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II. THE CRUSTACEA OF THE’ PLANKTON, JULY, 1894—DEC., 1896./ , £2 $4 r ERIS Fim. ie iy Ly, akg Reef EADY eg WR Louie Abe BY ~ fa Le BE x ae é i LON ae . A. BIRGE, Pu. D., Sc. D., AU On y a Professor of Zoology, University of Wisconsin. Cry (2 | hcp, REPRINTED FROM THE TRANSACTIONS OF THE WISCONSIN ACADEMY OF SCIENCES, ARTS, AND LETTERS, VOL. XI., PP. 274-448, } Qn v v dhalgia'S LON WITH THE ADDITION OF TITLE PAGE AND INDEX. [Issued December, 1897.] TABLE OF CONTENTS: Introduction, 274. Coefficient of net, 278. Temperature: Methods, 286. Results, Winter, 289. Spring, 291. Summer, 293. The Thermocline, 295. Autumn, 299. Annual distribution of the crustacea: General account, 301. Winter, 305. Spring, 307. Summer, 309. Autumn, 311. Table of crustacea, 313. Order of succession of leading species, 316. Largest numbers per cubic meter, 318. The species in detail: Diaptomus, 319. Cyclops, 326. EH pischura, 332. . Ergasilus, 333. ‘Nauplii, 333, Daphnia hyalina, 335. D. pulicaria, 340. D. retrocurva, 345. Chydorus, 348. Leptodora, 350. Factors determining numbers of crustacea Food, 352. Temperature, 358. Competition, 365. Horizontal distribution; swarms, 366. Vertical distribution, 375. Winter, 378. Spring, 380. Summer, 382. Autumn, 336. Vertical distribution of individual species, 391. Diaptomus, 394. Diaphanosoma, 347. Vertical distribution of: Cyclops, 395. Daphnia hyalina, 397. D. pulicaria, 399. D. retrocurva, 402. Diaphanosoma, 403. Chydorus, 404. Leptodora, 404, Nauplii, 405. Distribution in the upper meter; diurnal movement, 407. Distribution at the thermocline, 415. Factors determining vertical distribution: Food, 419. Temperature, 421. Chemical condition of water, 423. Light, 425. Wind, 427. Gravitation, 429. Age, 431. Specific peculiarities, 432. Literature, 483. List of plates, 435. Statistical tables: Dates of collections, 436. Average number of crustacea per cub meter, 437. Average number and percentile vertical distribution, 438. Diaptomus, numbers and vertical distri- bution, 439. Cyclops, numbers and vertical distribu- tion, 441. Daphnia hyalina, numbers and vertical distribution, 442. D. pulicaria, numbers and vertical dis- tribution, 444. D. retrocurva, numbers and vertical dis- tribution, 445. Diaphanosoma, numbers and vertical distribution, 445. Chydorus, numbers and vertical distri- bution, 446. Index, 447. PLANKTON STUDIES ON LAKE MENDOTA. II. THE CRUSTACEA OF THE PLANKTON FROM JULY, 1894, TO DECEMBER, 1896. E. A. BIRGE, Professor of Zoology, University of Wisconsin. INTRODUCTION. The following paper is a continuation of the work done by myself with Messrs. Olson and Harder, in the summer of 1894, published in the preceding volume of the Transactions of this Academy. (Birge, ’95.) The study carried on in that month showed a vertical distribution of the crustacea so unexpected and peculiar that it seemed to me worth while to continue the investigation throughout an entire year. A few observations were made in the latter part of August, 1894, and on Septem- ber 18th, regular observations were begun and were continued until the close of December, 1896. During the fall of 1894 ob- servations were taken on 28 days. In 1895 observations were taken on 110 days, and on 126 in 1896. The details of the number of observations and of the days on which they were taken will be found stated in Table A given at the close of this paper. During the late spring and summer months as many as three observations per week were taken. During the winter season, the late fall and early spring, observations were necessarily fewer in number, and occasionally a period of two weeks would pass without an observation. At this time of the year, however, the crustacea are not varying greatly in num- ber, so that small error results from these gaps. I had intended at first to carry my observations through one year only, but as a peculiar annual development of the crusta- Introductory. 2795 cea was found in the course of the year 1895, it seemed to me advisable to continue the observations through the season of 1896, in order to determine whether the course of develop- ment would be the same as in 1895. Until August, 1896, the number of the crustacea in each catch was determined separately, and the average catch for each two-week period was computed. After that date the catches for each two-week period were mingled together, and the average number only was determined. Up to August, 1896, therefore, the average, maximum, and min- imum catches are given for each period, in the tables of the appendix, but after that date it is possible to state the aver- ages only. This “two-week average” is the main number used in this paper. The net employed was that described by me in my former paper, and the method of counting was substantially the same, except that asmailer fraction than one-sixth was often used to determine the large number of crustacea from the upper levels of the jake—one-tenth to one-fifteenth being ordinarily employed, with a view to making the last figure of the resulting number 5 or 0, in order to facilitate adding and multiplying in subse- quent operations. The multiplications to reduce the catch to the number per square meter of surface were performed by the aid of Crelle’s Tables. The products are stated in this paper in thousands and tenths, in order to avoid the constant use of ciphers in the last two places. The result would have been quite as accurately ex- pressed in most cases if the nearest thousand had been stated, but in case of the smaller numbers it was necessary to state the hundreds, and as the products were read off directly in all cases in hundreds, I concluded to leave them in the printed results, although, of course, understanding that no reliance is to be placed on the exactness of the enumeration in the last place of figures if the total is large. The total number of serial observations was 333 besides 97 single catches, and as there were at least six collections in each series, and from three to eleven species of crustacea to be deter- mined, the number of single observations is very large — over 10,000. It has been my aim in preparing this paper to exhibit 276 Birge—The Crustacea of the Plankton. these results in agraphic form so that they might appeal to the eye, and to print only the summaries of my observations; rather than to confuse the reader by presenting him with the great mass of figures which would be needed to exhibit the results of the single observations. In preparing the diagrams which accompany this paper, the average number of crustacea for each two-week period was determined and was platted at the center of the space rep- resenting the period; and the averages of successive periods connected by a line. It has been found impossible to use the same scale in platting the annual distribution of the different species of the crustacea. Where numbers range from less than 25,000 to over 3,000,000 per square meter, it is not practicable to use the same scale for all species. The scales employed range from 25,000 to one ver- tical space, to 200,000 for the same distance. In all cases the scale is stated on the margin of the diagram. No attempt is made to show by a curve the rate of variation within the two- week period, since this variation is quite too irregular to per- mit a curve to be drawn with any accuracy. I had intended to introduce this paper by a preliminary account of lake Mendota accompanied by a hydrographic map. Some hundreds of soundings have been made by myself and by the De- partment of Civil Engineering of the University of Wisconsin, but the preparation of the map has been delayed, and it is there- fore impossible to insert the account at this place. I must therefore refer to the brief account given in my former paper, merely stating here that the lake is about 6 miles (9 kilometers) in length by 4 miles (6 kilometers) in greatest breadth, of a somewhat regular shape. No greater depth than 24 meters has been found; a large part of the lake is deeper than 18 me- ters, and the bottom is very flat without irregular depressions. The principal observing station was near the southern side of the lake, about 2,700 feet (850 meters, from te southern shore, and in 18.5 meters of water. The second principal station was about a mile and a half (2 kilometers) from the southern shore, and in 22 meters. The principal station was marked bya buoy, so that the observations were taken at the same spot. Introductory. 277 During the winter observations were made through the ice, the net being suspended from a tripod. While it is very easy to make a single haul of the net at any temperature in the win- ter, it is very difficult to make a series if the temperature is ma- terially below—6° C. At lower temperatures, or even at this temperature on a cloudy day and with northerly wind, the net freezes so rapidly that work is extremely difficult and slow, as time must be taken for the net to thaw in the water before a second haul can be made. The line also becomes so heavily coated with ice and so slippery and stiff that it is impossible to secure accuracy in the time of raising the net. While there- fore the pleasant warm days of winter offer the best possible oc- casions for working the dredge, the average work in winter is extremely disagreeable. It is, however, more difficult to secure continuous observations during the periods immediately pre- ceding the formation and the breaking up of the ice than it is in winter. The lake freezes near the shore so that it is difficult to get out with a boat, while the ice is still too thin to bear the weight of a man; and as there is no current in the lake, the breaking up of the ice in the spring is ordinarily very slow and there is always a number of days in which the ice is too weak for safety. After the breaking up of the ice a continua- tion of north winds may keep the sludge ice on the southern shore, and thus still further delay observations, as was the case in 1896. In carrying out this work it has been my endeavor to make a contribution to the natural history of an inland lake as “a unit of environment,” to employ Eigenmann’s appropriate phrase. (Higenmann ’95, p. 204.) I have, therefore, discussed somewhat freely the causes which seem to me to have contrib- uted to the peculiarities of the annual and vertical distribution of the crustacea. I do not suppose that my conciusions are cor- rect in all particulars, still less that they are compiete. The causes determining the biological conditions of a lake are far too numerous and various, and their inter-relations far too complex. to be understood at present with any accuracy. It has seemed to me, however, that the aim of plankton investi- gations should be to reach an understanding of these conditions, 278 Birge—The Crustacea of the Plankton. and I have therefore put out the suggestions of the final sec- tions of each part of my paper, with the hope that they will stimulate others to similar attempts and thus lead to an en- largement of our knowledge and to the correction of whatever errors may be present in my conclusions. THE COEFFICIENT OF THE DREDGE. One of the most difficult and unsatisfactory portions of plank- ton investigation has been the determination of the coefficient of the dredge. It is well known that the net when raised through the water offers a certain resistance to the passage of the water, so that a part only is filtered by the net, while an- other fraction is displaced. The determination of the relative amounts of water filtered and displaced is the determination of the coefficient of the dredge. Many attempts have been made to determine this quantity. The most elaborate investigations have been made by Hensen (Hensen, ’87, p. 11, and Appendix; 95, pp. 67-86). Reighard (94, p. 57) has also devised and car- ried out another method of determining the coefficient. Hensen has attempted to work out a formula by which the coefficient for a net of given cloth and given area could be determined, and has finally given the best and easiest method of determining the coefficient in lakes abounding in vegetable plankton (95, p. 92). Reighard’s method depends upon mixing with the water a known number of particles and determining the relation between those caught by the net when drawn through the water and the num- ber known to be present. This method was entirely inappli- cable to a net constructed like mine, and it was impossible for me to enter upon any elaborate investigation of the coefficients of the cloth which I used. I confined myself, therefore, to a de- termination of the coefficient of my net under the conditions in which it was used. In the serial investigations which formed the greater and more essential part of my study, the dredge was raised through a distance of three meters. The speed was approximately one half meter per sevond, although ordinarily a little less, the total time occupied by raising the dredge t aro 3 meters, being from 6.5 to 6.75 seconds. In order to ascertain the coefficient of the dredge I determined to ascertain the num- The Coefficient of the Dredge. 279 ber of crustacea in a column of water 3 m. in length and 10 cm. in diameter and to compare with this number the catch of the net. Tor this purpose a tin tube was made, of the size indi- cated. This tube was provided at the lower end with a slide in which was placed a carrier bearing a net and bucket. The car- rier and net could be slipped to one side so as to leave the open- ing of the tube entirely free, and by means of a cord reaching to the surface, they could be drawn back so as to hang immedi- ately below the opening of the tube. The slide and carrier were made of brass plates carefully scraped and fitted together, so that no crustacea could escape between the bottom of the tube and the top of the net, and the net was closely covered when slipped to the side of the tube. The tube was lowered into the water with the net moved to one side of the opening and was lowered slowly so that the water within the tube might remain at the same level as that without and no appreciable currents should be set up ‘in the water. The tube was also provided with a close fitting cap on the top, which could be closed after the top of the tube had sunk about one-half meter below the surface. When the tube had been lowered this cap was closed and the slide with the net drawn across the bottom of the tube. There was thus im- prisoned a column of water 10 cm. in diameter and 3 m. long. The tube was then slowly raised to the surface and lifted out of the water so that the contained water might be filtered through the net, leaving behind the plankton. Several successive hauls of the tube were made, and the number of crustacea so taken was compared with that obtained from a similar number of hauls of the net made at the same time and through the same dis- tance. The number of crustacea thus obtained was carefully determined, 7 to 7; of the number being counted where _ the number was great, and + where the number was small. In determining the coefficient of the dredge, it was assumed that the tube took all of the plankton in the column of water which it contained, and the number of crustacea caught by the tube was compared with that caught by the net. Since the opening of the net was four times that of the tube the catch ought to have been four times.as great, provided all of the water was fil- 280 Birge—The Crustacea of the Plankton. tered. Asa matter of fact, the net caught about twice as many crustacea as the tube, thus indicating that its coefficient is about two. In this method of determining the coefficient the quantities compared are by no means uniform; indeed, it is known that the number of crustacea caught in a given haul of the tube may be only one-half the number caught in a second haul within a few seconds. A single comparison has therefore very little value and accuracy in the determination of the coefficient by this method can be reached only by a considerable number of ob- | servations. In my own work I made use of six sets of obser- vations, taken on May 14th, October 12th and 25th, 1895, Feb- ruary 25, May 18th, and July 11th, 1896. By distributing the observations over so long a time it was possible to get at the coefficient of the net at different times in its life and under different conditions of plankton. In May the number of crus- tacea is at a maximum, and the amount of algae is small. In October the number of crustacea is considerable, but the veget- able life is at a maximum; while in February the amount both of animal and vegetable life is of course small. From four to six pairs of observations were taken in each set. The ratio of the catch of the tube to that of the net was computed for each observation in the set, and the average of these ratios was com- puted, using the method of least squares. As a result of these determinations, the following ratio was established : Tube : net :: 49.85 : 100. The probabie error of the deter- mination is +1. The appended table shows the general results Several facts appear from the table. It will be noticed that the amount of difference between the maximum and mini- mum numbers caught varies greatly on different occasions. It is plain also that the net shows no greater amount of variation on the whole than does the tube. On the contrary, on those occasions where the numbers are approximately constant in the tube, they are similarly constant in the case of the net; and where the numbers vary considerably in the case of the net, they vary to much the same degree in the case of the tube. There is therefore no reason to suspect any considerable irregu- larity on the part of the net due to the stoppage of its openings, or to any other cause. H The Coefficient of the Dredge. 281 TaBLE I.—fesults of determination of coefficient of net. oa Pairs wei, of Counte a CATCH OF TUBB. CATCH oF NET, catches. | ratios. |of catch. Make Min. Max. Min. 1895, May 14....... 4 16 1-10 2,910 2,400 4, 760 2, 920 Oct, 12..:... 4 16 1-5 1, 482 1,170 2, 292 1,770 Cet 25... 65. 6 36 1-10 8, 490° 4,290 14,520 10, 560 1896, Feb.25..... 5 25 1-4 1, 420 760 3, 500 1,750 May 18....... 5 25 1-10 5, 940 4,310 12, 100 10, 480 duly It. ).: 5 25 1-15 4,215 2, 480 8,370 5, 680 Total........ aaa A gash Minimum Ratio; Tube : net :: 21 : 100. Maximum Ratio; Tube : net :: 100 : 100. Average Ratio; Tube : net :: 49.85 +1 : 100. Area of opening of tube : area of mouth of net :: 1: 4. Hence coefficient of net = 2, approximately. Area of opening of net = 314.1 sq. cm. Hence to state catch of net in terms of sq. meter of surface, multiply catch by aX 2= catch X 63.6, which factor was used. In determining the number of crustacea caught by tube or net, each species was counted separately. The individual species show just about the same amount of variation as does the total catch; although in the case of less abundant species the maxi- mum number caught was not infrequently three times the mini- mum. In the case of the tube no difference could be detected in the range of variation of the numbers of species which are active, like Diaptomus, and those which, like Chydorus, or Cy- clops, are relatively slow in their movements. During the sum- mer of 1896 an attempt was made to determine the coefficient of the dredge from the number of spherules of Glototrichia, but as this plant is found mainly in the uppermost strata of the water on calm days, it proved an unsuitable object, and its variations in number in successive catches were greater than those of the crustacea. | It may be added that there was no constant position of maxi- mum or minimum catch in any series which was made, but the numbers varied in a wholly irregular fashion. 282 Birge—The Crustacea of the Plankton. In all of the work reported in this paper and done before the 1lth of July, 1896, a single net was employed. After that date the net was replaced by one of silk bolting cloth, number 16, containing about 3600 meshes to the square cm. This net was cut from the same pattern as the old one. In order to compare the two nets they were similarly mounted in the same frame, and a series of comparisons made to determine their relative coefficient. To my surprise the two nets showed practically the same co- efficient. The numbers caught necessarily varied considerably, but the average of each of two series of five pairs showed prac- tically the same number of crustacea; the silk net catching on the whole about 5 per cent. less than the old net. It did not seem necessary therefore to alter the coefficient of the dredge with the change of the net. On the 20th of August the dredge, with all its appurtenances, was lost by the accidental breaking of the line, and the work for the remainder of the year was done with a similar instrument of smaller size, having a square opening of 100 square cm. The coefficient of this net was de- termined by comparing it with the tube, one set of comparisons being made by determining the number of the crustacea. A second set was made by determining the bulk of the plankton caught by the tube and net when allowed to settle for the same length of time in similar tubes. Two other determinations were made by Hensen’s last method. (Hensen, 95, p. 92.) The net was fitted with a cover having an opening of 2.5 square cm. Ten successive hauls of the net were made with the small open- ing and their contents mingled. This was preserved and allowed to settle and compared with the amount of plankton caught with the full opening of the net, the two quantities being similarly preserved and allowed to settle in similar tubes. The result of these three methods of determination of the coefficient of the net was substantially identical, the coefficient varying from 1.81 to 2.04. The coefficient 1.9 was selected, and as a result the catch of this net is multiplied by 190 in order to give the num- ber of crustacea per square meter of surface area. An important question has been raised, first by Hensen (’87, p. 12) and especially by Kofoid (’97, p. 11) regarding the vari- The Coefficient of the Dredge. 253 ation in the coefficient of the net due to the accumulation of the plankton within it as the net is drawn through the water. Un- questionably the stoppage of the openings of the net by the accumulating catch raises the coefficient, and if the net accum- ulates a sufficient amount of plankton it will wholly cease fil- tering the water. In plankton—rich lakes, therefore, serious error may be introduced from this source. Since lake Mendota during the summer and autumn contains very large amounts of vegetable plankton, it was quite possible that the stoppage of the net should causé errors. In order to determine whether these errors existed, I regularly made hauls of the net from the bottom of the lake to the surface during the season of 1895 and compared the number of crustacea obtained in the hauls from the bottom with the sum of those caught in the six suc- ‘cessive levels of my series. I1 append a table showing the num- ber of Cyclops caught in the months from January to July, 1895, in order to compare the series and the single haul. It will be seen that the number of Cyclops varies, often considerably. ‘Out of 41 cases prior to July 1, the total haul exceeded the sum of the series in 24 cases and fell below it in 17 cases. There was thus no decided advantage on the side either of the series or the single haul. If the amount of variation in this table be compared with the amount shown in the catches of the tube in Table I, it will be seen that the differences are of much the same order as those disclosed by the tube. There is therefore no evidence that under ,these ‘circumstances the net suffered any stoppage in passing through the 18 meters of the lake which altered its coefficient to any marked degree over that of the net used through 3 meters. After the first of July Anabaena and similar small plants de- veloped rapidly in the lake, and the amount of vegetable plank- ton increased to a great amount. Under these circumstances the number of crustacea caught in the total haul varied widely and irregularly from the sum of the series, and soon became uniformly lower than the sum. It was found therefore that the coefficient of the net has been raised by the amount of algae _ present and the catches made by the total hauls were not em- ployed in reckoning the number of the crustacea after the first 284 Birge—The Crustacea of the Plankton. TaBLE IIl.