66 DO vere ee ee 14 oes Slee ee 
118 12.25 A ODS ley ete 0.00 1.82 2.75 | 0.86 
125 15.30 8.94 0.67 0.00 0.59 2.68 i Ofer 
130 24.40 16.93 1.44 0.00 Lay, 3.29 | 0.40 
202 17.82 6.74 OUAC ee ea e. 0.25 4.36 | 0.80 
207 9.90 3.45 OZ aa weeen naee 1.58 223. Vie 
213 12.98 1.63 OUS3u Seen: 0.94 6.80 | 0.23 
220 5.62 1.38 OSS a bee 0.76 1.63 | 0.35 
302 19.56 12.58 DOOR neues at: 2.00 1,60), 22024 
307 22.82 15.57 af taf Rey ee eas 2.65 1.16 
311 13.32 1.56 ORO ERE eae 2.54 5.43 1.4] 
325 11.01 6.06 QUASI a cece 1.48 1.03 1.24 
337 19.72 9.10 OLGSsl eee ae Deo 3.12 |p One 
350 33.60 25.58 OUSSr lee eae 1.45 2-00 | OCSt 
359 29.86 HIS SG GST Ee ated nn ess smc aya tee 4.75 | 0.94 
363 Aer BOSON se es ORS AT kee 2.01 | 0-90 
366 30.99 CPA Vie (a Pace a Mo Ge pe ee Up 4.01 | 0.82 
367 33.35 Daou beeen 0.28 1 yp ee aiihep a Is Sah 
401 9.64 DA hein kat 0.31 0.58 ow 1.03 
404 13.63 eA A iat te 0.30 LcBOC Se eee 
409 9.13 SECO uae tinal: 0.39 140 Soe, See eee 
502 Da he, BOE Gm (igh MY inal GA ECA ine Tate any MY DOS) teas ‘ 
503 19.65 hI ORES TEAM a a de or es aan Nea gt eee Dee Leave 

Monona... 330 6.20 0.38 QAO ses 1.28 1.56 | 0.81 
354 12.16 4.71 DS 40 epee Aika 0.83 1 7OUssOR2o 


191 


APPENDIX 


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192 PLANKTON OF WISCONSIN LAKES 


TasBLeE 17. Maximum, minimum, and mean quantities of material 
in the nannoplankton of Lake Mendota expressed in milligrams of dry 
orgame matter per cubic meter of water. Compare with the results 
given for the net plankton wm table 6. 


Number of 
Year Samples Maximum Minimum Mean 
1915 35 OG 944.2 1,622.0 
1916 ( 41 ’ $3,151.5 bE 962.3 1 ,640.7 
1917 f. 11 ~ £2,603.4 e 195.2 il G20 
E Eas Qi UR ee bs Sera ae wd 


TABLE 18. This table shows the maximum, minimum, and mean per- 
centages of nitrogen on an ash free basis in the nannoplankton of Lake 
Mendota and also the quantity per cubic meter of water. Compare 
with the net plankton wn table 7. 


Per Cent, Ash Free Milligrams per Cubic Meter 
Number. of Waiter 
Year ) |---|) | | | 
Analyses | Maximum | Minimum} Mean |} Maximum| Minimum | Mean 
1915 35 10.44 5.32 7.07 174.2 63.2)" | Tia 
1916 Al 8.77 4.24 6.27 249.7 49.8 102.9 
1917 11 10.66 5.91 8.27 22205 47.8 | 134.1 


Taste 19. The annual range of variations in the percentages and 
nm the amounts of crude protein in the nannoplankton of Lake Mendota. 
These results are based on those gwen in table 18. Compare with the 
net plankton, table 8. 


Per Cent, Ash Free Milligrams per Cubic Meter 
Number of Water 
Year of | | |_| |—__—_— 
Analyses |Maximum | Minimum | Mean || Maximum] Minimum] Mean 
1915 35 65.25 33.25 44.19 1,088.7 395.0 716.6 
1916 41 54.81 26.50 39.19 1,560.6 311.2 642.8 
1917 11 66.62 36.93 51.68 1,390.6 298.8 838.0 


TABLE 20. The range of variation in the percentage and wn the quan- 
tity of ether extract in the nannoplankton of Lake Mendota. Compare 
with the net plankton in table 10, 


Ether Extract, Per Cent Ether Extract, Milligrams per 
Number Ash Free Cubic Meter of Water 
Year of | $s | | A | | 
Analyses | Maximum! Minimum | Mean || Maximum| Minimum | Mean 
1915 35 15.77 3.02 7.38 355 .6 49.0 119.7 
1916 41 12.50 2.56 6.17 287 .7 31.2 101.2 


1917 11 TO" 2.21 5.36 184.0 17.6 87.0 


APPENDIX | 193 


TABLE 21. The variations mm the percentages and in the amounts of 


the pentosans found in the nannoplankton material. Compare with 
the net plankton in table 12. 