— Showing the number of Cyclops caught by the net at the same date and place in a series of six hauls of 3m. each, and ina single haul of 18 m. ——$_—. Sum of Date. series. 1895. Jan. 6.. 400 Jan. 9.. 378 PAW MG eee edon ace 505 Feb ras. 2.4 870 MODs Bowe eos cakes 2,350 MORE 62 wos cows owes 345 Meh Tiles e sete oes 678 Meh O12) 58. eee 719 Moh 238i occ. waswmes 780 Bs TO oh oiiaeee 690 PES 18k Me tat 1,000 PNT oy SAN) Ke Mee Ie 2,520 Apr. 23.. 2,925 N35 0 Gig.) eee ees ed aie 9, 055 Day Abc eo pee tie 15, 470 May 7 13, 630 May AZ ac qaseebrne 11,980 of July. The comparisons of net and tube show no appreciable difference in coefficient between the catches of October when the vegetable plankton is at its maximum, and those of February and May, when it is greatly reduced in quantity. There is therefore no reason to suppose that the coefficient of the dredge is appreciably altered by being raised through the distance It may be added that results similar to those obtained in the above table would be shown if any other species of three meters. Single haul. DUBO Sao a~ denice tee June 105.)..85 Geckos DUNES. oi. ed cciee ane SMNOAT,. Sewer UMS AB Aen Seca Sum of series. 11,940 19, 470 11, 780 12, 859 16,710 16, 220 13, 220 10,010 8,020 8,070 4,530 3, 809 4,760 3, 710 3, 299 3,190 3, 920 6, 105 3, 416 2, 960 3, 434 2, 791 Single haul. 14, 300 17,630 19, 200 16,000 11, 240 15, 625 17,900 15, 200 10,080 | 7,800 3 640 5, 600 3, 240 5, 680 3, 750 2, 120 2, 400 3, 700 3, 600 3, 300 3, 960 2,560 3, 080 3, 120 1,840 The Coefficient of the Dredge. 285 of crustacea had been selected, or if the total of all the crusta- cea had been chosen. There is still a third question relating to the coefficient to the dredge, namely, does the net function similarly on different occasions, or does its coefficient vary irregularly and in such a “way as to vitiate conclusions based on the hauls of the net? This question is partially answered by the determination of the dredge coefficient, as shown in Table I. A second answer can also be given. During the winter the numbers of Daphnia and Diaptomus do not increase by reproduction, and the successive catches should therefore show no very great variation. In a sub- sequent section, dealing with the question of swarms, I have given the figures for the catches of these genera during the winter of 1895, from which it appears that the variation in suc- cessive catches made within a short time of each other is no greater than may be found between catches made on the same day. “Still further, a diagram is given (Fig. 21), showing the numbers of Cyclops caught during the year 1895. This diagram shows plainly that when the average number of Cyclops is ap- proximately constant, the individual catches do not ordinarily vary greatly from the average, no more than would be expected from Cyclops’ necessarily somewhat irregular distribution in the lake. An examination of the maximum and minimum catches in the tables for the different species shows the same result. I do not pretend that I have determined the coefficient of my nets with absolute accuracy, nor that the coefficient of the net is exactly the same on different occasions; but the careful study whose results are summarized above has convinced me that the coefficient of the net is quite as constant as any of the factors entering into the determination of the plankton. The number of the crustacea certainly varies from point to point in the lake. Where a fraction only of the crustacea are counted, the deter- mination of the number caught is an approximation and is sub- ject to error. This error, is, of course, multiplied greatly in stating the number of crustacea in terms of square meter of surface. Among the variables and approximations which en- ter into the statement of the results of plankton work, I think it may fairly be said that the coefficient of the net is one of the 286 Birge—The Crustacea of the Plankton. most constant factors, and that it may be quite as accurately determined as any other. TEMPERATORES. Figs. 1-5. The following account of the temperatures of the lake is not: q intended as a complete discussion of the subject. My tempera- — ture observations were made at first with the aim of securing” q approximate results in order to determine the biological rela-~ — tions of temperature. The methods employed until July, 1896, | while accurate enough for these purposes, are not sufficiently q accurate for other ends. I have therefore refrained from print- ing the observations of temperature, and discuss chiefly the — temperature diagrams, which give the result of my observa- | tions by weekly or rather, quarter-monthly averages. A, Methods. Surface temperature observations were taken from the begin-. — ning of my study, and temperatures from all depths after Octo-- ber Ist, 1894. A water bottle and thermometer were the instru- : ments employed until July 27th, 1896, after which date a ther- — mophone was used. The latter instrument has proved extremely; useful and accurate. A full description of the instrument may be found in Science, Vol. II. of 1895, page 639. As constructed for my work, the instrument ranges from minus 5 to plus 30: degrees C., each degree being graduated into fifths. There is no difficulty in reading the instrument to less than 0.1 degree q C., and its readings are exceedingly accurate, agreeing exactly with those of a standard thermometer with which it has been constantly compared. Observations can be made very rapidly, the time of a single reading varying from one to one and a half minutes, according to the amount of change of temperature — from the last reading. Tha temperature bottle contained about 14 litres and hada _ small neck. It was lowered to the desired depth; allowed to re- q main from one to three minutes for the glass to acquire the tem- _ perature of the water; was then uncorked by a sudden jerk on the: ‘ "M0}40q ‘oUl] WexOIG :oovJAns ‘OUT [MA “GEST ‘sornye.rodute} (ur gt) u104}0q pue sovyang—T OL 0°: r : - - Ss ria iN , ' i : ‘ : : : 1 ss at | ! Qe eee “ \ ' D 1 5; ; Spee | : ) ee ' ' ' 1 1 ' oa e ! : : v ' ' ' i] i] a : ' 1 ' ' t 1 ; 5 a : ! ' ! y ' ! ' ’ ye ' : 1 1 i] i] ' ‘ 1 ! 1 ; ' ' ' ' r 1 L ’ U ' ' 1 gc - , ; : S Rca ae Sa ' 1 f 1 3 : ' ' 1 1 4 i ; ; 8 rene 1 ) = u Wi Pi ae te 1 ! ' ; ' 1 i ’ ol: : : set oT ! ’ 1 ! 1 1 f) ' 1 ’ { i] 4 7 d ; : : ; oe ML ON Rte at hae eer ge i L wo x poe ee ee ! : ’ U ! ' i i : oe Pe ae 1 i i ‘ i ; ; oy I ; ; ! : og — ; Ce ee a ' F : : : : ; oo | : | aaa ; oFZ eeeee Dia 5a ea Pats See jie es ond ‘AON ‘LOO ‘Laag “ony ‘RTO "TINO Sxcva ames "IIday “AX 938d "IX “TOA “‘PeOY “SIAA “SURI, SS SS ae —_—— oro rrr reeooa : : Same a a ea it i rat vere oy or "m10440q ‘oUT, UsyOAq foovsans oul] [Ny ‘96ST ‘Soinyeaodutoy (UI gy) W04JOq pue ooRFING —-"Z “OI ' ' ‘ ' r ' ‘ ( u i ' ' ' i i 1 ' u t ' ' 1 4 ; ' 1 ! ' ; ' ’ : L ' ' ' ; ' ' ' ' ! ' u s ' ' ’ ' ' 1 ) ' 1 a ' | ’ ' D 0 : U 7\ ¢ ' ! ' 1 ’ ’ i ' y, ’ 1 ‘ u 1 7) ' J U ’ UY { ' 7 ' 1 ' ' ’ t 4 1 ' ' 1 ‘ ‘ i) , ' ' ‘ ' ' 1 u ’ ' 1 1 U ni ’ ' ' ' : ' ' ‘ si i ' ' ’ ’ ' ' 1 ' ’ . f 1 1 1 } ; A ! ‘ ' 1 1 5 ' 1 1 ' ' 1 1 UJ ' 1 q 1 ' ' ' ’ ' ' ' ' 1 4 ' i) ' i ; : i] ' \ . ! XN ' ’ ‘ ' ' u ' Z D 1 i} , ‘ 1 ’ ' UJ ' ’ ’ ' ' 5 ’ ! ' ’ ‘ ' 1 1 ' ' 1 ’ I 1 . ‘ ’ ' 1 U . 1 ’ ’ ' ‘ ' J l) ' ) ‘ U 0 z, . ' ’ . 1 ' ‘ 1 - ‘ ‘ U : vs . ‘ . ‘ . . ’ ' ' ) A ' t ' ! ‘ ‘ . ' ' ' . ‘ . ° i] 1 . i . U i ‘ ’ I . ‘ , ' ' ' ’ ‘ ' ’ iy ' ' . ’ ' U] 1 1 ‘ ‘ ' ' ' 1 ' ‘ ’ ' S mes ay “TAX 9d ‘AON ‘LOO "Lag “pay “KTNG "ANA ‘AVW "TIUdV TX Oi PEO Vest SURRY sees oF 2 eevee 2G Oo sree gz wevre s+ eee ees aeus vee re Temperatures. 287 line, and allowed to fill. It was then drawn rapidly to the sur- face and the temperature read by means of a long-stemmed ther- mometer graduated to one-fifth of a degree. The time of rais- ing the bottle from the bottom of the lake was ordinarily about ten seconds; and the small size of the opening prevented mix- ture of the upper water with that in the bottle. The tempera- ture of the water in the center of the bottle, which was meas- ured by the thermometer, did not change perceptibly during the time required for the thermometer to set. The water from the lower part of the lake, however, was somewhat warmed by con- tact with the glass and the air in the bottle. This error was carefully determined by comparison with the thermophone, and is about one-fifth of a degree C., when the difference between surface and bottom is about 10 degrees. Errors much more considerable than this occur with the use of the temperature bottle at the thermocline. In this region the temperature may fall as many as nine degrees in a single meter, and not infrequently as much as three or four degrees in a quarter of a meter. It is impossible that the bottle should take in all of its water from the stratum in which its mouth lies as the escaping air sets up currents so that a mixture of the water occurs. A difference of half a degree may therefore oc- cur between the readings of the thermophone and the bottle in this region. In one case the error amounted to two degrees, where the bottle was opened a few inches below the upper level of the cold water and took in a mixture of this water with the lower part of the warm stratum above. The errors at this re- gion, however, while considerable, make little difference in the average results of observations, since their only effect is to make the upper level of the cold water appear to be a fraction of a meter lower than it really is. Since this level is subject to irregular variations, under the influence of the wind, which may amount to two or even more meters, the errors introduced by the bottle are insignificant in the average of a week’s read- ings. It was intended to correct the observations of the bot- tle by means of the thermophone and to introduce the correc- tion in the diagrams of temperature. It was found, however, that the amount of correction to be introduced in the diagrams 288 Birge—The Crustacea of the Plankton. was so small as to make it inadvisabie to insert it. In Figure 4 the change from bottle to thermophone is made in the last week of July, and it will be seen that the lines come together with great accuracy. Above the thermocline the bottle and thermophone agree exactly, except at the surface on calm, sunny days, when the reading of the thermometer is higher than that of the ther- mophone, since by means of the thermometer the temperature of a very thin stratum can be taken, while the thermophone coil is of such a shape that it reads only the average tempera; ture of a stratum some eight centimeters in thickness. During the period April— December, 1896, 189 sets of obser- vations were made on 135 days varying from 3 to 6 per week. In 1895, 196 sets of observations were made on 126 days in the — same period. The temperature observations were made at all hours of the day; rarely by night, and must be taken as representing the day temperatures of the water. Little difference, however, would be made in the diagram if the night temperatures had been introduced, as has been shown by an elaborate series of observations made in 1897. Observations were regularly made by single meters by the thermophone, and also by the boitle when the difference between single meters exceeded one-half degree C., and often when the differences were tess. After recording the temperatures, those for meters not directly observed were interpolated, and the average was taken of the observations for each meter and each quar ter-month. In preparing Figs. 3 and 4 the average temperatures for each meter and quarter-month were platted at the proper depth, and in the center of the space representing the quarter-month on the diagram. The position of the full degrees was then platted on the assumption that a uniform decline of temperature is found within a single meter. This assumption is incorrect in the region of the thermocline as the zone of the most rapid decline of temperature is frequently less than a meter in thickness, but as this zone varies in thickness and shifts its vertical position under the influence of the wind, little error results from using this method of platting the average observa- = a Temperatures— Winter. 289 tions of a week. lines were then drawn connecting the posi- tions of the full degrees. In 1895 the diagram is carried to 18 meters only, the depth at my regular station. In 1896 the temperatures were carried to 22 meters, observations being taken at that depth nearly every week. Two other temperature diagrams are given, showing the movement of the surface and bottom temperatures from April to December of the years 1895 and 1896. : B. Results. Winter Temperatures. Lake Mendota freezes at very different dates during the early winter in different years, and the time of opening also varies greatly. The lake is so large that continued high winds prevent its freezing even after long continued low temperatures, and as there is no large affluent, there are no spring floods to move the ice, which therefore remains until it is greatly weakened by the effect of the sun and is broken up by the wind. In 1894 the lake froze on December 28th, and opened April 8th, 1895, being closed for 190 days. In 1895-96 the lake froze December 6th and opened yack 28th. The first and last observations through the ice were made on January Ist and March 23d, 1895; and De cember 9th, 1895, and March 28th, 1896. In the winter of 1896-97 the lake froze December 29th, then broke up again and did not freeze the second time until January 7th, 1897. It op- ened on April 10th, 1897. The ice usually reaches a thickness of over 60 cm., and in 1895 became nearly 1 m. thick. During the winter the temperature of the surface of the water is, of course, zero. The water at the bottom when the lake freezes has a temperature which varies in different years. If the lake is prevented by wind from freezing during the first cold weather of December, it may remain open for days or even weeks, cooling very slowly. This was the case in 1894, and the temperature at the bottom on January 1st, 1895, was barely one degree, and at nine meters was about 0.5°. In 1895 when the ice on December 9th permitted observations, the tem- | perature was as follows: 0:5 m.,°0.3°; 5 m., 1.2°; 18 m., 1.7° 19 290 Birge—The Crustacea of the Plankton. It is, of course, possible that the lake should freeze when the bottom is at any temperature between 4° and zero. It is hardly probable, however, that it often freezes permanently when the bottom is lower than 1° or higher than 2.5°. Below the ice the temperature of the water rises rapidly, being half a degree or even more within less than half a meter of the ice, and below this level the temperature rises very slowly and regularly to the bottom of the lake, the difference between the water at 0.5 m. and the bottom rarely exceeding two degrees. The mud is or- dinarily decidedly higher in temperature than the water just above it. (See FitzGerald, 95, p- 81.) The difference between the temperature of the mud and the water half a meter from the bottom was sometimes found to be as great as 0.7-0.9° in 1894-5, and 1895-6, by the aid of the water bottle; while the thermophone in 1897 showed differences of 0.3-0.8°. This dif- ference varies in different parts of the lake without any assign- able reason. The temperature of the water of the lake rises during the winter, especially during the latter part of February and March (Cf. Apstein, ’96, p. 18). In 1895 the temperature reached nearly 2.5° at the bottom, and 1.5° close to the ice on the 27th of March. In 1896, on March 28th, the temperature at one- half meter was 2.9°, at the bottom (18 meters) 3.1°. This was a rise of from 1.5 to 2° during the winter. In 1897, the tem- perature on January 23rd was: 1 m., 0.6°; 18 my 16") a March 29th, at 1 m. the temperature was 1.4°, at 18 m., 2.1°. This warming of the water is due to the sun. If it were due to warm water coming from springs the bottom temperature would necessarily rise to 4° before the change appeared in the upper water. But this is not the case. The temperature at the bot- tom has not reached 4°, in any of the three winters during which observations have been taken, until after the breaking up of the ice in the spring. It would appear, therefore, that this warming must be due to heat which enters the water from above. While this rise in temperature is very gradual and is small in amount, it has important biological results. The reproduction of Cyclops and of the rotifers goes on very much more rapidly at a temperature above 1.5° than at a temperature near 1°. In- Temperatures—Spring. 291 deed, at the lower temperature the progress of the development of eggs is almost suspended, while at a temperature of 2.5 to 3° the development of eggs into nauplii and of nauplii into young Cyclops goes on with cousiderable rapidity, and at 1.5-2° it is present, though decidedly slower. The history of Cyclops in the spring, therefore, depends to a considerable degree on this warming of the water under the ice. If the winter is cold, so that the warming does not take place, or the rise is only slight, the number of Cyclops may remain almost unaltered during the winter; while conditions like those of the winter of 1895-96 permit the development of large numbers of young Cyclops ready to take advantage of the increased warmth and food in early spring, and so to develop enormous numbers of this genus. The spring rise of temperature. A glance at Figs. 1 and 2 will show that the warming of the lake in the springs of 1895 and 1896 was singularly alike. In each year the month of April was prettysteadily warm, and the surface of the lake rose rapidly and uniformly in tempera- ture for about six weeks following the breaking up of the ice. Immediately after the disappearance of the ice the temperature of the lake frequently falls, since the breaking up of the ice is often caused by a north wind accompanied by a much lower temperature than had preceded the breaking up of the ice. This fall in the temperature of the water amounted to over one de- gree in 1896. But this slight drop is quickly recovered, and if the weekly averages are considered it will be seen that the sur- face temperatures in both years rose rapidly and steadily. For a time the rise in temperature at the bottom is as rapid as that at the surface. The length of this time varies, of course, with the amount of wind. A succession of warm days, accompanied or followed by high wind, will mix the warmed surface water with the body of the lake and thus secure uniformity in temper- ature. In neither 1895 nor 1896 were these conditions long realized; the temperature of the bottom began to lag behind that of the surface, and by the middle of May there was a difference of 7° to 8° between the surface temperature and that of the bot- 292 Birge—The Crustacea of the Plankton. tom. In six weeks the temperature of the bottom had risen about 5° or 6°, while that of the surface had advanced about 15°. The relation of the wind to this warming of the lake is well stated by Whipple (95, p. 207). | In both of the years of observation, and aiso in 1897, there came in the middle or latter part of May a marked decline in tem- perature accompanied with high northerly winds. The effect of this was two-fold: first, the surface water was cooled; secondly, the wind mingled pretty thoroughly the water of the lake, thus caus- ing a sharp rise of temperature in the lower strata. On the 12th of May, 1895, the difference in temperature between top (15.6°) and bottom (7.7°) was 7.9°; on the 16th the difference was only 1.5°, and on the 18th only one degree (12.6°-11.6°). On May llth, 1896, there was a difference of 8.3° between top (18°) and bottom (9.7°), and a thermocline was evidently formed be- tween 4 and 6 meters. On May 17th the difference between top (15.6°) and bottom (13.4°) was only 2.2°. Thus in both years there was a rapid rise of 3-4° in the temperature of the bottom water. It is probable that if temperatures could have been taken at the most favorable time the lake would have been found nearly homothermous in late May, at a temperature not far from 11° in 1895, and 13.5° in 1896. — Thevemecs of the spring warming was therefore to warm a mass of water 18 to 24 meters deep from an average temperature between 2° and 3° in March to an average of 11° to 14° at the latter part of May; with the differences between the top and bottom not ex- ceeding 1° to 2° at the beginning and end of the period. From these facts it appears that the bottom temperature of the lake may vary greatly in different summers, and that the bottom temperatures of lakes of the same depth, in the same region and season may also vary greatly—much more than the temperatures of the surface. Four factors are effective in de- termining the bottom temperature; three constant, and one variable: (1) the depth of the lake, (2) its area relatively to its depth, (8) the shape of the lake and the nature of its surround- ings as favoring or hindering the influence of the wind, and (4) the amount of warmth and of wind during the spring and the times of occurrence of gales and the succession of warm and cold A Temperatures—Summer. 