Number Pentosars, Per Cent Pentosans, Milligrams per 
Year of Ash Free Cubic Meter of Water 
Analyses |—-—————— , A), Ss | A | —— 
Maximum | Minimum | Mean | Maximum] Minimum] Mean 
1915 a2 7.39 2.84 4.87 115.6 AGA 1 78.9 
1916 8 7.61 3.64 5. 28 144.0 53.7 86.6 
1917 2 Bie a 3.52 3.64 96.6 47.4 72.0 


TABLE 22. The range of the variations in the percentage and in the 
amount of crude fiber in the nannoplankton of Lake Mendota. Com- 
pare with the results for net plankton, table 13. 


Crude Fiber, Per Cent Crude’ Fiber, Milligrams 
Number Ash Free per Cubic Meter of Water 
Year of ——_———_ [|_| crm st ccr 
Analyses | Maximum | Minimum | Mean | Maximum| Minimum] Mean 
1915 34 5.36 27 3.02 85.8 24.7 49.0 
1916 17 9.27 4.20 6.46 2Naa7 57.2 108.0 


1917 4 LE S47 3.86 6.73 158 .2 78.8 109.2 


194 PLANKTON OF WISCONSIN LAKES 


TABLE 23. The amount of ash and silt in the nannoplankton of Lake 
Mendota given in terms of milligrams per cubic meter of water. The 
last part of the table shows four constituents of the ash stated in per- 
centages of the dry weight of the nannoplankton. 


Ash of FeO; 
Number Total | Bowl | Nanno-| Silt, | SiO. + | CaO,;MgO, 
of Date Ash, | Ash, |plankton| Mg. | Per |Al.O;) Per | Per 
Sample Mg. per| Mg. | Mg. per| per | Cent} Per | Cent} Cent 
Cu. M. | per | Cu. M. |Cu.M. Cent 
Cu. M. 
529-31 | June 8-11, 1915} 926.6)...... 216.0 |) 710611950)... | Oe ee 
535-37 | June 14-17 O64 Gl cee 192.3 CI2 A202 OU cca| c= ae eee 
541-45 | June 21-24 C2G20 bias 123.0 604 .9)24.51| 5.20) 4.5¢) 1.59 
549-51 | June 28—July 1 C20. 29 So aee 133.4 5940/2107 92) ls eee 
553-57 | July 6-9 OZING ERS er 141.0 (80. 61I9C TC Se eee 
561-65 | July 12-15 S2250)) apa: 158.5 664. No ol, See 
567-75 | July 19-22 E200 cS )e is eer 198.5 |1 ,062.3)10.44; 3.8C} 6.97) 1.47 
579-81 | July 26-29 bss 71590(| | eee 176.7 1,849.0) 6238). cal coco eee 
585 August 2 1 ,404.0)...... 186.4 |) 207.6) 883300. aan eee 
589 August 6 1, S49/G)0 ee 939 (0. (1 -110.6111.82|... 1, eee 
595-100} August 9-12 OLORZe eee 117.0 SOL S213 560). ee Rs 
5102-6 August 16-19 Hoe Oloke ean 105.0 ANT O\LS S38. 6) ee ee eee 
5110-12 | August 23-26 HSS Olan ss 123.0 465 .5/25.58| 6.65) 5.76) 1.65 
5116-22 | Aug. 30-Sept. 2} 741.0|...... 154.2 1° 586 8117.04... 1 ee 
5124-26 | September 7-9 50540). see 133.4 3696/14. 72). oe a ee 
646-52 | June 12-16, 1916)1 ,3871.0) 555.3) 117.0 698. F207 EO). Sale eee 
654-56 | June 19-22 1,577.7) 557.4) 164.6 855.71/18.4¢] 6.97) 8.1] 5.17 
668-74 | July 5-7 1 696.2} 554.0) 248.1 894.1)11.41] 5.77)18.7€| 6.12 
688-92 | July 24-28 1,767.8) 555.6; 204.0 |1,008.2) 8.25)..... LS STS 
694-98 | July 31-Aug. 3 |1,466.9] 556.0) 174.0 736.9] 7.36) 6.14)19.44) 6.21 
6100-6 | August 7-9 1 ,454.7| 64973) 18823 | 667. J}10:19)). - 2) eee 
6108-12 | August 14-18 {1 ,320.6) 566.0; 161.4 593.2} 9.52] 7.55/10.05) 6.25 
6114-18 | August 22-25 {1,297.0} 556.7) 1380.3 GLOOM 2 e220 2S tia ee 
6120-24 | August 28-31 {1,269.0} 550.0) 124.2 594. SLU s43) obs. eer eee 
702 Jan. 18, 1917 =|1,155.6} 761.6) 158.5 235.5) 5.90) 5.80/11. 6€] 9.35 
706 March 9 942 .5| 720.0} 111.0 112.5} 6.00) 3.72/15.2C)11.72 


195 


APPENDIX 


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APPENDIX 197 


TABLE 26. The quantity of dry organic matier in the total plankton 
of Lake Mendota during the different months of the year. The figures 
indicate the averages for the different months and cover the period 
from April, 1915, to June, 1917. The first part of the table gives the 
results for the deep water, that is, the area within the 20 meter con- 
tour line, and the second part shows the amount when the entire area 
of the lake is taken wnto account. The live weight of this material 
would be approximately ten times as large as the dry weights shown 1m 
this table. 