293 waves. The same factors are also the chief powers in deter- mining the position of the thermocline and its rate of down- ward movement. Very few of the inland lakes of Wisconsin are more than 25-30 meters in depth, and their bottom temperatures vary more with relation to their area than to any other one factor. In the Oconomowoc lakes, which are in the same region as lake Men- dota, andare of the same depth approximately, but are much smaller in area, the temperature of the bottom water does not rise much above 7° during the summer. The same is true of Cochituate lake, Massachusetts, having a depth of 60 feet and an area of less than one and one-half square miles. (FitzGerald, ’95.) Green lake and lake Geneva, Wisconsin, both of them not greatly differing in area from lake Mendota, but having a depthof 150 to 200 feet, have bottom temperatures of about 6°. In a lake of large area, like lake Mendota, and about 24 me- ters in greatest depth, the temperature at the bottom may dif- fer widely in different summers. In 1896 the bottom tempera- ture at 18 meters at the first of June was nearly 15°; in 1895 about 12°, and in 1897 about 11.4°. At 22 meters it was about 0.5° lower in each year. Had it not been for the gales in the latter part of May the bottom temperatures would have been much lower; possibly from 7° to 9°. The extreme pos- sible range of bottom temperature in summer for lake Men- dota in different years may perhaps be stated as from 8° as a minimum to 18°, as a maximum, and the probable range as from 10° to 15°. Summer temperatures. The temperature of the surface rose rapidly and evenly after the fall in the temperature and mixture of water in the latter part of May. In 1895 the weekly averagerose from about 13.6° to 22.5° in three weeks, a rate of nearly three degrees per week. In 1896 the surface rose from 15.4° to 25.1° in six weeks, rising some what less regularly and at a much lower average rate. The period of the summer maximum was reached about the middle of June in 1895, when the average temperature was 23.5°, and about 294 Birge—The Crustacea of the Plankton. the 1st of July in 1896, when the maximum was about 2.5° higher. The maximum surface temperature recorded was 25.2° ~ Aug. 1, 1895, and 27.8° July 28, 1896, both at 5p. m. After the maximum has been reached there follows a period in which the temperature of the surface is nearly stationary, and in which the weekly averages do not vary more than two degrees. This period was exceptionally long in 1895, lasting from the middle of June to the third week of September, about three and one-half months, in which time the weekly averages were between 22° and 24°. In 1896 it lasted only about six weeks, from the first week of July to the middle of August, at a temperature of 24° to 26°. At the close of this period the surface temperature falls and the decline once started goes on pretty uniformly as shown by the weekly averages, until the lake nears the freezing point. In 1895 the temperature fell 3° in aS many days at the last of September. In 1896 there was a fall of 4.4° during the last ten days of August. At the opening of the summer period the temperature of the bottom rises somewhat rapidly in the latter part of May, gain- ing perhaps 1.5-2° in two weeks. After this the bottom temperature is stationary or rises very slowly, not gaining a degree in three months. The bottom temperature at 18 me- ters lay between 13° and 14° in 1895; close to 15° in 1896, and near 12° in 1897. At the depth of 22-23 meters the temperature was from 0.4° to 0.6° lower in each year. Late in September the water of the lake becomes mingled from top to bottom and the temperature becomes uniform. At this time the bottom temperature rises rapidly by the mixture of the bottom water with the warmer water above. During the early parts of the period when the bottom tem- perature is nearly stationary, that of the surface rises until the difference between bottom and surface amounts to 10° and even 15° in late July or early August. As the surface tempera- ture declines, the difference between top and bottom becomes less and usually amounts to between 4° and 5° in late Septem- ber, just before the time when the lake is rendered homother- mous by the fall gales. ee ee ee ee ee ee ee Temperatures. 295 The Thermocline. | During the summer, then, the difference in temperature between the surface and the bottom may amount to 10°, 12°, or even 15°. The decline in temperature from surface to bottom is, however, not uniform as the depth increases. If a series of temperatures is taken about the first of August it will be found that there is a layer of surface water from 8 to 12 meters in thickness whose temperature is nearly uniform, the difference between that of the surface and that at 9 or 10 meters being usually only a fraction of a degree and frequently nothing. Immediately below this mass of warm water lies a stratum in which the decline of temperature is extremely rapid. This stratum may be two or three meters in thickness with a de- cline of as many degrees per meter. It may be only a meter or even less in thickness, and a decline of as many as nine degrees has been observed ina single meter. This layer in which the temperature changes rapidly may be known as the thermo- cline —the Sprungschicht of German authors. Below the ther- mocline the temperature decreases toward the bottom at first more rapidly and then more slowly as the depth of the water increases, but never showing the sudden transitions which are characteristic for the thermocline, the rate of decline rarely exceeding one degree per meter of depth. The thermocline was first noticed by Richter (’91) in a study of the Alpine lakes. Its origin was attributed by him to the alternate action of the sun warming the surface in the day, followed by a cooling at night. The alternation ‘of conditions resulted in the formation of a layer of water of nearly uniform temperature above the colder bottom water. I do not wish to argue against the cor- rectness of this theory as applied to the lakes which have been studied by Richter and others, but in lake Mendota the concur- rence of gentle winds and hot weather are essential to the for- mation of the thermocline. In other words, the warmth of the surface water, received from the sun, is distributed by the wind through a certain depth of the lake, a depth which is propor- tional to the violence of the wind and the area of the lake. (Cf. FitzGerald, 95; Whipple, ’95.) Itcan readily be seen that 296 Birge—The Crustacea of the Plankton. in a lake of the size of Mendota the water would be of uniform temperature from top to bottom if the lake were always agitated by violent winds. On the other hand, if the weather were per- fectly calm, the lake would be warmed only to the depth which the rays of the sun could directly penetrate. Asa matter of fact, the formation of the thermocline is due to the concurrence of gentle winds and a temperature high enough to warm the sur- face water rapidly. The temperature observations on lake Mendota have been made chiefly at a station about one-half of a mile from the south shore. On bright days in May, with a gentle north (on shore) breeze, it not infrequently happens that a thermocline is formed, there being a mass of water four or five meters in thickness of uniform temperature, below which there is a rapid descent in temperature to the cooler water below. When, how- ever, the direction of the wind changes and blows off shore, this warm water is carried to the other side of the lake, and the temperature shows a fairly uniform rate of descent from the sur- face to the bottom. If, however, this condition of warm weather and gentle wind continues, there is produced a mass of warm water on the surface, so thick that however the wind may blow there is always a warm stratum floating on the colder water; and when this condition has been established, a permanent thermocline has been formed. A study of Figs. 3 and 4 will show the formation and movements of the thermocline as disclosed by the weekly averages. It will be seen that in the early part of May the gain of heat is rapidly dis- tributed through the whole mass of water. The bottom lags behind the surface, of course, but the difference in temperature between them rarely exceeds 5° and the temperature of the surface water reaches the bottom in 10 days or 2 weeks. During the rapid warming of the early summer this condition ceases. The sur- face warms rapidly, the winds are not constant or strong enough to distribute the heat throughout the water, and the ownward movement of the isotherms no longer extends to the bottom, but they penetrate for an increasingly shorter distance into the water. In 1895, for example, the surface reached an average temperature of 15° during the last week in May, { OS Trans. Wis. Acad., Vol. XI. Plate XVII. Depth. May. JUNE, JULY. AuG. SEPT. Oct. Depth. 10° 14° 15° 20° pein tas 0m... \/ ] So con (0) reat 3m.. 6m.. | 9m.. \ 12m... 15 m... ; 18 m... ‘ : se cbt wae 10° 11° 12° 15° 16° May. JUNE. JULY. AuG. SEPT. Oct. Fic. 3.—- Summer temperatures, 1895. See p. 296. ee My ” oa ane Trans. Wis. Acad., Vol. XI. Plate XVIII. Depth, APRIL. May. SEPT. Oct. Depth. 10° 15° 16° 16° 20° v2 0m... ——4 5 aaa .. Om ' J i 3m Hoel ————, 3 m 6m.. \ : 6m hatter F . 9m 12m... eI Se tla 15 m... soll san 18 m... . 18 m 21m... ..21 m APRIL, May. JUNE, JULY. AUG. SEPT. Ocr. Fic. 4.—Summer temperatures, 1896. See p. 296. Temperatures—The Thermocline. 297 and the isotherm of 15° penetrated nearly 10 meters of the lake in a week; it went down 3 meters further in another week, but thereafter moved downward at a rate little exceeding one meter per month. In 1896 the 15° isotherm was in- cluded in the May depression of temperature, but in late May it moved downward nearly 15 meters in one week, 1.5 in the week following, and only one meter in the next two and a half months. As the temperature of the surface rises above 15° the warmth penetrates to a distance increasingly small and the isotherms accordingly bend toward the horizontal at a level nearer the surface. The gain of heat, however, becomes rapidly distributed through the upper water to adepth of 8 to 10 meters, so that the thermocline becomes permanent at about these depths. When the thermocline has once been formed it moves downward very slowly. Beginning at about 8 meters in late June, it descends somewhat rapidly to about 10 meters, but after that moves downward slowly and irregularly, its descent depending rather upon the wind than upon the tem- perature of the air. In both years the thermocline reached the bottom of the lake in the last of September, which would make its downward movement about 4 meters per month, but the last 5 or 6 meters were passed very rapidly in consequence of the gales of late September. In 1895 the 18° isotherm was near the center of the ther- mocline; it oscillated about the 9 meter level in late June, sank nearly 3 meters in July, about 2.5 meters in August, and 4.5 in September, the last 3 in the latter half of the month, In 1896 the 20° isotherm was near the center of the ther- mocline at the outset and crossed the 6 meter level about July lst. It lay at 7.5 meters during the first week of July, reached 9 meters about the 20th of the month, oscillated be- tween 9 and 10 meters for more than three weeks following that date— weeks of unusually hot weather—until the middle of August. At that time the weather changed and continued cool with much northerly wind, under whose influence the thermo- cline rapidly sank more than 2 meters during the last half of the month and continued this downward movement through Sep- tember until it disappeared in the latter part of the month. 298 Birge—The Crustacea of the Plankton. These temperature diagrams, which give the weekly averages of temperature, do not show the actual condition of tempera- _ ture, and especially the temperature of the thermocline, on any single date. The thermocline oscillates up and down under or- dinary conditions of weather through a meter or more; and the effect of averaging the observations of a week is to increase the apparent thickness of the thermocline and thus to diminish the rapidity of descent of temperature in it. Without any consid- erable change either of wind or temperature the thermocline may oscillate through 2 or even more meters. The action of severe wind is much more apparent. Fig. 5 shows temperature dia- grams for August 2, 24, 26, 27, and 28, 1896. It will be seen that the diagrams for the 2nd and 24th of the month were closely similar, although the surface water had cooled a degree or more and the thermocline had descended about 1 meter. On the 24th there was a decided fall in temperature of the air ac- companied by violent winds from the northwest. The surface water fell more than one degree in two days, while the thermo- cline was temporarily depressed at the observing station more than 4 meters. It lay on the 24th between 10 and 11 meters; on the 26th between 14.5 and 16 meters. The temperature at the bottom, 18 meters, was raised about 0.4°, at 14 meters 5.6°, at 12 meters 4.3°, at 10 meters there was a loss of about 0.6°. On the 27th, the wind having fallen to a calm, the thermocline had risen nearly 3 meters, while on the 28th, with a gentle south wind, it had risen still further, and the temperature curve had greatly changed in form. During these three days the temperature toa depth of 8 meters had varied very little — too little to show in the diagram. This example of changes which are going on all the time, shows the following facts: 1. The isotherms of diagrams 3 and 4 represent only the average posi- tion of the thermocline. 2. The decline of temperature in the thermocline is ordinarily much more rapid at any given date than is indicated by the average of the week. In other words, the thermocline is not nearly as thick as the week’s average would indicate. 3. The greatest daily variation in temperature during summer is found at the thermocline, where a range of 5 or more degrees may be registered in a day. These variations rans: Wis. Acad., Vol. XI. Plate XIX. 14° 16° 18° 20° 22° 24° 14 m...: ee. 26! : ie [a7 ayins = Pano earl : a 2]: 4m). ies —- : a= | ; ie: : ep : : We ask : : : : tae : ae gee ee oe hs aaa d : 2 phys : , ‘ aia fL : 10 m...: 2 pigs : Sees 1 = Pe Bedicce 4 5 5 Beet i: ; : | Ee Tae ye : 12 me: TLaEIEEEIIE aR EEE Coa fear a : ° ee ire : 1 / Zaues | 8 16 me... gulf 1S) saa" he Fig. 5.—Temperatures, August, 1896. See p. 298. The dates of observations are indicated on the temperature curves. Temperatures—Autumn. zoe are not caused by the warming or cooling of the water but by the fluctuations in the level of the thermocline. These fluctua- tions go on to a certain extent without an assignable cause, but the larger movements, at the station where observations were taken, are plainly due to the wind. 4. The upper layers of the cool water become mingled by the action of the wind with the lower part of the warm water above it and are taken into the warm layer. Thus the thermocline moves constantly downward during summer, while the water below it is little or not at all changed in temperature. 9. The water below the thermocline is practically stagnant during the summer, and is cut off from direct exposure tosunand air. As a result, it may become unfit to support most forms of animal life, as is the case in lake Men- dota. 6. The larger changes in temperature below the thermo- cline are due to currents caused by winds. Autumn temperatures. By the latter part of September the temperature of the sur- face water has fallen so that it excceds that of the bottom by Warely 98°. At this time also gales from the north are _vapt to occur whose effect is to break the thermocline and render the lake homothermous. ‘This result is reached at ‘different dates for different depths, but in both years the lake became homothermous in its deepest parts about two or three days after the time when a similar condition was reached at 18 meters. In each year the homothermous condition was reached at a temperature not much exceeding 16°; and in general the temperature for the Ist of October may be stated -as about 16°. The breaking up of the thermocline is accompanied by a marked rise in the temperature of the bottom water. In 1895 this rise amounted to 2.8° from the 26th to the 28th of September ; and in 1896, to about 1.5° in the same time. _ During October and November the temperature falls with ‘Singular uniformity, as indicated by the weekly averages, pass- ing the temperature of the maximum density of water late in November. The decline continues steadily until a temperature as reached between 2° and 3°, after which the cooling goes 300 Birge—The Crustacea of the Plankton. on very slowly. The difference of temperature between the surface and bottom of the lake during this time is very small, In the morning the lake is entirely homothermous. On bright, calm days, the temperature of the surface rises, and may become as much as 2° warmer than the bottom. This condition of things, however, is uncommon, and ordinarily it is difficult to find differences between the surface and bottom exceeding 0.1° or 0.29. It is.a feature of especial interest in lake Mendota that the fall homothermous period begins so early and at so high a temperature. The autumnal multiplication of many of the species of crustacea goes on after this period has been fully established, and their vertical distribution at this time is there- fore independent of temperature. In the deeper lakes, or in smaller lakes of the same depth the homothermous condition is reached much later. In Green lake, as reported by Professor Marsh (Marsh, ’97, p. 187), it occurs in November at a bottom temperature of 4.7°, and at a depth of about 45 meters. The rise at the bottom was 1.4°. In Cochituate lake, near Boston, at a depth of 18 meters, the homothermous condition is reached at about the same time, and at the same temperature. (FitzGerald, ’95, p. 74.) This lake has an area of less than one and a half square miles. ahs During the last of November and the early part of December cooling goes on very slowly. The surface temperature fre- quently falls to zero, as the result of a calm night, and the lake may skim with ice, which is broken up again by the wind. 4 i q f The Annual Distribution of the Crustacea. 301 THE ANNUAL DISTRIBUTION OF THE CRUSTACEA. I, General Relations of the Plankton Crustacea. Figs. 6-11. Lake Mendota has eleven species of limnetic crustacea, which may be grouped as follows: A. Perennial species — a, Appearing in great numbers — Copepoda. Diaptomus Oregonensis Lillj. Cyclops brevispinosus Herrick. Cyclops Leuckartii Sars. Cladocera. Daphnia hyalina Leyd. Chydorus sphaericus O, F. M. var. minor Lillj.! b. Usually appearing as isolated individuals — Copepoda. Epischura lacustris Forbes. Ergasilus depressus Sars.? B. Periodic species -—— a, Appearing in great numbers — Cladocera. Daphnia pulex DeG. var. pulicaria Forbes. Daphnia retrocurva Forbes.? Diaphanosoma brachyurum Sars. b. Appearing as isolated individuals — Cladocera. Leptodora hyalina Lill}. To these might be added Bosmina of which a very few indi- viduals appear, chiefly in winter, but of which there are never enough to make a fair determination of their number a possi- 1Sometimes absent but not properly periodic. _? The specific identification is not certain. * Formerly classed as a variety of D. Kahlbergiensis or D. cucullata. 302 Birge—The Crustacea of the Plankton. bility. Most of the littoral forms of crustacea also appear oc- casionally in the plankton, especially after storms, as also do Hydrachnids and Ostracoda. Of these eleven species, the isolated forms do not contribute any appreciable addition to the number of limnetic crustacea. Their combined number is rarely as great as one per cent. of the total crustacea present. They have, therefore, been neg- lected in determining the total number of crustacea, and this general account will deal with the eight abundant species only. The limnetic crustacea on lake Mendota show a rhythm of de- velopment quite complex, but recurring in closely similar form during the time covered by my observations, July, 1894 — De- cember, 1896. (Fig. 6.) Observations less numerous have been continued to the present date, September, 1897, and show a similar development during the present year. The following periods can be distinguished: | Winter mmm Wn 4155.4 seen cece cm ecieoe ne December to April, then increase to the Spring) MaximMUMi .,26als.c ceases ceeibanets In May, followed by a great decline to the Early summer depression..... .............. June or early July, Mid-summer maximum................-.0-%- July, Late summer MINIMUM... .... we cece eee ee Late July or August, Mog qosooey seakeh.dhondsngy Bane sna doo noooo OnGUS September and October, declining to the winter minimum, through late October,. November and early December. There are, thus, three maxima and minima which are of un- equal value. The spring maximum is by far the greatest, the crustacea reaching a maximum number of 3,000,000 per sq. m. of surface, and in 1896 reaching an average of nearly 2,500,000 for the first half of May. This maximum is due almost entirely to the rapid development of Cyclops brevispinosus. After the maximum has passed, this species rapidly declines in number, and the total number of crustacea sinks with it, so that by the middle or last of June the number is reduced to less than half the maximum. This is the early summer depression, which may be greatest at any time from the middle of June to the first week in July. A rapid, but slight, recovery follows, due chiefly to renewed reproductive activity on the part of the species al- ready present in the lake, leading to the mid-summer maximum, in July, Then follows a decline, usually somewhat slow, reach- peas Sa ws eS egy 1,600... 1,400... 0 Ore 1,000.. 800. . 600... 400... Trans. Wis. Acad., Vol. XI. MARCH. APRIL. MaAy. JUNE. JULY. AUG. SEPT, Oct. Novy. DEc. - A 6 c 5 e Ny oF ° Te ; : e Y) e ° . e f 2) . $ 6 : 3 5 : ; : : - 5 . é 5 8 : \ : : ° ‘ 3 : k . c 5 ; : 5 " i e = " e : ° 5 ° oe O . . e ‘ vex << e / / / Ferece é : ‘ Se / ° . — 3 >e > ReSing / 5 = S ise e 4 >a Se 7 : a § 3 B 5, . * 5 . Za : i 5 : : SS (7 . x Q - ° ° * Fic. 6.—Total crustacea, 1894-1896. Scale, 1 vertical space = 200,000 crustacea per sq. meters. USSGR ish erg eee SOW LH see ie fee, Sut 1895.... — ——@-— _— Plate XX. JAN, FEB. eee coe slocoeres | coe noes a2 roc artaseecge|ecece aw eeceoos See p. 302. The Annual Distribution of the Crustacea. 203 ing a point of greatest depression about the last of August. During this period of decline, most of the periodic species are - introduced, but their numbers do not usually compensate for the falling off in the number of the permanent species. In 1896, how- ever, Chydorus increased so rapidly during this time as to more than counterbalance the decline in other species. In September a rise in the number of crustacea begins, caused chiefly by increase in Daphnia of all Species and in Cyclops. This increase culminates in the last of September or in October. This is the fall maximum, which, in general, is decidedly greater than the early summer maximum, the crustacea at this time reaching a number perhaps two-thirds as great as that of the spring maximum. During the later part of the fall and the early winter, the number declines very rapidly at first, and then more or less slowly, until the winter conditions are established with the freezing of the lake in December or early January. The rapidity of the decline varies in different seasons, depend- ing upon the abundance of the periodic forms and upon the num- ber of young Cyclops and Daphnia hyalina, which are produced in late autumn. The climatic conditions also affect the rapidity of decline; the rate of fall of temperature, the storms, etc., hav- ing a decided influence in hastening or retarding the approach of the winter conditions. Near the last of December, however, these conditions are fairly established, and the crustacea pass through the winter with but little change in number and aver- aging from 100,000 to 200,000 per sq. m. of surface. A glance at Fig. 6 will show that this complex rhythm recurred with an exactness quite surprising. While the abso- lute number of crustacea present varies considerably, the shape of the curves indicating the movement of the limnetic popula- tion is strikingly similar. The resemblance is the more surpris- ing when we consider that these maxima and minima are due to the increase and decrease of eight species of crustacea, whose numbers are independent of each other, avi which appear in very different numbers at different seasons and at the same sea- son in different years. The lines of diagram 6 represent, there- fore, the sums of a number of independent variables, never fewer 304 Birge—The Crustacea of the Plankton. than three in winter nor more than eight in the period from July to October. In the study of this rhythm of development, three facts may well be noticed in the first place. First, the number of crus- tacea in lake Mendota is to a singular extent dependent upon the perennial forms. In other lakes it often happens that the periodic forms are the dominant members of the summer popu- lation. Of these forms, Bosmina is practically entirely absent from lake Mendota; Diaphanosoma appears in small numbers only; and Daphnia retrocurva only rarely equals in number the related species, Daphnia hyalina. There is, therefore, no great increase in numbers in summer dependent on summer forms alone. Indeed, the influence of the periodic species is not greatly felt until September, and the shape of the developmental curve would not be greatly altered, were the periodic species omitted. Second, Chydorus occupies a peculiar place among the plank- ton crustacea. It is properly a marginal form, and appears in the limnoplankton only under favorable conditions. Apstein has connected its presence in the limnetic region with that of | Chroococcaceae. My observations seem to connect its abundance in the limnoplankton with an abundant development of these and similar plants. In other words, it seems true for lake Mendota that periods when the diatoms and Ceratiumare the only abundant algae, are periods when Chydorus is present in small numbers; while in periods when the Schizophyceae or Anabena abound, Chydorus is also abundant. The maxima of this species, therefore, have occurred without close reference to temperature or season, and may come at any time from June to late October. These maxima are also very irregular in amount, number, and duration. Chydorus, also, is peculiar in the limnoplankton on account of its small size. It contains little more animal matter than a good-. sized nauplius, and decidedly less than an embryo Daphnia. While, therefore, a great abundance of one form of plankton crustacea usually affects unfavorably the number of other spe- cies, Chydorus appears to be more independent of the presence of other forms. It seems, as it were, superposed on the regular limnoplankton, rather than a part of the general limnetic life, Sc tea ee Ae city PLES > ng rn The Annual Distribution of the Crustacea. 