Deep Water Entire Lake 

Month Kilograms Pounds Kilograms Peunds 

per Hectare per Acre per Hectare per Acre 
JLo 3 456.5 407.3 251.6 224.4 
MEMIMAIY. 5 ss PAST er 230.1 141.4 126.1 
RPO ks es ee 325.8 290.8 179.6 160.2 
PMRUES ls ys ee od 505.5 451.0 278.7 248.6 
VLE 9 eo ae 462.0 412.1 254.6 aH oe | 
TEE Sat eee ae 414.0 369.3 228.0 203 .4 
July 406.3 362.4 223.9 199.7 
LES 315.6 281.5 174.0 15542 
September............ | 385.8 344.1 212.6 189.6 
MreOiels 2. eee. 468.2 417.6 258.0 230.1 
Piowemper......)..5.. 487 .2 434.6 268 .5 239.5 
Wecemper.. 2... 5. seas 521.5 465 .2 287 .5 256.4 


TABLE 27. This table shows the amount of organic matter in the 
net plankton and in the nannoplankton of Lake Mendota in the 0-13 
meter stratum and in the 14-20 meter stratum on August 7, 1915. The 
amounts are given in milligrams per cubic meter of water. 


i Stratum, Organic Matter, 
Sample £. Meters Mg. per Cu. M. 
Mesos, Wannoplankton. .....:.......... 0-13 1 ,686.4 
Hessol, Nannoplankton. .....2....4.6-.. 14-20 703 .6 
Newser, Wet plankton...:....0..05060.6- 0-13 264.2 
Wasser, Net plankton. <i: 0... 6. oc 6s. ees 14-20 22.3 


TABLE 28. Quantity of organic matter in the total plankton (net 
plankton plus nannoplankton) of the different strata of Lake Mendota 
on August 7, 1915. These estumates are based on the resulis shown 
wm table 27 and on the numerical data obtained in these catches. 


Stratum, Kilograms Pounds Per Cent of 
Meters per Hectare per Acre Total Amount 
0-5 W723 104.5 36.7 
0-10 214.6 191.4 67.3 
0-13 253 .6 226.2 79.5 
14-23 65.3 58.3 20.5 


0-23 318.9 284.5 100.0 


198 PLANKTON OF WISCONSIN LAKES 


TABLE 29. The maximum, minimum, and mean amounts of dry or- 
ganic matter obtained from the net plankton and from the nannoplank- 
ton of Lakes Monona and Waubesa. The results are stated in milli- 
grams per cubic meter of water. Compare with Lake Mendota, table 
6 and table 17. 


NET PLANKTON 


Lake Monona Lake Waubesa 
Number Number 
Year o Maxi- Mini- Mean of Maxi- | Mini- Mean 
Samples| mum mum Samples| mum mum 
1911 4 519.3 134.3 345.7 


Cs ss i er | 


ee eee eee lo ee eee sete eee se eae fe eee sae ee 


NANNOPLANKTON 
1915 8 1|3,746.5| 723.0 | 2,339.5, 4 | 6,143.2] 2,871.2] 4,260.2 
1916 | 13 . | 5,696.2] 672.1 | 2,355.6] 12 | 4,830.9] 801.0] 2,977.5 


TABLE 30. The range of variations in the percentage and in the 
quantity of mtrogen in the net plankton of Lake Monona. Compare 
with Lake Mendota, table 7. 


Per Cent, Ash Free Milligrams per Cubic Meter 

Year Numbe of Water 

oO - | COU Oa —s i sO, | rrr es 

Analyses | Maximum} Minimum | Mean | Maximum| Minimum | Mean 
1911 4 9.91 6.85 9.20 46.9 13.2 31.8 
1912 6 9.67 8.83 9.20 302.2 20.0 123.6 
1913 16 10.39 7.15 8.70 thera 24.6 73.0 
1915 8 11.71 | 8.09 | 10.71 237 .6 40.5 | 150.5 
1916 a 9.89 7.18 | 8.87 88.0 21.1 53.0 


TABLE 31. Variations in the percentage and sn the quantity of crude 
protein in the net plankton of Lake Monona. Compare with Lake Men- 
dota, table 8. (Crude protein equals N X 6.25.) 


Per Cent of Milligrams per Cubic Meter 
Number Organic Matter of Water 


———$§ — | | | ES 


Oo oe 
Analyses | Maximum | Minimum | Mean | Maximum]! Minimum|} Mean 


— | — | | | | | | 


1911 4 61.9 42.7 57.5 293 .1 112.5 198.8 
1912 6 60.4 55.2 ov .9 1,888 .7 125.0 772.3 
1913 16 65.0 44.7 54.4 985 .6 153.7 456 .2 
1915 8 73.2 62.6 67 .0 1,485 .0 253.1 


APPENDIX 199 


J 


TABLE 32. Variations in the percentage and in the quantity of ether 
extract in the net plankton of Lake Monona. Compare with the net 
plankton of Lake Mendota, table 10. 


Per Cent, Ash Free Milligrams per Cubic Meter 
Number of Water 
Year of —_——————_ | -—s )s Ss 
Analyses | Maximum | Minimum | Mean | Maximum] Minimum]! Mean 
1911 4 10.57 5.30 7.95 41.3 12.6 Zeiif 55 
1912 6 8.81 3.12 4.77 156.4 18.8 64.2 
1913 16 9.61 2.79 S202 143 .4 3.3 47 .2 
1915 a 7.95 4.20 6.38 171.8 o.2 68.8 
1916 2 11.16 8.09 | 10.15 142.2 69.0 | 105.6 


TABLE 33. A summary of the variations in the percentages of ash 
and of silica in the net plankton of Lake Monona. Compare with Lake 
Mendota, table 14. 


| Difference 
Per Cent of Ash Per Cent of Silica between 
Number|——————_|_——_ | —__|—____|—_—_-|—__ Means of 
Year of Maxi- | Mini- Mean | Maxi- | Mini- Mean | Ash and 
Analyses} mum mum mum mum Silica 
1911 4 i325 5.32 8.10 220% One 1.86 6.24 
1912 6 20.46 Eye 5) 10.75 13.82 ES 4.96 5.79 


1913 16 31.14 2228 Wi eee 20570 0.12 3.82 8.30 
1915 8 19.82 8.40 | 14.00; 14.17 1.47 5.06 8.94 
1916 13 33.19 9.76 | 2h-61 ) 27.05 HS2e 8.30 13.3 


TaBLeE 34. The range of the variations in the percentage and in the 
amount of nitrogen in the nannoplankton of Lake Monona. Compare 
with the net plankton, table 30, and with the nannoplankion of Lake 
Mendota, table 18. 


Per Cent, Ash Free Milligrams per Cubic Meter 
Number of Water 
Year of ————_ |] | a] ——_ 
Analyses | Maximum | Minimum | Mean | Maximum] Minimum | Mcan 
1915 8 11.28 5.92 10.10 412.5 44.1 236 .2 
1916 13 10.08 3.58 8.21 574.2 30.4 193.3 


TABLE 35. The range of the variations in the percentage and in the 
amount of crude protein in the nannoplankton of Lake Monona. These 
results are based on those given in table 34. Compare with the net 
plankton, table 31, and with the nannoplankion of Lake Mendota, table 


Per Cent, Ash Free Milligrams per Cubic Meter 
Number of Water 
Year of oe ee eee ee ee ee ee 
Samples | Maximum! Minimum| Mean | Maximum] Minimum|} Mean 


—<_| ——_—____ eS ee 
| | | 


1915 8 70.4 37.0 63.1 2019.0) 2t5.6 1 476.5 
1916 13 63.0 22.4 51.3 3,588.8 | 221.2 1 ,208 .2 


200 PLANKTON OF WISCONSIN LAKES 


TABLE 36. The range of the variations in the percentage and in the 
amount of ether extract in the nannoplankton of Lake Monona. Com- 
pare with the net plankton, table 32, and with the nannoplankton of 
Lake Mendota, table 20. 


Per Cent, Ash Free Milligrams per Cubic Meter 
Number of Water 
Year of |---| rr |__| 
Analyses | Maximum} Minimum! Mean | Maximum| Minimum | Mean 
1915 7 6.27 3.34 4.90 201.5 47.9 125.9 
1916 9 9.50 2.00 4.75 154.2 5178 95.5 


TABLE 37. The quantity of dry organic matter wm the total plank- 
ton of Lake Monona covering the period from May to November in 1915 
and 1916. The averages for the different months are shown as well as 
the mean for the entire series of 16 catches. The first part of the table 
gives the results for the deep water, that 1s, the area within the 20 meter 
contour line, and the second part shows the amount per umit of surface 
when the entire lake is considered. Compare with Lake Mendota, 
table 26. 