305 and its rise and fall seem measurably independent of the condi- tions to which the other species respond. A third fact concerns Daphnia pulicaria. This species had a biennial period of development about thirteen months long, ex- tending from July to August of the following year, and a period of rest, in which it was almost entirely wanting in the plankton, extending from late August to the following July. In 1894 a few representatives of this species were found in July, and it wholly disappeared in August. In 1895 they were an important constituent of the crustacean life from July on, increased greatly in late fall and early winter, and continued numerous through- out the winter. In April and May, they increased enormously, producing maies and sexually mature females, and then declined, practically disappearing in September. This species was there- fore a constant and important factor in the number of the crus- tacea during the last half of 1895, the following winter, and the spring and early summer of 1896. It was absent during the latter half of 1894 and the spring and early summer of 1895. I will now pass to a brief discussion of the general crustacean life as it appears in the different seasons. I shall reserve most of the discussion of the causes and conditions affecting the num- ber of crustacea to a later chapter. The Crustacea in Winter. _ All of the perennial crustacea are, of course, constituents of the winter plankton, and their numbers are not very unequal. The number is by no means small, averaging about 125,000 per sq. m. from January to the middle of April, 1895, and about 235,000 from January to April Ist, 1896. The following list shows the species present during the two winters in question. TaBieE III.—Species, with average number of each per square meter. 1895. 1896. PMS ae cie ce einen oe selene oa cece tine caer cs ence dete denn ocss ses 24, 500 34, 800 eRPMNN RNC Rh ea oe. wg NAL EBIEe We dee Soba ob cnet cuee a cdawbes 06 52,100 120, 900 0 RSE aE Oe SR ai ee ke aS ee a ae ee 46, 200 22, 700 DISET PENH doh he oie sala od Kisha(a 4 $id wisn d'a| Va,5 o.956\5)b odie vaidieleaemiliea's + Wamjelee 48, 400 : Be ioras Be eb eh atts) aisha khY seid crv cloths oie/tuis« eisjsiole's osisin ei idie'sis [in mabiannd ons e 7,9C0 ge een cas teen eae ~~ 322,800 | 244,500 806 Birge—The Crustacea of the Plankton. It will be seen that in 1895 there were present only three species, while in 1896 two others were added. In 1897 the con- _ ditions were essentially similar to those of 1895. Indeed, while the time from which my observations have extended by no means warrants any positive assertion in the matter, there seem tobe . distinct indications of a biennial periodicity in the plankton in respect to crustacea, algae, and rotifers. Observations must be continued, however, over a much longer time before any definite statement can be made on this subject. The winter numbers of each species are on the whole singu- larly constant through the season, as will be seen by reference to the tables giving the numbers of the several species. The death rate must be very low, During the period, January— March, the variation in the number of crustacea taken in twenty or more catches made each winter vary to an extent hardly greater than might be found in catches made close to- gether on the same day. It would be very difficult to prove any considerable decline in numbers of Diaptomus or Daphnia during the winter and they do not increase by reproduction. — Cyclops produces eggs much more abundantly than the other species, and the adults seem to become fewer in late winter and late spring, but their number is more than made good by young individuals. In 1895 Cyclops began to show numerous egg clus- ters in February, and about ten percent. of the specimens were egg-bearing females. These eggs developed very slowly, and few nauplii and almost no young Cyclops were seen. In 1896 the reproduction of the Cyclops hardly stopped at all during winter. In the middle of January nearly one-half the Cyclops bore eggs, and numerous nauplii were present. By the middle of March the nauplii had grown to young Cyclops, from three-. 5 fourths to seven-eighths of the total number of the species were immature young. The winter minimum therefore falls in the period before Cyclops has begun this winter reproduction. In 1895 the mini- mum came in January and in February in 1896. Yet through- out the winter months the numbers are so constant that no well marked minimum can be placed at any date. In 1897 the condi- tion of Cyclops was intermediate between those of 1895 and 1896. The Annual Distribution of the Crustacea. 807 Young Cyclops began to appear under the ice, but the condi- tion of the species in the middle of March resembled that in the middle of February in 1896, and the progress of the develop- ment was in general about a month later. The rotifers also show similar differences in reproduction in different seasons. Of this group there are regularly present during the winter, Triathra, two species of Notholca, Anurea _ aculeata, cochlearis, and brevispinosa, Synchaeta pectinata, and a species of Oecistes. All these reproduce more or less actively, and become quite abundant before the breaking up of the ice. Other species are present in smaller numbers. The difference in the reproductive activity of these animals in different years seems to depend upon the temperature of the water, as will be explained at length in a iater section of this paper. In allseasons there is an abundance of food. One of the chief winter algae is Aphanizomenon, which continues its development vigorously throughout the entire winter. Several species of the diatoms are also present, and in 1896 Fragilla- ria and Diatoma contributed largely to the plankton algae, but in 1895 and 1897 were insignificant in quantity, as compared with Aphanizomenon. There is no season of the year in which the crustacea fully overtake the food supply, except at the time of the spring maximum. During the winter the crustacea are ac- tive and fat, but those species which do not reproduce do not increase in size. Careful measurements of numerous individuals of Daphnia hyalina showed no appreciable increase in the aver- age size between December, 1894, and April, 1895. When the temperature of the water is between 1.5 degrees and 2.25 de- grees C., ‘Cyclops develops very slowly or not at all from the nauplius state to that of the immature Cyclops, but at tempera- tures above 2.5 degrees the development goes on, although, of course, more slowly than at higher temperatures. The Crustacea in Spring. Lake Mendota has no large affluent, and the breaking up of the ice is slow, since it is due to the combined action of rain, sun and wind. The date of the disappearance of the ice differs greatly in different years. In 1895 the last expedition on the 308 Birge—The Crustacea of the Plankton. ice was made March 27th; in 1896, March 29th. The first col- lection in water was made April 12th, 1895, April 4th, 1896.. In general, the lake opens either wholly or over the greater portion of its surface about the lst of April. The period im- mediately following the opening of the lake seems to be a time of trial for most of the limnetic crustacea. The temperature of the water increases very slowly at first, or, indeed, may be lowered temporarily; and the surface is, of course, agitated by gales which are so frequent in April. During the spring Cyclops ordinarily increases in numbers with a rapidity dependent on the rise of temperature in the water, and upon the reproductive condition of the species at the time of the disappearance of the ice. Diaptomus and D. hyalina do not begin to rise in numbers until after the first of May, as may be seen by reference to Figs. 8 and 9. During April these species are wont to decline in number, so that the smallest catches made during the year ordinarily come in the latter part of April or the first of May. Cyclops, however, increases with great rapidity. Reference to the diagrams and tables will show that in 1895 Cyclops increased more than fourfold in number during two weeks, and that this increased number was nearly quadrupled during the next two weeks. In 1896 Cyclops ad- vanced with even greater rapidity and about two weeks earlier than in 1895. In each year the increase in Cyclops was about @ month in advance of that of Diaptomus or Daphnia hyalina, and in 1896, about two weeks ahead of the multiplication of Daph- nia pulicaria. The spring maximum is reached during the month of May, either in the first or the latter part of the month, according to the temperature. At the maximum the population of the lake consists largely of Cyclops, about 70 per cent. of the total in 1895, and 80 per cent. in 1896 consisting of this species. The multiplication of the crustacea and rotifers during the Spring seems to be more rapid than that of the algae, and in late Spring at the time of the maximum, the algae are far less numerous with respect to the crustacea than at any other season of the year. In a word, the eaters multiply in excess of the food. This undue multiplication of the crustacea puts a check iil ig aa ag nl ae i ag a a a en, wi a % , d ¥ h SE ee I SE TOE ‘OTE ‘80g “"d00g = *acqyout "bs 10d vooeysn19 QOO‘NOT = deeds [vor}10A T ‘eTRog ‘Oa “IXX 97P[d ‘AON ‘LOO ‘Ladag ‘pny a} eurpeay “q IV "TIdd Vy ‘IX ‘TOA “PBoV “SIM ‘SUIT SE lr nage ** -O0T “+092 ** “008 OOF *- “996 ** 909 ~*00L ***008 "000° ‘OTS ‘80g ‘d 999 ‘ond "AON mr reye) ‘Ldag ‘pny "A TOP "ANO ‘VW ‘rojoul “bs tod veovysn1d 0Q0‘OOT = eoeds J ‘eeog ‘ggg ‘vedRJsnio SuIpeor]—'g ‘DI "laa y ns bes : ON Sir saa @ *BAIN90I1401 °C : : a Oe ae oe © 8 OTL OMG : p La Tg eg eae : ong cee ee ere 3 3 — —e— — —e:::::: snumoydeig “LOO “Lang “ony *IIXX 2%[d "ATAL ‘NO “KV TAL "T1ad Vy XO A SDE Ve SSA “SUBLT, --OOT “992 "008 “QF "008 ** "009 ***008 "006 °° "000 ‘T oo ‘OOL ‘T ee “002 ‘T pee aoe nnisheg a aa « ba Rha eRe Sade eee orate Swine > 4 ee fd a 3 Se ee he 4 ~ eee moet ze dia AS ~ Ha ee se aa ho 2 eae? ste ele 4 © oe * The Annual Distribution of the Crustacea. 309 on their development. At the time of the maximum Cyclops may number more than 2,500,000 per sq. m. of surface, but of this enormous number only a very small fraction ever become sexually mature. In any catch made at this season of the year, not more than five per cent. are mature, and not more than one or two per cent. are egg-bearing females. The great majority, therefore, of these Cyclops die without reaching maturity, and after the maximum has been passed the number of Cyclops de- creases even more rapidly than it rose. The decline may go so far that in June the number of this species is scarcely larger than in March. During this decline of Cyclops, the other perennial species are increasing in number, but their combined increase is more than counterbalanced by the decrease in the number of Cyclops, so that the late spring and early summer show a marked decline in the total number of crustacea. The Crustacea in Summer. The summer life of the crustacea begins with the decline from the spring maximum to the early-summer minimum. This de- cline is dependent in part on the decrease of Cyclops. In part, also, it depends on the fact that both species of Daphnia regu- larly decline after a brief maximum in late May or early June, and in 1895 Diaptomus showed the same decline. The total number of crustacea may be thus reduced to one-fourth, or less, of the number present at the spring maximum. The lowest point of numbers was about the middle of June in 1896, and about the first of July in 1895. In 1894, when observations be- gan, during the first week of July, the crustacea were apparently at their minimum, which was exceptionally low in that year, owing to the peculiar character of the vegetation during that season. It was not greater than the number in the winter of 1895-96. The crustacea increase in number after the early-summer minimum. This increase seems to be due to two causes. First, the development of species hitherto represented in small num- bers. In all years there comes at this time an increase of Cy- clops Leuckartiit, The numbers of this species differ greatly in 310 Birge—The Crustacea of the Plankton. different seasons. -In 1894 it was only a small fraction of the total number of Cyclops present, while in 1896 it was quite as - numerous as Cyclops brevispinosus. In 1896 Chydorus developed in great numbers in the latter part of June and early July. This development coincided with the presence of great quantities of Aphanizomenon. In 1895, which was characterized by a pre- dominance of diatoms among the plankton algae during the sum- mer, there was no marked development of Chydorus until au- tumn. The second cause of this midsummer increase is the re- newed reproductive activity of the perennial species, especially Daphnia hyalina. These species has a marked reproductive period and maximum in the spring, (Fig 16) at which time from five to nine eggs may be produced. After the production of the spring broods the reproduction is greatly checked, and the species de- clines rapidly in number; but when the summer temperature of the water has been established, the species again reproduces, so that its numbers increase rapidly. Only two eggs are, however, regularly produced at once during the summer. The result of these additions of new forms and increase of old ones gives a marked rise of the total number of the crus- tacea in late June and early July. This rise was very feeble in 1894, owing to the wholly peculiar condition of the vege- tation, as stated elsewhere. From this mid-summer maximum all of the species, except Chydorus, usually decline steadily and somewhat uniformly until the middle or the last of August. Three possible causes may be assigned for this decline: first, the exclusion of the crustacea from the deeper water of the lake; second, the increased tem- perature of that part of the lake inhabitable by them; third, the great development of Ceratium, which regularly becomes a predominant alga during this period, and which is much less available as food than the diatoms and Schizophyceae. Cera- tium exerts a more unfavorable influence on the number of the crustacea from the fact that the young crustacea are quite un- able to eat it. It is so large and its shell is so hard that they cannot master it, yet Ceratium occupies, with its enormous swarms, the upper strata of water, which naturally belong to the young crustacea. While, therefore, the adult crustacea may Sie ee ee The Annual Distribution of the Crustacea, dll find abundant food in the deeper strata, the young are unable to develop, and thus the total number of the limnetic crustacea slowly declines. The insect enemies of the crustacea, notably Corethra, are also very numerous at this time, but the number of these which I have found is not great enough to account for the decline in the number of the crustacea, and the increase of the crustacea begins in September, before the insect larvae begin to decline. I assign most influence to the first and third of the unfavorable influences which I have named. During this time the periodic species are added but their numbers are usually not great until after the first of September. The Crustacea in Fall. The number of the crustacea begins to increase with the opening of September (compare Figs. 6-9) and the increase continues during that month and into October. This increase is due in part to the increase in number of the perennial species. Daphnia hyalina and Cyclops brevispinosus multiply and reach a maximum in late September or in October. To these species are added the periodic forms, which are present in August, but ordinarily not in sufficient numbers to balance the decline in the other species. During September, however, all increase in number together, and bring the total number at the fall maxi- mum to a point more than half as great as that at the spring maximum. In 1894 the maximum, 821,000 per sq. meter was reached in the first part of October; in 1895, the maximum was 768,000, in the early part of October; in 1896, there were two maxima, one in early September, numbering 1,441,000, of which more than half was due to Chydorus. The other, the fall maxi- mum proper, was 1,368,000 and came in early October, or leav- ing out Chydorus, 1,123,000 in late October. The figures are the semi-monthly averages. The difference in these dates is apparently dependent upon temperature. If October is warm and pleasant, the development of the crustacea continues longer, and the maximum is greater than under other climatic condi- tions. In all seasons food is present in superabundance at this time of the year. The algae are at a maximum, and are enor- mously in excess of any demands made upon them by the crusta- 312 Birge—The Crustacea of the Plankton. cea. The species present are those which are most easily avail- able as food, so that both in kind and quantity of food, the crustacea find the most favorable possible conditions from early September to the latter part of November. Temperature is the predominant factor in influencing their development. In 1894 and 1896 Chydorus was present in great numbers. Both of these seasons were characterized by the great abundance of Aphanizomenon. In 1895 and 1897, when the predominant algae were almost exclusively diatoms, the number of Chydorus was extremelysmall. Diagram 10 shows the number of crustacea from July to December, after subtracting Chydorus. It will be seen that the form of the curves is strikingly similar in all years, and that the numbers are extremely close for 1895 and 1896, with the exception of a great rise in late October, 1896, which was due to the sudden multiplication of Daphnia hyalina at that time. From the fall maximum the number declines, at first rapidly, and afterwards more slowly toward the winter minimum. The rapidity of the decline depends upon several factors. Ifa large number of young forms are produced late in the season, many of them die as well as their parents, and the decline in num- bers is correspondingly rapid. The number of the periodic species also exerts a great influence. In 1896, when Daphnia retrocurva was present in large numbers, its sudden disappear- ance at the close of its sexual period aided to cause a rapid de- cline in the total number of crustacea present. The climatic conditions also exert a great influence. A rapid decline in tem- perature, accompanied by violentstorms, causes the numbers to sink more rapidly than a more equable approach of winter tem- peratures. In any case the number of the crustacea falls off rapidly during November, more slowly during December, and by the middle or last of that month the lake freezes and the Winter conditions are fairly established. The different species of limnetic crustacea enter the winter in very different conditions. Daphnia hyalina produces in the late fall large numbers of young, which serve to carry the species through the winter. The old individuals disappear during No- vember and December, very few lingering into January. Dur- Trans. Wis. Acad., Vol. XI. Plate XXIII. JULY. AUG. SEPT. Oct. Nov. 400... 300... 200... 100... Fic. 7.—Leading crustacea, 1894. Scale, 1 space = 100,000 crustacea per sq. meter. See p. 308. D. hyalina..... Cyclops. 2.2% 6 Diaptomus... Dw ies i Oe JULY. AUG. SEPT. Oct. Nov. DEc. 1,100... 2 / Pine ae 1,000... / ee : ( é / \ : : : e e : : nA \ 90053 : INDE, 7. 00.0 : u ; é : -\ 100.),." \ : uk) Baie \ om re \ y \ 600... \ a : .—— re ‘e 2 \ 500 f me : P--—..-@ / if ‘e : : : ras \ 400... Bes Aen MEE ee Nas : Bae ‘ N et : | : EN i . ; : : sae SO). os AEA : : : ~ » ; NG N wate Ne ona 200 ee ee SAE Se eS ee ee eee : 7 < 11 a eRe aay hea AS Oe SORA ATG eM . y : : { Fic. 10.—Total crustacea, July-Dec., after deducting Chydorus. Scale, 1 vertical space = 100,000 crustacea per sq. meter. See p. 312. The Annual Distribution of the Crustacea. 313. ing the same months, those individuals of Daphnia retrocurva disappear, which have survived the reproductive period. Diap- tomus begins its decline in September or early October, and seems to make no special provision for winter forms. Cyclops continues its reproductive activity through the year, at least in periods when the temperature of the lake is above 2° C., but with arate of multiplication declining as the temperature falls below 15°. Larval Copepods are present in great numbers at. all seasons, but their development into later stages is checked in winter. Chydorus seems to have the same habit as Cyclops; but, for causes as yet unknown, it almost disappeared in the winters of 1894-5, 1896-7, although abundant in the preceding autumns, and present in considerable numbers in the winter of 1895-6. Daphnia pulicaria had a marked reproductive period in early December, and continued reproduction at aslower rate through- out the winter. Diaphanosoma disappears in October, and Leptodora in late November or early December. TaBLE IV.—Average number of crustacea for each two-week period and their sum, stated in thousands and tenths per sq. meter of surface. : D Diap- a clos. [teeta | anee"| retto- | goby, | hano- | Total 1894. TOS he a 242.2} 39.8 6.4) 19:8 a a a | 306.2 July 16-31 . 2959 1150.0) Sa{ 193.) a a 0.8| 472.3 Waeust 1-15...02......... eer | 218.7 | 151.0 1516.6 a a | 6.3 | 401.1 August 16-31............ 87.4} 200.3 0.8) 60.7 a 15.0 | 18.0} 382.2 eRe EBay eS eee stein Soci feas «cao [een onsite foe ss eee es oe ese Le icone eee a a ase September 16-20........ 54.6 | 190.1 a 148.4 a |278.9 | 19.6 691.6 October 1-15............ 67.6 1347.1 a | 207.6 a | 193.3 | 5.2 | $20.8 October 16-31........... 93.3| 261.3] a |252.5] a | 202.0) 3.0) 757.1 November 1-15.......... 44.0 | 246.4 a 183.1 a 97.9 a 571.4 PMR MERIC ieee We cae bd 2 Se MS oe ellen eats call Seen naee hp sca ens e|ne abinnlell meas idigels December 1-15.. 23.9 75.0 a 121.5 a | 9.5 a 219.9 December 16-31.......... (16.7)| (44.5) a (49.0) a | (1.65) a (111.9) 1895. » ganuary 1-15 ............ 17.65 21.5 a 40.8 a 1.3 a 81.1 January 1-15. 