Deep Water Entire Lake 

Month Kulograms Pounds Kilograms Pounds 

per Hectare per Acre per Eectare per Acre 
Maya eee 398 355 160 143 
DUE: heparin 483 431 194 173 
Sully eae en 310 276 124 neta 
AUIGUSt Beene 563 502 226 202 
September... . 798 (12 320 285 
Octobers2 sere Pete? 1 ,063 478 426 
November.... 678 605 272 243 
Meantes.ten- 666 594 267 238 


TABLE 38. The range of variation in the percentage and in the 
quantity of nitrogen in the net plankton of Lake Waubesa. Compare 
with Lake Mendota, table 7, and with Lake Monona, table 30. 


Per Cent in Organic Milligrams per Cubic Meter 
Number Matter of Water 
Year of | | Se | | 
Analyses |Maximum| Minimum | Mean | Maximum| Minimum | Mean 
1913 2 8.00 7.44 8.00 162.0 139.0 150ea 
1915 4 8.12 5.14 7.45 343 .7 81.2 239.6 


1916 12 8.20 5.82 7.50 217.6 36.1 83.6 


APPENDIX S01 


TABLE 39. The range of variation in the percentage and in the quan- 
tity of crude protein in the net plankton of Lake Waubesa. Compare 
with Lake Mendota, table 8, and with Lake Monona, table 31. 


Per Cent of Crude Protein Milligrams of Crude Protein 
Number in Organic Matter per Cubic Meter of Water 
Year of SSS SS 
Analyses | Maximum! Minimum | Mean | Maximum| Minimum | Mean 


——————| EEE En 


1913 2 =>. 58.6 46.7 50.0 1,012.5 868 .0 940.2 
1915 4 50.7 32.1 46.6 2,148.1 507.5 1 497.8 
1916 12 o1.2 36.4 46 .9 1,360.0 225 ,6 522.5 


TABLE 40. The range of variation in the percentage and in the quan- 
tity of nitrogen in the nannoplankton of Lake Waubesa. Compare with 
Lake Mendota, table 18, and with Lake Monona, table 34. 


Per Cent of Nitrogen in Milligrams of Nitregen per 
Number Organic Matter Cubic Meter of Water 
Year of ee 


1915 4 10.00 7.10 9.03 555.5 217.9 3 
1916 12 9.27 5.36 7.41 448 .0 54.6 Z 


TABLE 41. The range of variation in the percentage and in the quan- 
tity of crude protein in the nannoplankton of Lake Waubesa. Compare 
with Lake Mendota, table 19, and with Lake Monona, table 35. 


Per Cent of Crude Protein Milligrams of Crude Protein 
os ot in the Organic Matter per Cubic Meter of Water 
ear ) a 


| — | ee | | | 


1915 4 62.5 44.4 56.4 3,471.9 1 361.9 
1916 12 57.9 33.5 46.3 2 ,800.0 341.2 


2 404.4 
378.1 


TaBLE 42. The range of variatien in the percentage and in the quan- 
tity of ether extract in the nannoplankton of Lake Waubesa. Compare 
with Lake Mendota, table 20, and with Lake Monona, table 36. 


Per Cent of Ether Extract | Milligrams of Ether Extract 
Number in the Organic Matter per Cubic Meter of Water 
ear of ES ERs De Sear ee ees ee eee ee eee ee 


1915 4 5.28 Sea 4.61 308.0 102.7 193.0 
1916 8 8.13 1.48 4.68 134.7 61.5 85.1 


PLANKTON OF WISCONSIN LAKES 


202 


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203 


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211 


APPENDIX 


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213 


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PLANKTON OF WISCONSIN LAKES 


214 


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215 


APPENDIX 


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APPENDIX 917 


TasLE 50. This table shows some of the inorganic constituents of 


the ash as determined for eighteen samples. 
percentages of the dry weight of the sample. 


Lake 


Monona. . 


Mendota. 
Devils ... 


Monona.. 
Mendota. 
Mendota. 
Mendota. 
Green.... 
Devils ... 
Monona. . 
Waubesa. 
Monona. . 
Mendota. 
Mendota. 
Mendota. 
Mendota. 
Mendota. 
Mendota. 


Date 


VII 11, 1914 


IX 19, 1914 
X 8, 1913 


VII 6, 1916 
VI 15, 1916 
VII 15, 1917 
XII 10, 1918 
VII 22, 1913 
V 15, 1916 
IV 4, 1914 

V 10, 1917 
VIII 5, 1913 
VII 28, 1915 
IX 1, 1915 
VIII 28, 1915 
VII-IX, 1914 
IV 30, 1914 
VII-IX, 1916 


The results are stated in 
See table 49 also. 