15.9)| (40.0)} a | (55.9| a | @.0)} a | Cit.8) . ros 1-14. (44.5)' (80.8) a (75.3) a a a ' (200.6) 314 Birge—The Crustacea of the Plankton. | TaBLE ITV.—Continued. ; : ; signe ise) £2, [Pepe Dy sotro- | OMY: \‘hano-| Total, | 1895. February 15-28.......... 23.0} 3.1] a 65.8| a a a | 16.9 & Marek 105 We Bei! 28.3) 55.7] a nr a a | 118.7 @ March 16-31 ............. 34.7 66.2 a 63.6 a a a 164.5 . PN Bs er ae pn Oe aor 14,041 BAD Wigan BIN. ve a a | “3 @ April 16-30, ....0.60s26e0-5)) |; 20.6) 1 242.5 a 16.3 ae seat. a 229.4 May tls, feo Wie 34.4 | 364.9| a | 289) 9 | G2 ee May ts-o10 0) ee 207,9|944.4| a | 250.7; a | 165] a |14l9.5 DUNG WaT ioe viseeciveree ee as 285.0; 616.9 a 319.2 a 36.7 a |1256.6 Tange 16-30 eee 190.6 | 262.6| a | 135.6|Scat.| 21.9] a | 610.7 July 1-15 ................| 187.4 | 323.6 | Scat. | 139.9 9.7 | 156.8 | Scat. 817.6 JWy 1G-Bl oe oa secaswes | OLIGO doles 11.6 |275.3 31.5 | 163.4 6.9 837.9 August 1-15.20... -...2....1 11015)" 107.6 19.9 |273.0 68.2 78.6 31.5 689.1 August 16-31............. 101.3 | 129.6] 38.1.| 252.8] 50.1} 18.7] 32.2} 6228 September 1-15.......... 224.6 | 142.0] 33.8] 202.8] 23.8] 15.6| 27.1] 669.7 September 16-30......... 331.5 | 226.0] 98.2] 201.6| 53.6| Scat.| 17.2] 928.1 October 1-15............. 148.4|327.5| 26.9| 180.5] 72.5 8.6 3.4] 167.8 October 16-31............ 79.7 | 219.7] 23.5] 76.6] 70.9 8.1 a 418.5 November 1-15 .......... 55.8 | 144.7] 49.6] 56.2] 59.3 | 25.9 a 391.5 November 16-30......... 46.0) 135.4] 58.3] 48.2] 24.2] 19.7 a 331.8 December 1-15........... 33.6 90.2) 141.1 35.0 5.0 15.9 a 320.8 December 16-31.......... 58.0! 89.1] 99.8) 44.6 0.7! 28 a 313.1 1896. January 1-15............. 48.6 | 111.9 88.2 36.2 a 10.1 a 294.1 January 16-31............ 23.31 151.0 24.8 17.3 a 19.5 a 240.9 February 1-14...........| 38.9 91.6 64.1 19.6 a 4.8 a 219.0 ‘February 15-29........... 35.0 82.0 43.9 27.0 a 3.8 a 191.7 March: $905 i605. cise caek [Sec ue cllvacs caeldees apact saeco sl] Serle © seiell aerouimeera aa March 16-31.............. 33.3 | 212.5 20.9 13.5 a 1.4 a 281.6 April $955.05 5. Meee BBLS A OOLT Baroy | aaee a 1.9 a 480.4 April 16-30...............{ 29.9 1,011.2} 118.2] 15.2] a 98} a | 1,184.3 May 1-15.................| 102.3 |1858.4| 284.9] 1246] a 23.0] a |2398.2 — May 16-31.....000 00000000 360.2 | 705.9 | 533.6 | 270.8 a 30.8 a |1901.3 Sone Tas 343.5 | 189.5 | 168.6 | 55.6 a 87.6 a 844.8 June 16-90...............]386.2| 358.7] 78.2| 211.1] a | 230.8| a | 1,265.0 Maly 400 jean Wee ake 202.9| 371.0] 39.3/319.0| a |/382.0| a | 1,314.2 q Duly 16-8860 2 ee 152.1) 317.5) 11.8' 63.5 2.5 245.1 |Scat. 176.5 The Annual Distribution of the Crustacea. 315 TABLE IV.—Continued. Diae.| Obs, [BRME PART zotro- | be, | Rane: | Total 1896. PAC MSG THUG... ea see ene. 91.9 | 326.8 3.7 95.2 27.6 | 406.5 8.9 960.4 August 16-31............. 167.0 | 209.0 5.9 60.9 57.1 | 426.0 | 147.4] 1073.3 September 1-15.......... 1259 | 157.1 23.5 | 120.4 | 157.7 | 748.6] 108.3 |1440.9 ‘September 16-30......... 163.4 | 228.6 3.4] 1925 | 228.6) 263.0 32.9 |1112.4 Wetober 1-15 ............ 52.8 | 364.8 0.4 | 228.0 | 199.3 | 423.7 0.4 |1368.4 _ October 16-31............ 48.& | 469.5 a |511.5| 92.7] 191.9 a |1314.8 November 1-15...........| 29.8] 267.7 |Scat...] 314.6 9.9 62.7 a 684.8 November 16-30.......... 23.5 | 173.9 |Scat...| 266 0 a 69.3 a 537.7 December 1-15...........| 29.3 | 115.5 |Scat...} 182.8 a 38.2 a 365.8 -December 16-31.......... 24.7 93.1 |Scat...| 138.9 a 28.1 a 284.8 In this table maxima are indicated by bold faced type and Minima by italics, a, means absent; scat., scattering individ- uals not enough to count. Parentheses indicate that observa- tions were made on a single date in the two week period; —, ‘indicates no observations. Although the general course of the development of limnetic erustacea is so nearly the same in successive years, yet the com- position of the crustacean population may differ very widely. This will readily be seen from the tables, and still more easily by the diagrams which show the numbers of the individual ‘species of crustacea in the different years. A single illustration is given in Figs. 11, 12, and 13. These diagrams represent the average number of the crustacea in the latter half of Sep- tember, 1894, 1895, and 1896. The area of the circles is pro- portional to the total number of crustacea, and the size of the ‘several sectors is proportional to the number of the individual ‘species. It will be seen that while the total numbers are not very widely different, there is a great divergence between the individual species. Diaptomus, for example, is by far the most numerous in 1895, while in 1894 it is the next to the smallest. In 1894, on the other hand, Chydorus is by far the largest; while in 1895 it is not represented at all. WD. retrocurva is one 316 Birge—The Crustacea of the Plankton. of the most important species in 1896, and had a fair develop- ment in 1895, while in 1894 it was wholly absent. No reason can be given in most cases for these variations in individual Species; but where a cause can be assigned, the subject is dis- cussed in the section which deals with the Single Species in. — detail. Diagrams 8 and 9 show on Single charts the numerical rela- tions of the most important limnetie crustacea during the sea- sons of 1895 and 1896. Several facts become very plain from. these diagrams. First, the development of Cyclops precedes. that of Daphnia and Diaptomus by nearly a month, and precedes. that of D. pulicaria by something more than two weeks. This. relation held in both years, although the development of all. the crustacea was some two weeks earlier in 1896 than in 1895. Second, in both years Daphnia hyalina and Diaptomus began. their development together in the spring and rose together to. the spring maximum. This coincidence was probably due to the rapid warming of the lake in both seasons. Figs. 1 and 2 Show that the temperature of the water rose with much the same rapidity in the two years. Diaptomus requires a higher tem-. perature for its development than does Daphnia, as is shown by the fact that it declines steadily after the lake falls below a. temperature of 20°, while Daphnia has its great autumnal pe- riod of reproduction in the month of October when the tempera- ture is below 15°. In the Spring of 1897 the warming of the. lake was slower than in either of the two years covered by my Study, and the development of Diaptomus lagged decidedly behind that of Daphnia. I am not able, however, to give the: exact numerical relations. Diagram 9 shows also that Daphnia pulicaria began its course: of development about two weeks in advance of Daphnia hyalina, Another fact is disclosed by Figs. 8 and 9, namety, that in. each summer some one Species of limnetic crustacean appears. to take the lead, and decidedly dominates the other forms. In 1894, as shown by Fig. 7, this species was Diaptomus, In 1895, as shown by Fig. 8, Daphnia hyalina maintained its: numbers full through July and August, gradually declining through the autumn, and being nearly twice as numerous ag. Trans. Wis. Acad., Wool Xie Diaptomus D.hyalina JS 148 Ch ydorus 279 Fa. 11.—Crustacea, Sept. 16-30, 1894. Diaptomus 332 Fic. 12.—Crustacea, Sept. 16-30, 1895. Dthyalina 193 Diaptomus 163 Diaphane. 33 D. PEM gaigg Diretrocu rva 229 Fic. 13— Crustacea, Sept. 16-30, 1896. See Table IV and p. 315. Plate XXIV. ‘ ; ‘ bs ' n ‘“ ‘ ' 4 4 ’ \ ’ . « ia t Pa . ) ‘ é " = ~ : . > » . ri - La ‘ * * rn F ; > : \ 4 i q l * ’ * e S j “ A a! ie ‘ i . The Annual Distribution of the Crustacea. 317 _ the other two leading genera. In 1896 Cyclops held a similar place, recovering rapidly from its early summer depression and maintaining its numbers full throughout July and the early part of August. The diagrams show further how all the species of crustacea increase in September, and that the rise persists to different dates in the Jater autumn. In 1895 Diaptomus showed a max- imum in late September, and that of Cyclops came in the first half of October. In 1896 Daphnia hyalina and D. retrocurva rose together from the latter part of August to the middle of ‘October, when the former species had a period of enormous re- production, while D. retrocurva, which had produced its ephip- ‘pial eggs, rapidly declined in number. The increase of Cyclops in ‘this year also continued until late October. The diagrams show further how all species rapidly decline in number in November, and then more slowly during December, reaching their perma- nent winter condition in December, or at latest about the first of January. The feature of the annual distribution of the crustacea which ‘surprised me most in the progress of my work is the great dif- ference between the numbers of the same species of crustacea | present in successive years. Ido not refer so much to the larger or smaller numbers of forms like Cyclops, for whose variations causes can be assigned, at least in part, but rather to such facts as those shown by Daphnia retrocurva and by Diaphanosoma, which are either absent, or present in very small numbers in ‘one season and appear in great numbers in another year. For Such variations it is very difficult to assign even conjectural ‘causes. A similar fact has appeared in‘the succession of the algae. It is not true for lake Mendota that the forms of algae suc- ceed one another in a definite order in successive seasons, so that one can be sure of finding certain forms at certain times of year, as would be the case with plants of woodland or prairie. For example, in the winter of 1894-95 Aphanizomenon and Clathrocystis were the predominant algae after the early part of January. In the succeeding winter these plants were almost entirely absent and Diatoma was the predominant form. In the 318 Birge—The Crustacea of the Plankton. / ee winter of 1896-97 Aphanizomenon together; the latter form being more abundant at the opening of the winter and the former relatively increasing towards. spring. Asterionella has been regularly present in all years as. a small part of the summer plankton, but never has been predomi-: nant except during a short time in the spring of 1897. Ceratium. has been a leading alga in the summers of 1895 and 1896, but in. 1894 and 1897 there was no Ceratium period. Lyngbya predom-. inated in July, 1895, but scattered filaments only were present. during the succeeding two seasons, while in August and Sep- tember, 1897, it was again present in considerable numbers,. though nowhere near as great as in 1894. The summer of 1895. was definitely a diatom season, as was also that of 1897, very few of the Schizophyceae being present; while in 1896 the latter plants predominated, although a considerable number of dia- toms were always present. In the autumn there has always. been a diatom period, but the predominant forms have been — Diatoma, Fragillaria, and Melosira in different seasons. The: first alga to develop in the spring is one of those which has. predominated during the winter, but the order of succession in the forms which follow is wholly uncertain, as the few illus-- trations given above sufficiently indicate. LARGEST NUMBER OF CRUSTACEA PER CUBIC METER. The following list shows the largest number of crustacea found: per cubic meter. It is computed on the assumption that the ani- mals are equally distributed through the three meter space cov-- ered by each haul of the net and gives the average per cubic: meter for the distance of three meters. In reality the maximum. at the stratum of greatest abundance would be greater than the table shows. Probably 600,000 would not be too high as the maximum for the total number in a cubic meter. The numbers are given as thousands per cubic meter. All, except. D. pulicaria are from the upper, or 0-3 meter level. and Diatoma were present Largest Number of Crustacea per Cubic Meter. 319 Table V. Diaptomus. Cyclops. D. hyalina. Afibbays) 2 Yio tot) Skee 88 | October 8, 1894......... BOUT Saliva 2s SO ea ene 101 tte A207 1895 oc ees ee SA May 185489025 26.52 ssees 180 | Aug. 21, 1895 ........ ee LOZ September 16, 1895....... 98 | May 8, 1896............. 290 | June)29; 1896.05.06... 2... 145 PIO D | ISOG fe 6.25 eet done OPA RS eI ita Sr oy IF age July ASI) akee" ween tes 170- dune 10; 1896 2.22. ......-% TR Aledo ce ROGER pra esr rare Oet.26, 1896. 3.05 5. se Sasi 122: D. pulicaria. Chydorus. Total crustacea. Aug. 22, 1895..41, 9-12 meters] Sept. 22, 1894............ 71 | May 9, 1896 ............. 347 Sept. 22, 1895..41, 15-18 meters] July 12, 1895............. AD May 18s) 1896. ss eee 392: Dec. 23. 1895..78, O- 3 meters] June 22, 1896............. 96 | June 19, 1896............ 415 May 18, 1896..78, O- 3 meters} July 7, 1896............. 131 | June 22, 1896............ 337 ROME ETN el Nei Siaupis's.@ islajeist yin nie Aires) Gi ASIG a cess MED uly Ty 1896) eo ates. 426) It thus appears that where most thickly massed, the crustacea. number nearly one to 2 cem. of water. Diaptomus Oregonensis Lill]. Figure 14. Table D, Appendix. The numbers of Diaptomus have varied from season to season less than those of any other species of the limnetic crustacea and they are also the least variable in daily numbers. Possibly the greatly developed locomotor organs of the animal aid in securing uniformity of distribution and also enable it to obtain so much food in times of scarcity, that its numbers remain con- stant when others decline. _Diaptomus does not reproduce during the winter and its num- bers show little variation during that time, as the following table will show. 320 Birge—The Crustacea of the Plankton. TaBLeE VI.— Diaptomus. Average number expressed in thousands per square meter of surface. aaa eee SSSSsSaw—————— 1894-5.|1895-6.| 1896. Ot et VG is AU clehee Os Whleiel eins aK ienelala tia aiclants ate olay a 67.6 148.4 52.8 Weto bor 16S essa Seu tha Sis ala td as war tunw a, nievea) soralalaienae ae ealtare els 38.3 79.7 48.8 INSTRU V1 D5 caeee ae eimai ata i bls Aiemlaraieleye; pie mallee aietela tie eahs 44.0 55.8 29.8 IN@veMiber HG Ta0 osicie fe \eh Gidatste Aiciays, waite ruinialls alsin arahenets «arene die win a/ey] im eater 46.0 28.5 Deca ber ly ese s he cave eis erate wikia diohstamal oe cheats eisai 23.9 - 33.6 29.3 WMscoinlber MOG yi ws arccivse mace toe cnet esa oe phakcwane estes te (16.1) 58.0 24.7 MANU ACV PUD ee ce ws as ed VES alia ch otaRa wecaitete a Ace cla ceia] veers iota ake 17.5 43.65" | faaaee MUR GSU eis aN GU AOU At ives i II aed iat (15.9) 28.535 e's aunenen ee JOYS oF cL ies WAZ Lo) SV ae pete nen ga ae URI ATA lor ee HCA BEN (44.5) BS Oil eee ABS DTUARY, TORE ke wicae ate cin tie leciahohale ole te Ciaub Bye inveterate veraiae 23.0 BH me PRON SE 3 3 2p eb TLDs Sele ciciaaiaewas ante eae ne ima Aualamcinielas stellata sina aat 28.3 March 16-31........ | 34.7 33:3) pes ssceeaer PRTC DED hls ASL Ud wictete) ested wie cies She yw Stata: efedesed onal a nese elaiialae Gievese 14.0 95.2) | Usa aseeer OTE AC BO ee iso state aacreniebane tice a eis ine wielaya ehelarays Aa cobs rite Gia dan onal 20 6 20.9 «fo visdieaeote EES st AN eR CAO Se IES ie aie Say ae eRe 34.4 102.3 |} cecaeeieae EIS AG (SED A AER) URE Dae ae eile ae ak ga ala a etn ea 207.9 S60. 0 Ms cceaceues Numbers enclosed in a parenthesis rest on observations made on a single day during the half-month. These figures show that Diaptomus begins to decline toward its winter condition early in the autumn. There is no marked reproductive period in the fall which supplies the individuals which are to live over winter, but the numbers steadily and rather rapidly decline after the time when the lake has decid- edly cooled from its summer temperature. The table also shows that the mortality must be very small in winter. In spite of the fact that there is no reproduction, the numbers show very little decline after the winter conditions are fairly established, and only a slow decrease in the late autumn. Indeed from the middle of October until the first or middle of May, the semi- monthly averages show no more variation than might easily appear in two catches made on the same day at the same place. This persistence of the numbers of the species must be attributed to the absence of competition and of enemies during this season. The food supply is ample for the winter stock of crustacea and Trans. Wis. Acad., Vol. XI. Plate XXV. APRIL. May. JUNE. JULY. Ava. SEPT. Oct. Nov. DEc. 1894.... 10 © omen 0 0 Go 6 emucoe ‘Agee +a _ & * r ft 7 ; oe i “a | { aoe vot |e gid le ere ee awae 7 , id. Diaptomus. 7!) Ook the reproduction of the crustacea in winter is slower than that of the algae. It is not impossible that the slight decline in numbers noticeable in 1895-6 may be attributable to the multi- plication of Cyclops in that winter. The decline in Diaptomus is too small to allow of certainty in the inference, but the adult Cyclops fell off rapidly in March of that year as they did not in ‘the preceding winter when little reproduction took place. Food ‘also became much more scanty in the spring of 1896 than in the preceding year. The amount of food material in the spring of 1895 was estimated as at least four times as great relatively to the number of crustacea present. The chief enemies of the crustacea are the larvae of insects and the young fish, both of which are absent or few during the winter. Leptodora also, though living chiefly on Cyclops and Daphnia, must devour some Diapitomi during the summer; while it is wholly absent in winter. At this season the perch, which also feed on the small crustacea, are at the bottom and apparently do not feed at all. There seem therefore to be no enemies of. the crustacea during the winter and their numbers are corres- pondingly constant. Throughout this season also Diaptomus is fat — fatter than in summer, as the drain on tissue for reproduction is absent. In April after the ice breaks up the crustacea are wont to de- cline in numbers. This is especially true for those species whose reproductive period comes somewhat late in the spring, and in which only the incividuals which have lived all winter are pres- ent in the spring. These find the conditions of the open water of the early spring harder than those under the ice, especially as they are exposed to the competition of the increasing swarms of Cyclops and sometimes of D. pulicaria. The smallest catches of Diapiomus which are met during the year, are obtained in the latter part of April when the number of Cyclops has risen greatly — more rapidly than the food has increased. In May there comes a great increase in the number of Diapto- mus. It shows itself first by the presence of a great number of immature animals in the upper strata of the water. In both years the appearance of these new members of the species was very sudden, as will be seen from the following table. 21 822 Birge—The Crustacea of the Plankton. TaBLE VII.— Showing the actual number of Diaptomus caught during May. 1895. 1896. NEA NOs Meee ACBL Giga sie nia 270 May 2iiie del. ok dehen eee 730 May FS 3's a Saas Meares etan deere 410 May 4. sds). osc ee eee 660 5 EE Re Ge MAE toe aol oe 710 MAY Gis scien» caeintoeuleiee en 980 ate ae A ae ete nae 780 MSY By wis sic oaitic dsjnil std aie eee 600 Ia ES all (cae PEE NG EOP S s eays OH Peta 2,200 May Oi eh uuieodnpativedes coke ee 560 May ZO er ee eh eke hec ial 1, 650 May AL ica s seine etic 6) side! see 1,945 Maye eels cian ints teen alee ariafe 3,820 8) May 25... ..cie ids «is 0% es ae eee 6,110 May 18 5 os. cies os 'ecrs'es coat meee 10, 250 Miaiy 20 a sio. ik: drome caper eee (svete noo 3, 690 It will be seen that these catches divide very sharply into two sets, the division coming between the 16th and 18th of May in 1895 and between the 9th and 11th in 1896. Catches earlier than those given in the table show the same general character as those given, as also do those taken later. There is no earlier eatch which is larger than 1000, nor one later in May smaller than 2,000 in 1895 or 3,500 in 1896. There is no reason to think that the increase of numbers is due to small, local aggregations of the species. The increase persists without intermission for long periods of time during all conditions of wind and weather. This alone shows that the large numbers must occur over great areas of the lake. On May 15, 1896, observations were made at different points, and the numbers were found practically constant at a distance of 2.5 kilometers in various directions from the regular place of collecting. It will be seen that the spring increase came just a week earlier in 1896 than in 1895—on May 11th and May 18th, re- spectively. This acceleration of development, which was shared by all of the crustacea, was chiefly due to the higher itn sie ture of the water in the latter year. In 1895 the ice went out on April 8th, in 1896 on April 2d. In each year cold and rainy weather followed the departure of the ice and at the middle of the month the temperature of the water was almost the same in both years, I ee Diaptomus. 323 April 15. 1895. 1896. SiPTS ETD) dA bee BOA PODS GLEE OI PREC HE SEATS uns IE Iie 1a UDR GTA) VP I Me EE 4.59 4.0° TEYOERROVIT sc) ey oe RS RG CIOTSEE ITAL Se ESL EER SES ATS 2 CRN ena CO 4.2 3.9 Later the temperature showed a nearly parallel rise at the surface, but a marked acceleration at greater depths for 1896, the following table shows: SURFACE. Borron. 1895. 1396. 1895. 1896. PATTEM LES rene state te yet ca elevara li sicreic are cb cieye wala: shell elcveis veleteiel Siete 6.0° 72° 5.0° 6.1° WMG ee oe a alalisie! = a 2015 lnie)alansigiaiaciulslabwigioe,eieicie na aio 8.0 8.0 eS 7.4 PORNO Ace CMe s citip iiclsainScelsleicctewacce eevee of . LES 9.6 6.9 8.5 ERE Geicias ca cies cain auisioeeis Ce peds belawneee does 10.8 16.4 6.3 9.9 PAPE Penasco a pidcivciegesvceteiceseceseancasst | 10.5 15.2 11.2 13.4 It thus appears that the average temperature of the water was decidedly higher in 1896 than in 1895, and to this fact I attri- bute the earlier appearance of the spring swarms of crustacea. There was nothing apparent in the increase of the algae to make any difference. When the young Diaptomus appear the number rapidly rises toa maximum which is maintained for some weeks, as the table shows: TABLE VIII.— Average number of Diaptomus during late spring and sum- mer stated in thousands per square meter of surface. 1894. 1895. 1896. TIMES) HATH Jo unoh sabe Wa ie ME aie I aa a aaa Geen RRS” | 102.3 PBLaea rs Here eet deena nue eRe mess ete uiductlay ou (Pewees oe] 207.9 360.2 Cibo g bo | or Hae ee ee oe Pa esa ee eeicns hcgaicilanee eee eet) Zoe 343.5 (ORIG) TARTS Ue eR RN SC os Ao ener Ree (eee ees 190.6 386.2 OTIS TRIB Oe RSV AIR ER AS AUDA UAE 242.2 | 187.4 202.9 TELLS TESS a NG EO ERO OE et or ae 298.9 217.8 152.1 PSE EGE ae Ue me ESCs katoassaeh 21808 110.5 91.9 [OTE TIS 1 (PES) Lele SNORE Se AUN SS ARR) Se 87.4 101.3 167.0 It will be seen that the numbers found in all three years are closely parallel. Indeed the July averages for the three years 324 Birge—The Crustacea of the Plankton. differ no more widely than catches might differ though made on the same day and close together. In each of the two years where the conditions of the preced- ing winter were known, the summer maximum was close to ten times the winter average. In all three years there was a marked decline of numbers to a late summer minimum in Au- gust; at which time the average number is 3 to 4 of the max- imum. In 1895 there was a very marked drop in numbers about the first of July; while in 1896 the maximum number was maintained throughout June and early July and then there was a steady decline for amonth or more. In 1894 observations began on the first of July. Diaptomus was practically stationary during the month and rapidly declined after the early part of August. These variations in number in different years are at present without completeexplanation. Yet the most singular fact — the notable drop in numbers about July first, 1895—-certainly extended to the species all over the lake. Observations were made between the first and tenth of July in that year even in the remoter parts of the lake, and with substantially uniform results. What- ever the cause it was probably the same as produced a similar fall in the numbers of Daphnia hyalina at the same time. The autumnal condition of Diaptomus varies with the temper- ature of the early fall. In 1894 and 1896 there was substan- tially no recovery from the August minimum. 1896, indeed, showed minor variations of number but on the whole the num- ber did not increase. In 1895 on the other hand there was a very marked rise of numbers in September, culminating in the third week of that month. We shall hardly be wrong in at- tributing this additional brood of Diaptomus in 1895 to the higher temperature of the water in that year. There was very little decline of temperature until the very last days of the month as the following observations will show: 1896. 6a. m. 6 a. m. 6a. m. O MOCOIS so5 co cicteresnie cow ha rtas earls mietectve eae ia anencteoy ie ereieiotoare 21.9° 20.0° 16.3° LO MOtOLS! ee Tea eu A RHE Lie CPS raul Secle ko bee Pe RI 20.9 20.0 16.5 PS Meters cece hus remo ore bob teteicla cubes ere craic rarest 13.9 17.7 16.5 u - - Re ee Si i seg Diaptomus. 325 Thus the decline of temperature for the month occurred in the last three days. ‘In 1896 the temperatures at the opening and close of the month were much the same as in the preceding year, but the decline was pretty equably distributed. ; Sept 1, Sept. 17, | Sept. 28 1896. 9:30 a. m. 4p. m. oon, f BREET AC TS en rea eis et slcirieg ava ofailoeinias he lnlieleveate oie eve ese: eislerdl iets s 21,22 1&.4° 16.0° RUSBTETES COS or aoe eosrey Sea dara Vip os Yovaaiiarereieh clavole!/bio eiei'e eieisl eve eiezevaieie eveve 4 20.2 18.2 15.75 PM SSPRCERESNES 4 oD Se RS A ICH 15.8 16.1 15.6 It therefore appears that the long continued warmth of 1895 gave Diaptomus a chance for an additional brood which did not appear in 1894 or 1896. Food, of course, is always present in superabundance during September. TaBLeE [X.—Diaptomus. The autumnal numbers stated in thousands per square meter of surface. 1894. 1895. 1896. PMR ETI SEEN ADE say Viet ayaa IE, Selate a elvajeloms| sie babe eae wan faves seine cees 224.6 125.9 SSPE ADERENE EO OO Yo.as: js 'v.n lene kre) ccivivie-e ajcie < wele Sle wesGeceGaeecees 54.6 331.5 163.4 MEMOIRS ls eee ecische wiaiaiece:eh o/c'eb cin eGewineleaie cine eaet capes 67.5 148.4 52.8 MEIC GOD ait 5) Sree wives + a 49.6 s METERED EGY 039 2 Scislc is winjn ives susiaia's w atesaigisienee bie hee 008 os a 58.3 s 1 NDOT DS og So ee eee ee a 141.1 8 2 DOLE ETL SES Se ea oa ee a 99.8 8 a, absent; s, scattering individuals only. 342 Birge—The Crustacea of the Plankton. The following statement shows the general numerical relations of the species, observations beginning in July, 1894: Season. 1894. 1895. 1896. 189%. SMU sevclexceatet tee ie ens oe sires ? Abundant ..| Absent.......; Abundant....| Very few... Early sammer ..2 30.0 ese sas ?Ephippia ..| Few.......... Adult males and females| Increasing. uate. summer 5... 26 6areee PO Wek a cea Abundant....| Few.......... Abundant.. PAMELIININT 202. aus owe role einarcrteeets Absent........|/ Abundant....| Very fow.. s.5.)ssscsuneeeeeee Winter sic e site ace cnie weldiecaerce oie [A DSON Giaccone Abundant....| Very fewiesscnisseeceeeeeeeee As was stated in my former paper, (Birge, Olson, and Har- der, 95, p. 473), this species is found through the summer in the deeper water only. Scattering individuals may be found extending to the surface, but even where one-sixth of the total number of crustacea was counted, the number of this species found rarely exceeded one individual; and in my studies during 1896, no individuals of the species were found from the upper levels of the lake. As will be stated more at length on the sec- tion on vertical distribution, D. pulicaria is confined in lake | Mendota during the summer to the space immediately about the thermocline. It is unable to rise higher on account of the high temperature of the water, and is unable to descend lower on account of the impurity of the deeper water in late summer and early autumn. This fact limits greatly the num- ber of the species during the warm season of the year, and in lakes whose bottom water is cold and not contaminated by de- composition products the number of the species is far greater during the summer months, and the period of active sexual reproduction is a much longer one. This species varies much more in numbers from day to day than does any other of the species whose numbers are at all considerable. The station at which most of the observations were made was not far from the southern shore. As a result of the action of the wind the thermocline is subject to considerable variation. A violent southwest wind, especially has the effect of driving out the warm water near the bottom of the lake, and thus temporarily raising the temperature of the Daphnia pulicaria. 343 deeper levels at the station. Under these conditions the mem- bers of this species which ordinarily live between the station and the shore become driven out from their ordinary place of - abode, and the numbers at the observing station are corres- pondingly increased. Thus on August 21, 1895, the number of this species caught was 493, a number not far from the average of the month up to that time. On the next day, the wind being strong from the southwest and the thermocline lying at an unusually low level, the number caught was 2,600. On the following day 954 were taken, and four days later only 85. The following table shows the details. TABLE XIX, Date. Wind. Depth. Temp. No. D. pulicaria. 1895. Above 9m. 0 9m. 21.4° 9-12 m. 480 PN ee ois coke ona Southeast........ 12 m. 18.4° 12-45 m. 5 15 m. 154° 15-18 m. 5 18 m 13.8° NEN hs ia) aires teiainic, &'esics oa e's Southwest. Above 9m. 9% Strong all day...| 9m. 21.7° 9-12 m.2,120 12 m. 20.4° 12-15 m. 360 15 m. 17.3° 15-18 m. 18 18 m. 14.7° Above 9m. 9 PONS eRe ie: ora ja, sleieth dis cince\'e a 03 Nearly calm. ....) 9m. 22.08 9-12 m. 640 12 m. 20.82 12-15 m. 220 15 m. 14.8° 15-18 m. 40 18 m 13.8° PREPS Ts ahais aia shorted aeitiiee eousiain Or. boy UE ae Grim. 6k 22.0? Above 9m. 0 12 m. 20.89 9-12 m. 0 15 m. 17.3° 12-15 m. 80 18 m. 13.9° 15-18 m. 5 In September of the same year 415 specimens were taken on the 18th, 2980 on the 22nd, and 3615 on the 25th. The condi- tions of temperature in the deeper water were much the 344 Birge—The Crustacea of the Plankton. same as on the former occasion. The rise in numbers shown — by the tables and diagram in the latter part of August and in _ September are therefore due to these unusual accumulations of the species and do not indicate a corresponding average rise in numbers extending over any considerable area of thelake. The ease is wholly different with the increase which comes in late November and December. This is occasioned by a very rapid multiplication of the species. The brood-sacs contain from 5 to 9 eggs. This reproductive period does not begin until after the temperature of the lake has fallen below 10°, and mul- tiplication continues, although at a slower rate, throughout the winter. In the spring comes the main period of reproduction; and during May, 1896, the numbers were uniformly large, yet even here they were subject to very considerable variation. At the time of the maximum, the species was the most abundant of the limnetic crustacea, with the exception of Cyclops, and since the individuals are so much larger than Cyclops, the species was the most important constituent of the crustacean plankton. It would seem necessary to suppose that the ephippial eggs deposited in June and July of one year remain unhatched for nearly a year. This is a very long period, and I have no direct observations which would make the conclusion certain. I am sure, however, that the species was practically absent from the plankton after August, 1894, since it was carefully looked for and only one specimen was found, and that in December. There _ was also no reproductive period in 1896 after the first of August, the increase in numbers in September of that year depending on an aggregation of individuals corresponding to that in 1895, there was no reproductive period during November or De-— cember, and the species declined in number, so that it was not practicable to enumerate it in the plankton. The winter eggs of Diaphanosoma must remain unhatched from about Oct. 1 to June of the next year. The peculiar history of Daphnia pulicaria in lake Mendota is conditioned in great part by the fact that the species is unable to live in the cooler water of the lake below the thermocline. In lakes which are relatively plankton-poor, the epg *d cog ‘un “bs sod eaovysnzo 000'¢g SSRIS te re ae ee ne [BOI}AOA T ‘opeog “9ERT ‘GERT ‘HoHNGraystp jenuUY “eAINdor404 ‘G—'gT ‘DIVq OS PIP Rc IOI OLLI LP LOI A OO - Sy TCI A AO MII ee ——--@°** — Or FEST we eet) "GEST 4 acs 5 : . — ieee : eee | : : peas : 0 — 8 — — 0» gg07 ii : : ve : ° 9 e Fy Ve: ° 3 | 3 5 ag "7E°d cog ‘10jour “bs «od \i ? ; BEde4SH.19 (OO'GZ = sords [voT}IeA T ‘oTwog “9BRT “GEST - ‘F68T ‘UOMNQIIASIP [enuUy ‘eMosourydeIq—'é6y— ‘DIA ' : . . ° a . ° 6 5 \ 1 ee A : 5 ‘ i] . . a e 3 | 5 ® ; / . 2 ° . \ ‘ e U \ bs ° « ; : ; \ COKE . \ ; ‘ 1 7 : ee a cL : ) : 4 I : : oat! : 2 002 vee l Sapa, ohare : Y : | : OOT i : \ : ‘ ; ; 0&2 ‘ Sere : { ‘ 1 es : : : : oe } { ; \ pe : : eae \: | ; ee : ; or es ete meee teres Toot ee, me OO ae) ee : a ee ; : : = : : Sore tGOR te ° ® ° i U . 8 ° y : paeNoe! 2 i vs: : : : ‘: . ae EN : ; ; ! Wo : : : . . e Le LF . Vv . « . ‘ eer - eeoeee (a "AON ‘LOO "Laas ‘pay welcray "AON ‘LOO "Laas ‘pny "RING is "XIXX 94° d "IX ‘TOA “proy ‘SIAA “SUvIT, Daphnia retrocurva. 345 species is found in far greater abundance during the summer in the cool, deeper water, and extends to the bottom of the lake. In the lakes of the Oconomowoc group, this species is abundant and is by far the?most conspicuous of the crustacea which are found below the thermocline. Daphnia retrocurva. Figure 18.— Table H, Appendix. TABLE XX.— Number per sq. meter of surface stated in thousands. 1895. 1896. eT A ene se le cases sabes sues atieroes s a mR TREE Re aisle cis ato wlaicicin claly vidicinin act neve weeeiasled amas ces 9.7 8 Duly 16-B1......... 200 ceee cece renee neces cece rece cen ceeeceeetccees 31.5 2.5 ERNE RRSP re er ioieiat ce Sicse aielciciainl eiciec civac)s Sasa waa wleaaesicis oe ob aces 68.2 27.6 REET SU ae Ce ue Coe tice sislee Uvles dee Soda awe duller cwiasnee 50.1 57.1 SMA SENE INTE IY yao 0 206 90> (erat clofaaipisisfai miele Nisinioie aie eidleein aavie @aew Ses ene 23.8 157.7 Se RSBMNACN PRES eM 21h 5 aie pisin.a/lcipnnic: 2b un es eles oe Feaie se lsecs esieeies 53.6 228.6 MRE Me ete ctels ayotiayoaituimnnieic es ioicajes seins mn Seascale shes ay. ay 72.5 299.3 MOREE CREE GE et oie crdin ara lore ete aunic.ois! Sievecaveie die sieicioiein'sie v siays ofelesic 70 8 92.7 _ REL Day iss tt 8 Oe re Bethe Merde aiiatens 59.3 9.9 November LOSE con GE ee eee ae 24.2 8 UPnP) 200 Lela da eis Se ee on ee ee ieee 5.0 s PISCCMMOST ISBNs 2. oe vos cone cane ve ee coee were ce en cone cece tees cee: 0.7 a Daphnia retrocurva belongs to the periodic crustacea, and its numbers have been very different in the three seasons of my study. In 1894 the, species was practically absent; two Specimens only were seen in July, and none were found in later months. In 1895 it was present in moderate numbers, and in 1896 the numbers in September and October were very consid- erable. The small number in 1895 is probably the result of the absence of the species in 1894. Perhaps also the competition of Daphnia hyalina had something to do with preventing the in- crease of the species in 1895. In that year Daphma hyalina was present in large numbers throughout the late summer and the autumn. In 1896 D. hyalina declined greatly in numbers in 346 Birge—The Crustacea of the Plankton. August, and in the latter part of the month both retrocurva and hyalina were practically equal and their numbers rose together during September and October. It is quite possible also that the lower temperature of the water in September, 1896, as com- pared with the same month in 1895, favored the development of both species. In 1895 the summer temperature of the lake was maintained until late in September. The result of this was apparently a great increase in the number of Diaptomus, and a steady decline in the number of Daphniae. D. retrocurva first appears in the latter part of May. The numbers are small, but two or three specimens can be found by search in almost every catch. During June it apparently disap- pears, or is much more rare than on this first appearance. It is not possible to estimate its numbers with any accuracy be- fore July or August. The males begin to appear in late Sep- tember or in October. They were first noticed on September 17th, 1895, and October Ist, 1896. The ephippia developed during October, and the species declines rapidly in November, and finally disappears from the lake by January Ist. The ephip- pia float, and many of them are doubtless driven to the shore, so that if the level of the lake is much lower in the spring and summer than it was in the fall, these ephippia may fail to develop, and thus cause a scarcity of the species. The maximum of this species coincides with the presence of the males. These, when at their greatest abundance number from 18 to 50 per cent. of the full number caught. They are always more abundant, relatively, in the upper strata of the water than are the females, agreeing in this particular with the young of most species of the limnetic crustacea. The food of this species agrees with that of the other members of the same genus. It eats Anabaena and diatoms in prefer- ence to other plants. It makes very little use of Ceratium and avoids Clathrocystis whenever possible. Marsh (’97, p. 210) assigns the maximum of D. Kahlbergiensis. to late October, thus agreeing with the corresponding species in lake Mendota. He does not say anything about males and since the species was present during the winter of 1894-5 it would seem to belong to the perennial crustacea of Green lake. Diaphanosoma brachyurum. 347 Zacharias (96, p. 53), gives August and September as the maximum, and also says nothing about males. The species was only occasionally present in the winter. Apstein (’96, p. 179), gives August as the date of the maximum for all species of Daphnia. He does not mention a sexual period, though he gives no especial attention to the subject. Had there been such a period as is shown by D. retrocurva it could not have been missed. Dianhdnosoma brachyurum Sars. Figure 19. — Table I, Appendix. TaBLE XXI.—Average number per square meter of surface, stated in thousands and tenths. 1894. 1895. 1896. BR fee ese lee ons 31s e waicleiAwiciciaislaie sobs neasvelenvcesleccccves ais s S TELS) Ue et PMNS ods Saisie x cieisl ak sles 6 0.8 6.9 S PMT EREU SGI Mo ecieret ni ceyariic vic) ois clerelaiciss cieiw aie cielo a cele Selsie earcla sees 6.3 31.5 8.9 22) LR ELLge See Oe 18.0 32.2 | 147.4 BRS ESC EURES OTOL oN hy percie tes sale) 9 21c/a1 aie ovelielSie nilola’sie t@.g/aisl a a'eiete'ee ove oa No obs. 27.1 108.3 CPSP ELON Go yas ee elaine aivdertisiettile Sikcicte Sieide Oogei wicroe weiae 19.6 17.2 32.9 MARR BeD Ue eter leven F 5 cis apniciarere < aleiersisie sieinisiciaeic. nent dees 5.2 3.4 0.4 ESM SI Ale) sichatisicisl “alsiaicin Wioie'<\s/emys cisie'v.av s-dees 6 3.0 0.0 00 This species is the least numerous of the limnetic crustacea which appear in large numbers, and has the shortest season. Scattering individuals may be seen as early as the middle of May, but they do not become a regular constituent of the plank- ton catch before the middle of July or the earlier part of Au- gust. They disappear in October, and are greatly reduced in number by the cold storms which usually come in late Septem- ber. Males appear about the middle of September, and the win- ter eggs are then produced. The species was far more abundant in 1896 than in either of the two preceding years, which agree with each other fairly well. For this difference I can assign no _Yeason. The numbers were constantly greater in 1896, so that the increased number was not the result of a few large 348 Birge—The Crustacea of the Plankton. catches. The life history of this species practically belongs to the period when the temperature of the upper water of the lake is above 20°. Apstein ('96, p. 166), Fri¢ and Vavra (’94, p. 103) find the: relations of Diaphanosoma quite as I have done. It does not. seem to belong in lake Ploen. Marsh (’97, p. 215) gives the species as present from June to November in Green lake. All. find it a little earlier in the spring than I have done. Chydorus sphaericus O. F. M. Figure 20.— Table J, Appendix. TaBLE XXII.—Chydorus sphaericus. Average number per square meter expressed in thousands. 1894, 1895. 1896. SAN ALY: Dei sy ied few cewieeldaiaien cus cieeieaeees f ay crotaneicers : 1.3 10.1 SPTAAVATEY Ml meh dete eles mc eres ian ne eo Atel waetevepal ajecsvaheiororcr meres (ay : a 19.5 Me Daa ry tal sock SA ici cIerasascioiamnd Siew icles ioeiaia ecient anions a 4.8 Me brilary loss esc ceule cae cak iio iste Goa ereeiamiererite terse a 3.8 IM ae Fi AND ce otha Me cicie ois aralvrarele cys. Otel ejard + scareiwichetaic cuore rani esciniate a No obs. Wilieaiae FUG Beats. | cie,s sie sistarn's 'gctose, aie bs pier oie breve esaistaaae nce tetoras a 1.4- 7A) Wh) Es i ane) Rn Ae 9 gre Tp AK a 1.9” April 16-30 ...... uasle uisloldttialoe aslo mw choke patos Cramteneserale ancy mine 4 s 9.8 May ID eit cis cues ots aise ia siciere aie niente SEO Ut che CEe 3 12.1 28.0> My TEU E fu cect Dea EM Le LO anata ae : 16.5 | 30.8. Toma dethe sis wa ol Va ta rc) 36.7 87.6 ‘Tajine AGO) en eR ESN A i el A 21.9 230.8 Daly Mos. sos cies se chevew tarscewl soe iowmoeo meee eee e ee ae s 156.8 382.0 Duby: TG BU iets ese oi stains ais rarascioie ie Susyainie wie stenta la qeaeene sale mis ele s 163, 4 245.1 August 1-15.......... By EDM OL OO ERNIE ead PECL s 78.6 406.5 - August 16-31................ coe alonishorap Stoo aes Ue ale as easier 15.0 81.7 426.0- September aoc as cio aysle vata cic sree eretete-tictetetocateteeters No obs. 15.6 748.0: September WOM et iat dye oe eee ee 278.9 ‘s 263.0: Octobar in Se a ee Rea te eee 193.3 8.6 423.7 Debian BOB. Ps ie eitiche invere satel b's bo: a dlale etaictiress aralntsie! AR aye 202.0 8.1 191.9" November 1-15........ Er OMe uR roo rac Uae SAMY e eB 97.9 29.9 62.7 November AB=30 isch 2nd dc inc ataiioied veeblen tvestaails sot No obs. 19.7 69.3: December 1-15 ............00 Boao te aan er ching Serb ao | 9.5— 15.9 | 38.2: Dacamber SG-S1.. .dcc-ckisince nemesis. sti pietice Ghisise see 1.6 20.9 |. 28.1. * Dd) Clee eee Ee ay ee s>~s> Trans. Wis. Acad., Vol. APRIL. MAY. % = see - XI. Plate XXX. JUNE. JULY. AuG. SEPT. Oct, Nov. 7 Scar e o ry 4 ‘ t . ) ‘ 0 L ) ‘ ’ e 4 0 LS gf Ly) ‘ *e ) ® 6 1 r) ee pee. ‘ p ai Q H : 4 L ! ot 1 = C} o ry 1 / ql rt 4 U ° @ ° « 5 e ° ) ry 6 r) / \ ° t 1 8 0 2 ‘ i) a 4 + a fase : offi. \ ® . ' T = t ° r) 2 cee ee no afl ; Ke x 3101p. con Wr Roney ase eee \ : ve Avs , ye ee : 3 200. .... Ae 2 is Soar : cee : ye ea : HHO ae d SS Fia. 20.—Chydorus. Annual distribution, 1894, 1895, 1896. Scale, 1 vertical space = 100,000 crustacea per sq.m. See p. 348. . 1894.... 1895 eaes pe ee 1896 .... @-—- Oo ee Chydorus sphaericus. 349 The above table shows that the number of this species is sub- ject to very great variation; yet there is a certain degree of reg- ularity in its appearance. The years 1894 and 1896 resembled each other in having a maximum in autumn, which was wholly absent in 1895. A large number was also found in July, 1895 and 1896, while practically none were present in 1894. In the winter of 1895-6, Chydorus was regularly present; while in that -of 1894-5 there were found only isolated individuals from time to time. I believe that these periods of abundance are correlated with the abundance of Anabaena and allied algae in the water. “The autumn of 1894, and the whole season of 1896 were charac- terized by a great abundance of these plants; while they were exceedingly rare in 1895 after the spring and early summer. The summer of 1894 was marked by an enormous development of LLyngbya, an alga quite too large to serve as food for Chydorus, and at the same time occupying the upper stratum of the water ‘to the exclusion of the smaller algae. The development of Chydorus is therefore dependent on the kind of food to a degree unusual among the limnetic crustacea. It is also dependent on temperature. In both 1895 and 1896 it was the last of the perennial crustacea in its development, no marked rise occurring before the last of June or the first of July. ‘This is the more noteworthy, since eggs may be found in the brood *Sac at any time during the winter. In 1894 and 1896 the maximum came about the middle of ‘September, while in 1895 only one small maximum was present, and that was in July. In 1896 there was no decline of the species in August, but rather an increase, and in this season Anabaena and allied forms were abundant throughout the ‘Summer. : , In 1894 the number increased very greatly between the 6th -and 10th of June, as is shown by the following record of the umber of individuals caught. PMCREL cS eee ae etalol Salen niciaree ays cei eisi oc osinislc: wiaieieieis/ejaleisiee wisi e'eicielaeie'® oe cisislsicrs 90 SPAM PETE eas Cart Best) ce AD 5 he iyiciald itis ‘clufow sh ofa aie Bte\scdia..» cidie vk mis 450 Juneé....., 120 LOSE LDS ashe ab ae es ee eee eer ree ee 4, 200 STERN Spee Ne MPN (cs aerate Lea i wi cle Web S's w nial bn wiser siete ar ercrstoren de sain'e witele 4,430 June 17.. Ra Re Pia Ey fas Nate Os aso clsninebiatalenraigeae pe 1,740 350 Birge—The Crustacea of the Plankton. Karlier and later catches agree with those given. On the 8th and 9th of the month there was a violent wind from the north and northwest, which probably brought this species out from shore water where it had been developing. . ; These facts indicate that Chydorus is not properly a limnetie: form but that it gets into the limnetic region by accident and maintains itself there so long as suitable food is present. I agree: with Apstein in regarding this form as characteristic for lakes. abounding in Chroococcaceae or, perhaps, Schizophyceae. He- has not observed its dependence on the seasonal appearance of these plarts in the lake, as is the case in lake Mendota. In the- limnetic region the species is acyclic so far as my observations. | go. The largest catches of this species were 440,000 per sq. m. | Sept. 21, 1894; 221,000, July 28, 1895; 661,000, July 7, 1896;. j 674,000, Aug. 15, 1896. Leptodora hyalina Lill}. TaBLeE XXIII.— Leptodora hyalina. Average catch per square meter of surface. 1894. 1895. 1896. SUMO! ENG roa ci aioe eect nace, » Sane ae aioe ere Seen | Ne I 63 Ss PUNE LE=BO se ee Seat a she Gat es rae te ciara sees No obs. 680 254 - Sly PID sees el besos ce ones eae sctisenme GA eC eR eme aeree ae 324 986 1, 208 . DUWly AO-BU shee oie ik PRES Tec sae ON, 2 Ce eee aa 362 827 585. ANeRGEAGUS, cidschis'st Gia ne eee Si kee ee cae a oe eees 445 2,512 642: AUSUSE AG—Bhge i caches cipies weenie avi ebeiateuleed s Sabre nateeles 1,081 3,078 1,881 September Wat5 oeGos cee ccs is cwece isis aa eia ete crs aiealdepistecie eal ue eNOLODS: 1,068 2, 850° September 16-30 aie sic oc! enceiaeiais de cis ae winren aocieine werdievaiunrg 871 775 2,945 October Tata esi sep cns seltiekiv nie elma oe Sin cies tnelatetaiweutarea) nslole 1, 469 457 - 2,375- October 16-31....... 