Organism CaO | MgO 
Microcystis 0.92) 0.63 
Anabaena 1.42) 0.70 
Anabaena and 
Coelosphaerium | 4.66) 0.27/.....]..... 0.82) 0.63 
Volvox 1.10) 0.93 
Cladophora 3.26] 1.62 
Myriophyllum 4.28) 1.34 
Cy Clops ee hata esse o 0.78) 0.75 
Limnocalanus 0.53) 0.42 
Daphnia pulex 9.89} 0.49 
Daphnia pulex 2.25) 0.89 
Daphnia pulex 2.01) 0.47 
Leptodora 3.22) 0.95 
Cambarus 63|17 .82| 0.57 
Hyalella 7314.82) 0.35 
Hirudinea 73| 1.00) 0.70 
Zygoptera 50) 0.84) 0.03 
Sialis 46} 0.16) 1.12 
Chironomus tentans} 5.14) 0.32)..... 10} 1.30} 0.46 


218 PLANKTON OF WISCONSIN LAKES 


TABLE 51. Results of analyses of marine and fresh-water organisms 
as given by other investigators. The sources of the data are indicated 
in the first column of the table. The results are stated in percentages 
of the dry weight of the material. All of the samples taken from 
Brandt, except one, consisted of marine organisms; the series num- 
bered II to X were catches of marine net plankton and contained vari- 
ous kinds of orgamsms. Brandt considered the amount of nitrogen in 
protein as 15.61 per cent and this gave him a protein factor of approxi- 
mately 6.41 instead of 6.25; his data for crude protein, therefore, are 
somewhat higher than those given in this table. 

The other samples shown in table 51, as well as one of Brandt’ S, CON- 
sisted of freshwater forms. Volk gives no data for mtrogen in the two 
analyses recorded by him; instead of protein he designates this material 
as ‘‘muscle and other tissue.’’ Apparently he determined the ether 
extract, crude fiber, and ash, and listed the remainder as ‘‘muscle and 
other tissue.’? The latter, therefore, includes the crude protein and 
the nitrogen free extract; an asterisk has been used in the table to call 
attention to this fact. 


Nitro-| Crude | Ether | Crude 
Authority Organism gen | Protein | Ex- | Fiber | Ash |Silica 
(N X6.25)} tract 


| Ss | | | 


Whipple and | Microcystis 


Jackson! (Clathrocystis) $.30|0 SUL Siiale sae ae ec aeel eee e 
Anabaena, 9.60 |} 60.00 
Asterionella Be DOG Seo vee eee eienege 57.52/49 .48 
Spirogyra 4. BO 2812 ol. cock coh See 
Hyams and _ | Oscillatoria, young O00 SG 2257 eco ee eee 4.50] 1.46 
Richards? | Oscillatoria, fully grown] 7.90 | 49.88 |.......J]...... 6.40} 2.90 
Turner? Oscillatoria S.1Oe)), ok t19 2 AS low te € SOG ae 
Brandt! Peridinians (Chiefly 
Ceratium) 2.03 12.68 1.30 [41.50 | 5.20). 2. .: 
Diatoms (Chiefly 
Chaetoceros) 1.56 9.75 fis hal lapeae eh 65 .20/54.50 
Freshwater Copepoda | 9.16 | 57.25 6:01 | 4.54.) 9320) ee 
Marine Copepoda 9.187) 257.87 AQUI Senor 10921025: 
Marine net plankton 
II 3.41 | 20.06 VA al eae as 9.94) 4.95 
III 3.16 | 19.75 2 AGUS. 8.55] 4.59 
IV 3.28 | 20.50 eka ieeaaces 15.71) 9.59 
V 3.14 | 19.62 A Sy Soros 29.68/16 .33 
VI 1.80 | T1.25 Dea See 60.08/47 .16 
Vil 2.438 | 15.18 2 Pet 61.41/51.26 
Vill 5.61 | 35.06 150 4 ie ee ae 38 .77|27 .00 
IX 5.24} 32.75 oan ale eae 2 19.41|10.95 
x 3.19 | 19.94 BeOS Sas 36.14/26.40 
Volk Burytemora: i Rie oe (8.4¢7* 4. 6.20 | 11.07) 4.24). oo. 
Bosman i ey Tet ites oe 7.82" |: 10/66 |S. Si So 2a0h ae 


Jour. N. E. Waterworks Assoc., Vol. 14, 1899, pp. 17-18. 

Technology Quarterly, Vol. 17, 1904, p. 275. 

Jour. Amer. Chem. Soce., Vol. 38, 1916, p. 1402. 

Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 3, 1898, pp. 45-90. 
Verhand. d. Naturwis. Vereins in Hamburg, 3 Folge, XV, 1907, p. 45. 


ON HS 9 £9 po 


—— ——— 


APPENDIX 219 


TABLE 92. This table shows the length of radius required in the 
spherical type of curve to represent numbers ranging from one mndiwid- 
ual up to 864 million. The table is based on a value of one indwidual 
equals a radius length of 0.25 millimeter. The length of the radius for 
any number 1s determined by diwiding the number by four, extracting 
the cube root of the quotient, and then multiplying the cube root by 
the value selected for one indwidual. Let the number be 6,912, for 
example; dwiding it by four gives 1,728, of which the cube root is 12, 
and multiplying the latter by 0.25, the umt of value selected for this 
table, gives a radius of 3.0 millimeters. The construction of the curve 
is discussed on page 58. This table is taken from Lohmann’s report 
on nannoplankton (Wissensch. Meeresuntersuch., Abt. Kiel, N. F., Bd. 
3, 1908, p. 361). 

The original diagrams used for figures 22 to 26 and figures 
32 and 33 are based on a value of one individual equals a radius length 
of 0.25 millimeter, but the original diagrams have been reduced to one 
quarter size in reproducing them. 


Length of Number of Length of Number of 
Radius Individuals Radius Individuals 
0.25 mm i 16.0 mm 1 ,048 ,576 
0.5 32 17.0 1 ,257 ,728 
1.0 256 18.0 1 ,492 ,992 
1.5 864 19.0 1 ,755 ,904 
2.0 2 ,048 20.0 2 ,048 ,000 
2.5 4 ,000 25.0 4 ,000 ,000 
3.0 6 ,912 30.0 6 ,912 ,000 
3.5 10 ,976 35.0 10 ,976 ,000 
4.0 16 ,384 40.0 16 ,384 ,000 
4.5 23 ,328 45.0 23 ,328 ,000 
5.0 32 ,000 50.0 32 ,000 ,000 
5.5 42 ,592 55 .0 42 ,592 ,000 
6.0 55 ,296 60.0 55 ,296 ,000 
6.5 70 ,304 65.0 70 ,304 ,000 
7.0 87 ,808 70.0 87 ,808 ,000 
(eu 108 ,000 75.0 108 ,000 ,000 
8.0 131 ,072 80.0 131 ,072 ,000 
8.5 157 ,216 85.0 157 ,216 ,000 
9.0. 186 ,624 90.0 186 ,624 ,000 
9.5 219 ,488 95.0 219 ,488 ,000 
10.0 256 ,000 100.0 256 ,000,000 
11.0 340 ,736 110.0 340 ,736 ,000 
12.0 442 368 120.0 442 ,368 ,000 
13.0 562 ,432 130.0 562 ,432 ,000 
14.0 702 ,464 140.0 702 ,464 ,000 
15.0 864 ,000 150.0 864 ,000 ,000 


INDEX 


Alumina, 53, 128 Curve, spherical, 58 
Amoeba, number, 84 Cyclops, analysis of, 166, 215 
Anabaena, analysis of, 159, 215 number of, 129 

number of, 136 weight of, 155 


Anisoptera, analysis of, 174, 215 
Ankistrodesmus, analysis of, 161, 215 
Annual cycle, 1 
of plankton, 27, 30, 69, 105, 113, 126, 
130 


Daphnia, analysis of, 168, 170, 215 
number of, 111, 129 
weight of, 155 

Determinations, chemical, number, viii 


Anuraea, number of, 129 ; : 
Aphanizomenon, analysis of, 159, 215 naseeae an of, 166, 215 


istributi f, 85 
Aphanocapsa, distribution of, weight of, 155 


9, 136 
eee oe tae Diatoms, analysis of, 163, 215 
: Discussion of results, 150 


Apstein, on ash determinations, 54 - 
fe Blount of, 150 Dissolved oxygen, 2, 3, 20, 22, 156 


of nannoplankton, 80, 83, 117, 135 
of various organisms, 179 ~—-—~=«| Ephemera, analysis of, 174, 215 
8 , Epilimnion, 1, 3, 20, 22, 156 
Asplanchna, weight of, 155 Fither cree Ba 
5] / 
in nannoplankton, 75, 116, 134 
in net plankton, 40, 107, 127, 140 


i le of, 88 
Bacteria, role of, Eurytemora, analysis of, 167 


Bradley, 53 
Brandt, 55, 162, 164, 167, 178 
Bureau of Fisheries, U. S., ix. Food and feeders, 4, 155 


Food for planktonts, 155 
Food relations, 4, 155 


ps 128 Fragilaria, number of, 56, 111 


Cambarus, analysis of, 171, 215 
Carbohydrates, 42 


Carbon dioxide, 2 Grasshoppers, analysis of, 176 
Centrifuge, 12 Gyrinidae, analysis of, 175, 215 
bowl of, 14 Hemiptera, analysis of, 176, 215 


effect of, 64 
efficiency of, 14 


Ceratium, analysis of, 162 Hensen, 7, 8, 55 

number of, 56, 129 Hirudinea, analysis of, 173, 215 
Chironomus, analysis of, 174, 215 Holopedium, analysis of, 170, 215 
Chlorochromonas, number of, 85, 135 Hyalella, analysis of, 172, 215 
Chydorus, number of, 129 Hyams and Richards, 161 
Ciliate, anaerobic, 22 Hypolimnion, 1, 3, 22, 156 