966 661 1,026. INOVeHRi ber 1S10 sce erndisciac wieoe sane sua ce we cen Nae seme 95 292 247 November 16-30. ower cisncs ve oatcieats ann cn wbcaaetsice’s wales | sau mess tem 25 31 The table given above shows the average number of Lepto-- dora during the seasons of 1894, 1895, and 1896. The species. first appears in May, being first cbserved May 29th, 1895, and’ 1896. The nauplii must appear earlier, but I have never seen: | Leptodora hyalina. 301 one, although careful search was made for them in both years. The number of the species is so irregular that the average per square meter represents very little. On August 22nd, 1895, the species was present in the upper meter at the rate of nearly 2700 per cubic meter. These were all young females, either without eggs or having the eggs just laid. On October 6th, 1894, three sets of observations gave respectively a catch of 9, 38, and 13 individuals. On July 19th, six catches, at different hours, gave 0, 34, 11, 4, 3, 0. On August Ist and 2nd, there were taken: 4, 24, 16, 10, 4, and 2 individuals at different hours. These examples are sufficient to show that the figures for Leptodora are subject to a far greater variation than those of the other crustacea. For this reason, and also because the size and habits of Leptodora are quite different from those of the other limnetic crustacea, the species has not been included in the total number of crustacea. The maximum catch was 79, Aug. 7, 95; 75, June 22, 96; about 5,000 per sq. m. Males of this species appear in October, the numbers decline rapidly during November, and no individuals were caught by - the vertical net after November 26th in either year. Horizon- tal collections, however, show that they were present until after December first. The limits of this species, therefore, ex- tend from the middle of May to the first of December, and the maximum numbers occur in late summer and early fall. It is worthy of note that in no year does the maximum number coin- cide with the production of males. This is to be expected, as the large summer catches were due to the presence of num- bers of young or half grown Leptodora at the place where the net was hauled. It is therefore not surprising that these _ swarms should be irregular, and they would not be expected at the time when the adult females are producing the winter — eees. | Many observations were made upon the food of Leptodora, and it was found that they eat chiefly Cyclops and Daphnia. The attempt of the animal seems to be to squeeze out and swal- low the interior of the prey. In a considerable number of in- _ stances the intestine or the ovary of Daphnia, nearly entire, was seen in the stomach of Leptodora, and only occasionally 352 Birge—The Crustacea of the Plankton. were any parts of the skeleton of this species found. The legs and similar appendages of Cyclops were not infrequently seen. Large Daphnias have ordinarily a shell so thick that the weak jaws of Leptodora are unable to pierce it, and a very large pro- portion of the Daphnias seized by Leptodora escape apparently uninjured. Apstein (96, p. 175), notes that this animal in the Hinfelder See was very large, over 1 cm. long. It is not at all uncom- mon to find specimens measuring 18 mm. in lake Mendota. The average size is dependent apparently on the abundance of food. In Green lake and the Oconomowoc lakes the length is decid- edly less than 1 cm. FACTORS DETERMINING THE ANNUAL DISTRIBUTION. Our knowledge of the conditions of limnetic life is at present far too fragmentary to permit any complete explanation of the factors which determine the number of crustacea present in the plankton. Certain provisional results however, may, be reached as a result of this study of the crustacea. The following factors are present and combine to determine the total number of the crustacea present at any time and the number of the members of each species. 1. The food, both in quantity and quality. 2. Temperature. 3. Competition. Food. It is plain that the quantity of available food must set an upper limit to the number of crustacea. Available food must be carefully distinguished from plant material, since all plants are by no means equally edible by the crustacea. Gloiotrichia, for example, is present in lake Mendota in considerable num- bers from the latter part of July to the early part of Septem- ber. It is never the dominant alga, as it is apt to be in the plankton-poor lake. Butitis often the most prominent alga — to the eye, and is present in such numbers as to form on calm ~ days a thin scum on the surface. It does not appear, however, that any species of crustacea regularly eats it. I have given q ( very careful study to this point during three seasons, and have Factors Determining the Annual Distribution. 353 never seen any evidence that any of the limnetic crustacea feed upon it. Of course in cases of necessity it may be eaten, but even where other food is comparatively scanty, Gloiotrichia seems to be avoided. It should, therefore, be subtracted from the quantity of available food. Clathrocystis and Coelosphaerium appear also to be far less readily eaten than other species. I have made very numerous observations upon Daphnia of all three of the species present in lake Mendota and have uniformly found that while the dia- toms, Anabaena, and Aphanizomenon are greedily eaten, the colonies of the genera first named are uniformly rejected. Dur- ing the autumn and winter of 1894-5, Clathrocystis and Aphan- dzomenon were almost the only algae present. The food of Daph- nia was almost exclusively the latter species, and I have seen hundreds of Daphnia persistently rejecting Clathrocystis, while greedily collecting and devouring Aphanizomenon. Daphnia eats freely all of the filamentous diatoms, including Fragillaria, Melosira and Diatoma, while Diaptom us seems to prefer Ana- baena and Aphanizomenon to the diatoms, when all are present in large numbers. Since these preferences for various _ kinds of food are so strikingly marked among the crusta- cea, it may easily happen that a period when vegetation is su- perabundant in the lake may be one of scarcity for the crusta- cea. The most conspicuous case of this sort occurred in the summer of 1894, when my observations on the crustacea began. In July and early August of that year a species of Lyngbya ‘overgrew all the other species of plants, constituting more than 95 per cent. in bulk of the vegetable plankton. It was so abundant as to constitute a thick scum on the surface of the lake during calm weather. The filaments of Zyngbya are large and perhaps for other reasons than size are little available as food. The Daphnias present were carefully examined and hardly a single filament of the species was found in them, nor could I find any evidence that the other species ate it, although the re- mains of diatoms and other species of plants were found in their intestines. The number of every species of limnetic crustacea, except Diaptomus, was far smaller during this period than in _ other ce as the following table will show: 304 Birge—The Crustacea of the Plankton. TaBLE XXIV.—Number of limnetic crustacea during July, 1894-1896, stated in thousands per sq: m. of surface. July. 1894. 1895. 1896. MVE POTATIS Polos isin Bis Slate cealeyae einve aioe eis ted coenet pec olin Pato ce see 260.5 202.2 177.5 CREO TAS eis eee k ule Garth Sak base Wee RET RE Liman rere 95.4 227.8 244.2 Daphnilas hyalina ss: secc cies ceti cise ways winre he cca minnsiewele MADE 15.5 207.6 192.2 Chay ons 1655) Bare Lathe Meio ad ea ea Soe Maem ae ae me acuta onl ees ae morals 160.1 313.5 Daphnia retrocurva was entirely absent in 1894, while begin- ning its regular development in the two latter years. It seems quite evident that the presence of Lyngbya in the lake was the determining factor in causing the numbers of all species except Diaptomus to be so exceptionally small. The influence of this alga is not by any means confined to the adults. It is even more important in its action upon the young. In all the species of crustacea the immature forms are found near the surface, and during the day the upper one-half meter, or there- abouts, is occupied by immature crustacea. This is the same. region as that in which the Lyngbya is most abundant, and since Lynybya is wholly unmanageable as food for the immature crustacea, its presence in the upper water exerts a very unfa- vorable influence upon the development of the new broods which may be hatched while it is the predominant alga. It is note- worthy that Diaptomus, which maintained its numbers through the Lyngbya period, is the species of crustacea which combines great locomotive powers with effective means of collecting food. Daphnia has the most effective food collector, but is inferior in locomotive powers. Cyclops is inferior to both species in both ways, but ordinarily has an advantage in its omnivorous habits and its greater adaptability to different conditions of life. In late July Lyngbya began to decline, and Aphanizomenon and Melosira began to develop. Parallel with this change in the character of the algae, Cyclops and Daphnia hyalina in- creased rapidly, and in late August, when Melosira was the predominant alga, Cyclops and Daphnia were the predominant crustacea. Chydorus had fairly entered upon its period of rapid multiplication at this time but its numbers only became large as Aphanizomenon multiplied in September. ee Sige ae Factors Determining the Annual Distribution. 355 Ceratium offers an instance of an alga which, while not ab- solutely unavailable as food, is far less rapidly eaten than other species. So far as my observations extend, the adult Cyclops devour it more freely than do any other species of crustacea. Cyclops, indeed, is the most omnivorous of the plankton crustacea. It seizesand devours rotifers, nauplii, and other small animals, as well as plants. I have seen it pounce upon and devour Cer- atiwm several times, while I have never seen Diaptomus do the same, and have only very rarely found fragments of Ceratium in the intestine of Diaptomus. During 1895 I did not find in a single instance Ceratiwm within the shell of Daphnia, but in 1896 I found it in avery fewcases. Ceratium is a prominent alga during the summer, and at some time ordinarily becomes the dominant form, so that there is fairly a Ceratiwm period. In 1895 this period fell from the middle of June to the middle of July, and for a week on each side of the first of July, Ceratiwm constituted more than 90 per cent. of the plankton algae. In 1896 this period was later, coming in August and early Sep- tember. It was present in large numbers from the early part of the summer, but seemed to be hindered in its development by the great numbers of Aphanizomenon, which were present in the water. For nearly a month it seemed doubtful whether there would be a Ceratiwm period at all, but finally in August, Ceratiwm predominated decidedly over Aphanizomenon, although a considerable quantity of the latter species and Anabaena was always present. Ceratiwm, like Aphanizomenon, occupies the upper strata of the water, and its presence there is a hindrance to the development of the young crustacea, since it is so large and its shell is so hard that it cannot be eaten by them. The Ceratium period in 1895 marked the beginning of a decline in the numbers of the crustacea. The same was true to a less marked extent in 1896. Ihave no doubt that the presence of this alga in great quantity is one of the factors which influences. the late-summer minimum in the numbers of the limnetic crus- tacea. In 1894, Ceratiwm was present, but its numbers were always far inferior to those of of Lyngbya. The quantity of food also exerts an influence on the number of the crustacea. In a lake in which the plankton is so abun- 356 Birge—The Crustacea of the Plankton. dant as in lake Mendota, the quantity of algae is ordinarily in ex- cess of the demands of the crustacea, and any scarcity of food is wont to be brought about rather by changes in the quality of the algae than by an inadequacy in the total supply of vegeta- ble material. There is, however, one line of facts regarding the quantity of food to which sufficient attention has not as yet been given, namely, the correspondence of the relation of the rhythm of development of the algae with that of the crustacea. « As is well known, the successive species of plankton algae come on in waves of development, and between the periods when given species are plentiful, there are intervals, longer or shorter, when the food supply may be small. This relation may be best seen in lake Mendota at the time of the spring maximum. The crustacea, during the spring, increase more rapidly than the algae, and when the crustacea are at their maximum, the mass of plankton appears to the eye to consist of little except crustacea. Under these circumstances the food supply must be inadequate, the number of crustacea must fall off, and, especially, their re- productive power must decline. If the rate of increase of the algae coincided with that of the crustacea, so that the time of maxi- mum amount of food agreed with the time of maximum needs on the part of the crustacea, this quantitative oscillation would be of little importance; but, if at any time the decline of the dominant algae coincides with the reproductive period of a species of crustacea, it may be long before the species recovers from the injury thus caused. This relation between food and crustacea is one of the most important, and at the same time one of the most difficult to investigate, and one to which as yet but little study has been given. It is plain, however, that the number of a species of crustacea must be determined—so far as determined at all by food—by food relations when most unfavora- ble, and that the quantitative relations of food and crustacea must be followed from day to day, if this relation is to be un- derstood. Zacharias ('96, p. 60) expresses his surprise that the small crustacea do not increase beyond a certain number when they are provided with so abundant food throughout the year. To this question he states that there is at present no answer. I Factors Determining the Annual Distribution. 307 am very far from supposing that I can answer the question completely, yet Zacharias’s own figures show that at certain times of the year the food supply must be exceedingly small. For example, his figures show that the quantity of plant life is apparently abundant during the spring and early summer, but that in the late summer the amount of vegetation is small in proportion to the number of eaters. On August 20, 1895, the number of crustacea (L.2@65) ps 45) was nearly 1,360,000 per square meter of surface, the diatoms less than 30,500; Dinobryon, Eudorina, and Ceratium 459,010; and Gloiotrichia 70,650. Thus, including Gloiotrichia, there was less than one colony of algae to 2.5 crustacea. On Sept. 20, there was hardly more than one plant to 10 crustacea. Under these conditions a daphnia would have to strain a good many liters of water to satisfy her eternal hunger. It never happens in lake Mendota that the ratio of food to erustacea falls as low as these observations in lake Ploen, and while I am convinced that the occasional scarcity of food is an important factor in limiting the number of crustacea, I am equally sure that there must be other conditions, still unknown, which at times are even more important. My studies on the vertical distribution of the crustacea in 1895 and 1896 show that all or nearly all of the increase of the crustacea which causes the fall maximum is brought about by the increase in the num- bers of the crustacea in the deeper part of the lake from which they are excluded during the summer. In other words, the num- ber of crustacea in the upper three meters of the water remains nearly constant from a date near the close of the spring max- imum to the decline in numbers in late autumn. In 1896 the number of the crustacea in the upper strata increased somewhat during the autumn, owing to the occasional presence of large numbers of new-hatched individuals, but even in this year more than three-fourths of the increase in the number of the crustacea was due to the increase of the population of the lake below the nine-meter level. In the upper water, however, the increase of plants is most rapid. It begins in August at latest, and the quantity of vegetation goes on increasing, for two months at least, until in October the amount of food may easily be four 358 Birge—The Crustacea of the Plankton. or five times as much as in mid-summer. During this period the conditions of temperature are by no means unfavorable for reproduction, and it is at present impossible to see why crus- tacea should not increase more rapidly and thus reach a greater number at the period of the fall maximum. Temperature. The temperature of the water, as such, independent of its in- fluence on the food supply, determines the reproductive powers of the crustacea and the rate of their development, and thus limits their numbers. Perhaps, also, it exerts an influence on the length of life of the adults, although this influence is less certain. The different species of limnetic crustacea differ greatly in their relation to temperature. The periodic species are neces- sarily more greatly influenced by it than are the perennial. Diaphanosoma brachyurum is the most stenothermous of the periodic species. The first scattering individuals appear late in May but the species does not become a regular constituent of the plankton until late in July or early in August. The species increases in number throughout August and early September. The males appear towards the middle or last of September, when the species rapidly declines and wholly disappears from the plankton before the 1st of November. Its active period, there- fore, lies during the time when the temperature of the water of the lake to a considerable depth equals or exceeds 20° C. The individuals found in October are the survivors of the Sep- tember swarm, which show no reproduction and which disappear rapidly. Daphnia retrocurva comes next in its relations to tempera- ture. The species first appears late in May, but develops very slowly, and does not become plentiful enough to be counted as a regular constituent of the plankton until late in July or early in August. Its appearance thus coincides approximately with that of Diaphanosoma, but its autumnal history is quite dif- ferent. The species continues to increase sexually until mid- October. The immature males appear late in September or early in October. The females begin to develop ephippia in the first Factors Determining the Annual Distribution. 359 half of October. The first ephippial females were seen on Octo- ber Ist, 1895, and October 12th, 1896. By the middle or last of October nearly all the females bear ephippia, and the ephippia are cast off before November Ist. After this date the species rapidly declines, and the last females practically disappear about the first of December, although scattering individuals may re- main until after January lst. The sexual period of this species, therefore, instead of coming, like that of Diaphanosoma, when the temperature of the lake is still in the neighborhood of 20°, does not begin until the temperature has fallen below 15°. It should be remarked that in all these cases of an autumnal sexual period, scarcity of food can play no part in bringing it on. At this time the lake is crowded with algae of those species which are most greedily eaten by the crustacea, and in the case of the Daphnias there is always present a large mass of food materiai between the legs. Leptodora is closely parallel to Daphnia retrocurva, although of course, its numbers are far smaller. I have never been able to see the nauplius of this species, though I have looked for it carefully. The young females appear late in May. The species reaches a maximum in late August or September. The males appear in late September or early October, and the species dis- appears about the middle or last of November. In the perennial species the effect of temperature is chiefly seen in its action upon reproduction. Cyclops brevispinosus is by far the most indifferent to low temperatures. Its chief re- productive period is in the spring, and the young may appear during the winter beneath the ice, when the temperature of the water is below 3.0° C. The rate of reproduction increases as the lake warms, but the maximum of the species is reached by the time the surface of the water has been warmed to 15°. During the summer the species makes no marked recovery from the spring decline. In Pine lake this species is found during the summer in great numbers, close to the thermocline, living chiefly in the colder water just below it. It seems probable, therefore, that the species is unable to reproduce rapidly in the warm water of lake Mendota, to which it is confined during the summer. The young of the fall reproductive period do not ap- | 360 Birge—The Crustacea of the Plankton. pear in large numbers until after the lake has fallen below 15° C. The production of eggs and nauplii continues throughout the year, but the development goes on with increasing slowness as the temperature of the lake falls. When the temperature of the lake has fallen below 2.0° C., there seems to be little or no development of the nauplii into young Cyclops, but as the water of the lake warms toward the spring, the development goes on once more. There is, however, no time in the year when female Cyclops may not be found in considerable numbers bearing eggs. In summer the number of Copepoda is smaller than that of the nauplii would lead us to expect. It is fair to conclude that at this time the temperature is higher than the optimum for their development into the adult forms. Diaptomus does not reproduce during the winter, although a very few females may be found in late February or March bear- ing egg-sacs. No nauplii of this species have ever been seen during the winter, and the total number seen with eggs has not exceeded a dozen during the three winters of my study. Nor does reproduction begin immediately upon the disappearance of the ice. Females bearing eggs are seen from the middle of April on, but the young Diaptomus do not appear in numbers until the water of the lake, to a considerable depth, is near 15° C. Although the numbers of the species vary through the sum- mer, it remains on the whole more constant during the heated term than any of the species, and the late-summer decline in August is apt to be less marked than in other forms. The number of eggs is less in summer than in spring. It may be as great as 30 early in the season but declines to 10-15 later. In 1895, there was a marked rise in the number of Diaptomus during September, which was not seen in 1894 or 1896. Since in all years food is abundant at this season, we must look for the cause of this exceptional increase in 1895 to the persistence of the warm weather during September of that year. -----|-0- - 6 2--|----- rm=e@---|----- Sees sa Naseer 17 = -]- oo oe a eee Seen a IN ke NA el Teg D Pata I |i Pk I ated te Soa JN Yn Ig a ba et eh | PI ka es | i a eee sis oS ee eee ‘ Se Net aol ed (ct | nn en JUNE. JuLY. AUG. SEPT. Oct. Nov. Dksc. McH. Apr. May. FEB. JAN. The curve indicates the aver- Scale, 1 vertical space = 100,000 crustacea per sq. m. See p. 370. Number of each catch, 1895. Fic. 21.—Cyclops. age. F, 2 ’ ” «3 . ‘ i fo 7 > ¢ ‘ ¥ / at ! i ‘ 4 . : ‘ 4¢ i , ‘ « = ; vm ae e ° A \ Le “ > ; ; = y az r\ - ‘ 1 a f 2 - ; 2 / , ~ if : a P ‘ cs _ | P “a iJ bs . Horizontal Distribution—Swarms. 