Circulation of water, 2 

Cladophora, analysis of, 162, 215 

Clarke and Salkover, 179 

Constants of ether extract, 168 

Corethra, analysis of, 174, 215 - 

Gigs ‘fiber, oe te a,’ 43 ) June bugs, analysis of, 176 
in nannoplankton, 79, 117, 134 


Iron, 53, 128 


in net plankton, 108, 127 Laboratory, sketch of, 13 
Crude protein, 5 Lachnosterna, analysis of, 176 
in nannoplankton, 74, 114, 132 Leeches, analysis of, 173 
in net plankton, 32, 35, 107, 126, 139 Leptodora, analysis of, 171, 215 
ratio to organic matter, 115 Lifetime of organisms, 153, 156 
Crustacea, economic importance, 154 Limnocalanus, analysis of, 166, 215 
food for, 155 Lohmann, 9, 10, 58, 62, 90 


Cryptomonas, number of, 85, 136 Lyngbya, analysis of, 159, 215 


222 


McHargue, 176 

Magnesium, 54, 128 

Manganese, 53 

Melanoplus, analysis of, 176 

Melosira, number of, 56, 111, 129 

Mesolimnion, 1, 22, 156 

Methods, 8, 16 

Microcystis, analysis of, 159, 215 
number, 111, 129, 136 

Mollusks, 154 

Monads, number of, 118 

Moore et al., 96 

Myriophyllum, analysis of, 165, 215 


Nannoplankton, defined, 63 
in Mendota, 63 
in Monona, 112 
in Waubesa, 130 
organisms in, 83, 118, 135 
ratio to net plankton, 96 
summary of, 146 _ 

Net, description of, 10 

Net plankton, 20 
in Kegonsa, 139 
in Mendota, 25 
in Monona, 104 
in Waubesa, 124 
organisms in, 55, 110, 128 
ratio to nannoplankton, 96 
summary of, 142 

Nitrogen, forms of, 40 
in nannoplankton, 70, 114, 131 
in net plankton, 31, 105, 126, 139 
in water, 73 


ratio to organic matter, 39, 71, 106, 


114, 126, 132 
Nitrogen free extract, defined, 80 
in nannoplankton, 80, 117, 134 
in net plankton, 44, 109, 127 


Oligochaeta, analysis of, 173, 215 
Organic matter, defined, 16 

in nannoplankton, 67, 112, 130 

in net plankton, 25, 104, 124, 139 


ratio to nannoplankton, 96, 120, 138 


in sea water, 96 

in total plankton, 97, 121 

live weight of, 121, 123 

per unit of area, 98, 122, 139 


ratio to nitrogen, 39, 71, 106, 114, 126, 


132 

Organisms, analyses of, 158, 215 
in nannoplankton, 83, 118, 135 
in net plankton, 25, 55, 110, 128 
overlapping of, 100 
weight of, 57, 155 

Oscillatoria, analysis of, 161 
number of, 119 

Oxygen. See dissolved oxygen 


INDEX 


Pentosans, 16 
in nannoplankton, 79, 117, 134 
in net plankton, 43, 108, 127 
Peridinians, analysis of, 162 
Phosphorus, 53, 128 
Photosynthesis, 3 
Plankton, defined, 7 
losses of, 153 
production of, 4, 17, 151, 156, 157 
quantity of, 3, 23 
standing crop of, 4, 151 
turnover of, 5, 151, 154, 156 
variations in, 27 


see nannoplankton, net plankton, and 


total plankton : 
Potamogeton, analysis of, 164, 215 
Protein, see crude protein 
Putter, 73 
Pumps, 9 
Purpose of investigation, 17 


Reproduction, rate of, 152, 156 
Rotifers, food for, 155 


Schroederia, number of, 119 
Schuette, viii, 16, 31, 40, 42, 54, 168 
Sialis, analysis of, 174, 215 
Silica, 52, 109, 128 

Silt, 82, 118 

Size of organisms, 89, 153 
Spirogyra, analysis of, 162, 215 
Stephanodiscus, number of, 86, 129 
Stratification, thermal, 2, 20 
Sulphur, 54 

Summary of results, 141 


Temperature of water, 23 
Thermocline. See mesolimnion 
Total plankton, defined, 92 
chemical composition of, 97 
mean quantity of, 97 
per unit area, 98, 102, 122, 139 
quantity of, 92, 119, 137, 148 
Turner, 161 


Vallisneria, analysis of, 165, 215 

Volk, 167, 171 

Volvox, analysis of, 162, 215 
number of, 111 


Water, nitrogen in, 73 
quantity of, 23 
temperature of, 23 
Water bloom, 18 
Weight of organisms, 57, 155 
Whipple and Jackson, 58, 161, 162, 


Zygoptera, analysis of, 174, 215 


164 


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