371 The following table gives the variations of the total number of the crustacea during three months of 1896. It will be seen the variations are somewhat smaller than are those of the single species but are of a similar character, and also resemble those of Table XX VI. TaBLE XXVII.—Total crustacea, May —July, 1896. Average. |Maximum.| Minimum. el ante, aD 2,398 2, 966 1,615 8 May 16-31... .... 0... see cece eee cece teen cee 1,901 2, 963 1,177 8 PRIN PE Maa taciers elee secs Sid bg casece save ewe’ 845 1,977 561 9 SU TEROML GOO Reta ral civicbs = Oe arsictaray\e' sie, sie'e eieteles 1, 265 1,908 890 9 PNT AE re atts Sia ickews cialis esis ese ololdse bje'e oie) eral ofs 1,314 2,302 1,005 6 PACH oN et a ee eee Na Sliculae ei giad 795 1, 266 511 11 I think that I have given here and in the tables of the ap- pendix, sufficient evidence to enable the student to undertands the extent of the variation in the distribution of the crustacea. I do not know whether the figures will be interpreted as show- ing an equal or unequal distribution. I judge that Marsh, from his discussion of the subject (’97, p. 218, ff.) would regard the dis- tribution as irregular. 1 think that it is quite as uniform as Apstein would expect. For myself, I have never supposed that every square decimeter of the surface of the lake covered an ecual number of crustacea. I have been surprised that a net 20 cm. or 10 cm. in diameter should disclose such a uniform number as it actually shows, especially in the case of organisms so ltighly organized as the Entomostraca. On the other hand, there is clear evidence for swarms in cer- tain species of crustacea, and at certain times. (1) The dis- _ tribution of Daphnia pulicaria is very irregular, far more so than that of any of its congeners. This species in lake Men- dota is confined during summer to the region of the ther- mocline, and as this stratum works downward through the lake in summer, the area inhabitable by the species is contracted around the edge of the lake, and the crustacea as they move out _ from the shore to keep in the cool water, may accumulate in MM Swarms. These have already been mentioned in connection with 372 Birge—The Crustacea of the Plankton. the species. The most conspicuous case occurred in August, 1895. On the 21st of the month the catch of Daphnia pulicaria was somewhat under 500; on the 22d it was nearly 2,600, and on the 27th it was only 85. This aggregation of the species was due to the wind carrying a current of warm water through the deeper levels at the point of dredging and so driving into deep water the individuals near shore, and the decline in number was due to the removal of the large numbers by currents rather than to the final scattering of the swarm. When a species has once aggregated in this manner, the aggre- gation may last for a considerable length of time; and Daphnia pulicaria always showed a greater range of variation in its numbers than did any other species, apparently due to these tem- perature aggregations in summer. For example, on April 18th, 1896, at one point in the lake, 3,060 of this species was caught; while another catch, at a distance of some two kilometers, showed only 230. On December 23, 1895, two catches were made of 260, and 3,440 respectively. See also the lateral dis- tribution in Table X XVI, above, which discloses a similar want of uniformity. A distribution so irregular as this, it seems to me, fairly warrants the title of “swarm.” I may add that late in the spring the species become more uniformly distributed, and when at its maximum showed a variation of less than three- fold in 10 catches, distributed over 21 days. (2) Apstein has found no case where a swarm has been seen. I have observed true swarms of Daphnia hyalina on at least three occasions. On October 17th, 1895, about 9 a. m. a large swarm of this species was seen at the surface near the dredging sta- tion about 800 meters from the shore. The water was perfectly — calm, and the sun was bright. The Daphnias were aggregated at the surface to a depth of about 5 cm. or less and within that depth the water was completely filled with them. The swarm was about 50 meters in width, and its edges were perfectly dis- tinct, as the boat passed slowly in and out of it. The length of the swarm was probably three times the width. All of these : animals were adult, so that they were easily seen with the naked — | eye. The occurrence was the more unusual as the bright sun ‘ | should have kept this species well below the surface. . Horizontal Distribution—Swarms. 373 Two similar swarms of the same species were seen in 1896 on October 3rd, and on November 3rd; both days when the lake was perfectly calm. On the first occasion there was a fog on the water; on the second occasion the sky was clear. These swarms were nearer the shore and were much more exten- sive. On the first occasion the Daphnias occurred in patches of irregular extent and shape— perhaps 10 meters by 50 meters, and these patches extended in a long belt parallel to the shore. The surface water was crowded by the Daphnias, and an im- mense number of perch were feeding upon them. The swarm _ was watched for more than an hour, during which the fog passed away, and the water could be seen disturbed by the perch along the shore as far as the eye could reach as one was standing in a boat. After a time a light breeze sprang up and, of course, prevented further observation. On this occasion the number was determined to be 1,170,000 per cu. m. in the densest part of the swarm. On November 3rd a similar swarm was seen, and water was again dipped up from the denser part of the swarm. The crustacea were crowded into an extremely thin layer, not more than 2-3 cm. thick. The surface water only was allowed to fall into the vessel and the number determined _ in 6 catches made by straining 10 liters of water, was from ~ 800,000 to 1,492,000 Daphnias per cubic meter, about 99 per cent. adult. In addition there were present about 1,000 Cyclops i! per cubic meter, but nothing else was found. On this occasion - one ephippial female was present, the only one that I have ever seen in this species; the ephippium was fairly developed, but - no eggs had been deposited in it. No males were in these swarms. The highest number is found nearly ten times the maximum number of this species per cubic meter, as derived from the three-meter hauls. It is also nearly fifty per cent. more than _ the maximum catch of this species as obtained from a depth of 18 meters, and nearly five times as great as the average for _ November 1-15. On November 3d, catches were made below ; Ke i the swarm from 0.3m. to 3.3m. The average of two gave per i. iby - cubic meter: *, IDEN DID TOC GENS Baas SOLS OSU DE OBOE Ee COE OD OCOAO OS SORE Or ODo Oro Ice 4,900 iy, SMO Set erate aac as aie on eats ay we tioie Wor wias «vies ae sie V\aee dja nesaie se aa see's 26, 600 UTE RDE TIED seh crs caer ss 2) aca UT raise cache teat oa eo ae whe eee ee wileate wide el 18, 200 COROIEIFU So S05 CBSE GoGo one DBE LOO DAE Oo OE On GOCE IDOE ODOR COOP EE OIS OGL eo Ocr 15, 700: v4 Birge—The Crustacea of the Plankton. The average of D. hyalina in the 0-3m. level for the first half - of November was 32,200 per cubic meter, of which at least half were immature, so that the catch of November 3d was not an exceptionally low one. These facts show that the swarm in question was a lateral aggregation and not merely a gathering at the surface of the individuals ordinarily below it. Great numbers of individuals broke through the surface film of the water on all of these occasions. This aggregation of Daphnia hyalina in swarms is probably more frequent than the number of observations would indicate. The swarms are found in the surface water, so that they are dislodged by the slightest breeze, and it is impossible to see them unless the water is entirely smooth. This condition isnot often reached, and I have felt myself exceedingly fortunate in being able to observe this phenomenon on so many as three oc- casions. I may say, however, that during the autumn of 1896, I looked for these swarms on every calm day when it was possi- ble for me to go outon the lake, but found them only twice. The significance of these aggregations is difficult to state. The habits of the animal are completely reversed in one respect. The adults are strongly negative in their relation to light, and under the conditions of all these occasions should have been found at a depth of one-half to one meter below the surface. It is possible that these aggregations represent the remains of a former sexual period. This may be indicated by the presence of the ephippial female. I have no doubt that Daphnia hyalina had at one time two sexual periods, inspring and fall, of which these swarms may be a remainder, but since the few males which appeared in the fallcame at a time decidedly later than the earlier of these aggregations, I do not feel warranted in positively interpreting the swarms in this sense. These swarms of Daphnia seem to be phenomena of the same order as those described by Francé (’94, p. 37). In one case the swarm was near the littoral region, as were those described by him. In the other cases they were well out in the limnetic re- gion. The swarm was confined within vertical limits even nar- rower than the one meter named by him and in all three cases the swarm was “von weitem erkennbar.” The Vertical Distribution of the Crustacea. 275 While, therefore, I find swarms occasionally present, I find also that the crustacea of lake Mendota are in general distrib- uted with marked uniformity. Marsh (’97, p. 220) finds an ordi- nary variation of ten-fold in the numbers of Diaptomus and an even greater variation in the case of other limnetic crustacea. With the exceptions already noted the range of variation in lake Mendota has not often exceeded four fold. The number of ob- servations, therefore, necessary to give a fair average for the population of the lake is not so great as that spoken of by Marsh. The examination of my records shows that the general development of the crustacea can perfectly well be determined by catches taken at intervals of a week and that the vertical distribution, if computed from such observations, would agree very closely with that reached from the very much larger number actually used. Of course the larger and rarer forms, like Hpischura and Leptodora, vary in number very greatly. No one would at- tempt to compute the population of a lake from the presence of a single Leptodora in the catch, or from the occasional presence of half a dozen, or more, but the numbers of the crustacea which are the regular constituents of the limnoplankton vary within comparatively narrow limits in lake Mendota, and I feel confident that my averages fairly represent the crustacean population. The variation of the numbers of the crustacea in lake Mendota _ does not support extreme views either on the side of uniformity of distribution or the opposing theory of swarms. In connection with reconnoisance observations it may be well to remember the following: Exceptionally large catches are due to the presence of great numbers of young, and exception- | ally small ones usually contain few young. A catch containing | great numbers of young may therefore be suspected to be un- | usually large and one with few young, if taken in summer or i fall, to be small for the lake from which it comes. yi THE VERTICAL DISTRIBUTION OF THE CRUSTACEA, In making collections to determine the vertical distribution of the crustacea the same general method was followed as that de- | scribed in detail in my former paper. (Birge, 95, p. 429.) The 5 PTOI emerge! ToS NG ite a ARS ‘ang me 2: 376 Birge—The Crustacea of the Plankton. dredge was lowered to the bottom of the level from which speci- mens were to be taken, raised through the proper space, and then closed by means of a messenger sent down the line. It was then drawn to the surface, washed out, and the collection preserved for future study. My observations show so much variation in catches made at the same place and in succession that 1 have little confidence in the differential method of determining vertical distribu- tion; unless a very large number of observations is made and averaged, so as to eliminate the chance of variation in the sin- gle observation. See p. 281. The distance employed in all of my collections was three meters. This interval was selected because it divided the lake at the point of observation into six levels of uniform thickness, and also because of the close correspondence be- tween three meters and ten feet. Experience has shown that the distance was fortunately chosen as the number of crus- tacea begins to decline rapidly between 2 and 3 m. from the sur- face. The place of regular observation is about 850 me- ters from shore, where the water is about 18.5 meters in depth or somewhat more when the water is highest in the spring of the year. The greatest depth observed in the lake is between 23 and 24 meters. The slope of the bottom in the deeper water is very gradual, and a depth substantially greater than 18 meters is only reached at a considerably greater distance from the shore. If observations had been made in the deepest part of the lake, the distribution as shown in thousands per cubic meter would not vary from the facts as shown in the tables, nor would the summer percentile distribution be altered, since dur- ing the summer the deeper parts of the lake contain no crustacea. During the fall and winter months the distribution is nearly uni- — form in the lower water. The average percentile distribution would, of course, be changed by the addition of one or more levels during winter, and the aggregations of crustacea, espe- cially Cyclops, which are found in the bottom levels, would of course, be moved from the 15-18 m. level to those lying be- low. Observations were made occasionally in the deeper water, as often as once a week during the summer and fall months; less The Vertical Distribution of the Crustacea. 377 frequently during the winter. But as the observations were few in number in comparison with those made at the regular point of observation, they have not been used in the preparation of the tables. During the last half of the year 1894, 75 serial observations were made, 127 during 1895, and 131 during 1896. These were most numerous during the summer months. In general it may be said that on every day on which observations were made as stated in Table A of the appendix, a series was taken, and on Some occasions more than one. The general distribution, of © the observations, however, can be ascertained from the table. At least five were made in each two week period from the mid- dle of April to the middle of November. During the winter of 1895, some observations were made by six meter intervals in the lower water of the lake, and the result of these observations was equally divided between the two levels covered by them. In Table B, accompanying this part of the report, the popu- lation of each level is given in thousands per cubic meter, the _ total population of the level being divided by three on the as- sumption that the crustacea are equally distributed throughout the level. Under some circumstances this assumption is incor- rect. In the 0-3 m. level, the upper meter contains more than one-third of the crustacea, especially when there are large numbers of young. It may contain twice as many as any meter below. On the other hand, on bright calm days, when few young crustacea are present, the upper meter may contain less than one-third of the total catch from the upper level. In the level which includes the region of the thermocline the population of the single meters varies greatly, as will be shown later in this paper; the crustacea being found in considerable numbers above this stratum and practically absent below it. A third error arises at times when large numbers of crustacea are settling to the bottom and dying. This occurs with Cyclops during the winter and spring, and with Daphnia hyalina in the early part of June. At such times the lower meter of the lower level would contain more than one-third of the crustacea present in that level. These variations from an approximately uniform dis- tribution are however so varying themselves that it has not been 378 Birge—The Crustacea of the Plankton. thought wise to attempt to distribute the crustacea among the three meters of each level on any other assumption than that of - uniform distribution. THE GENERAL VERTICAL DISTRIBUTION OF THE CRUSTACEA. Figs. 22-28, Tables B and C, Appendix. Winter —January, February, March. The months during which the lake is covered with ice show a great equality of distribution on the part of the crustacea. This is due to several facts. First, the lake is thoroughly homo- thermous, at least in a biological sense. Differences exceeding a degree between the temperature of the water at one meter from the surface and at the bottom of the lake are only found in late winter. Second, the food has no such concentration toward the surface as is found in the summer, though the algae are moré abundant in the upper strata. Third, the action of the wind is removed, and the influence of the sun is :greatly reduced, both by the snow and ice and by the low temperature of the water. Fourth, there is no reproduction of most species of crustacea and consequently no difference in age to influence dis- tribution. A few forces act in the other way: First, the food is more plentiful near the surface, as the algae reproduce more abund- antly there. Second, when Daphnia pulicaria is present it is far more abundant in the upper strata of the water than below. Third, Cyclops often appears in swarms near the bottom of the lake. Fourth, If Cyclops reproduces during the winter the young are more numerous toward the surface. Tables B and C of the appendix show that during January, Feb- ruary, and the early part of March, 1895, there was very little dif- ference in the population of the four upper levels. In January of that year the lower strata were decidedly poorer in number than those above; while in the latter part of the winter they were the most populous, owing to the accumulation of Cyclops in those levels. In the winter of 1896, the 0-3 m. level was at least twice as populous as any below, owing to the large num- Trans. Wis. Acad., Vol. XI. Plate XXXII. 0-3m Mca. APR. May. JUNE. Juuy. AUG. Fic. 22.—Population of the 3 m. levels, 1895. Scale, 1 space = 100,000 crusta- cea. The 25,000 and 50,000 divisions are indicated. See page 387. iirans. Wis. Acad., Vol: XI: Plate XXXIITI. 0-3m Mcn. Apr. May. JuUNE. Juny. AuG. Serr. Ocr. Nov. Dac. Fic. 23.—Population of the 3 m. levels, 1896. Scale, 1 vertical space = 100,000 crustacea per sq. meter. See p. 387. « y AY . - * ‘ bic: Gat) ’ ~ i , , ‘ . L ope: 4 = : ing ey oe A cen somtedicnre ’ rer ib tig " Hy A Ay Tan r \ i ide he patie, : ae i bi 2 Vertical Distribution of the Crustacea— Winter. 379 ber of Daphnia pulicaria present in that winter. The 15-18 am. level was the second in population, except in the early part of January, owing again to the accumulation of Cyclops in that region. The middle strata of the lake were the poorest in popu- lation in both years. Some illustrations may be added showing the concentration of ‘the two species in question in the lower and upper water of the lake respectively. On February 15th, 1895, out of 870 Cyclops taken by the net, 570 were below 12 meters; on the 19th 880 out of fieisd. On March 9th, 1,017 were found below 15 meters, out of a total of 1,650; on March 12th, 485 out of 710. This aggre- ‘gation at the bottom was not seen in January, and some few -eatches of later date did not display it. In 1896 the same tendency was shown, and began as early as January. On the 7th of that month 1,250 Cyclops out of 2,070 were below 12 meters, and similar catches were made through January and February. In March the old Cyclops were greatly reduced in number, aggregated only about 640 indi- viduals for the whole depth, and showed no tendency to col- lect at the bottom. At this time the young Cyclops were pres- ent, averaging over 2,000 to the catch, and the 0-3 m. level contained about twice as many as any other. Daphnia pulicaria was absent in 1895 but was numerous in 1896. During January and until the middle of February there were at least five times as many in the 0-3 m. level as in any lower one. As the numbers declined in February they fell off chiefly where they were the greatest and the 0-3 m. level be- came about twice as populous as any below. Thus the tables of distribution in winter for 1895 and 1896 show resemblances and differences. In 1895 the 0-3 m. level ‘shows no noteworthy excess over those below, while in 1896 it is about twice as populous. Between 65 and 70 per cent. of the ‘population of this level in 1896 are due to Daphnia pulicaria. ‘In both years the bottom water is more populous than that at the ‘middle of the lake, due to the settling of Cyclops. This species furnished from 75 to 85 per cent. of the population of the bot- tom level in both years. The average population per cubic ‘meter is much greater in 1896 than in 1895, especially so in 380 Birge—The Crustacea of the Plankton. January; but the population fell off more rapidly in the latter part of that winter, and there was no very noticeable difference in March. Taste XXVIIL.— Average percentile distribution for the winter —Jan- uary, February, March. PER CENT. IN EACH 3 M. LEVEL. Aver- |9_3m.| 3-6. | 6-9. | 9-12. |12-15.|15-18. age No TSO De Rees ee eee reaee sweet eas COO un oa 19.3 13.7 12.8 15.8 20.3. MSGR awk chine Geile cir ee ated state votes 237,000*| 34.1 15.7 14.8 10.8 10.3 13.6. * Chydorus omitted on account of its rapid decrease in late winter. Spring— April and May. Tables B, C, Appendix. The distribution of the crustacea during the first half of Apri is on the whole fairly equal in the different levels of the lake, ~ but with irregularities which mark it as an accidental distribu- tion. The ice breaks up in the first days of April, and the lake is consequently exposed to the action of the wind. The tem- perature is fairly uniform at all depths, and the algae hardly begin rapid multiplication much before the middle of April. The water at this time has a more active circulation than at any other, as is shown by the presence in the net of numer- ous particles of vegetable debris from the soft mud at the bot- tom of the lake. During this time Cyclops begins its rapid increase towards the spring maximum, if the multiplication has not already begun under the ice. Its swarms of young are in the upper strata of the water. It may be laid down as a general rule that large numbers of young of any species of crustacea appear first in the upper levels of the water, and the animals later pass toward the middJe of the lake; and later still, occupy the water toward the bottom. It may be said, therefore, in general, that the presence in the upper water of a very high percentage of the catch of any species indicates the beginning of a period of re- ‘ ¢ ’ t) EE i ea 6 = ae L L] 0 i] { 6 t UJ 1) U t] 1] ¢ r) e ) ? u e U 9 e e 4 ‘ 0 L) 6 ‘ ’ U 0 : e e ‘ ‘ 6 7) ‘ ‘ t) ‘ 8 (] C ' “AON ATX ld . oc ese we fee eee «2 @e0 @e0eoe0 0008 sees efe 2 35.2 S093 = 5 7 ‘ony *** UT QT-6 "Ug -© ure -( ‘KIO eecceerljoosoce ‘rojoul arenbs rod vooeysn.io (Q0'0G = e0edS [vOI}I0A T ‘gfeog "CERT ‘UOT}NQLAYSIp [BO1}AeA—"FS “OTA "TIHdY ‘peoy “SIM ‘SUBIL, eo ec*#f ee eres ee eo 6 *** TL QT-6 SSS Tae ek ee ipa eee ku Geer a ure -9 "18g ‘do0Q ‘asojou ‘bs 10d voovysn.19 QQ0‘OG = sovds [vOT}AIOA T ‘OTVOG = “OGST SUOTZNQIAYSIp [eVdT}19A-—'GZ ‘OI ‘ ; , ; ; : 1 ry e ee ae 1 eevee : ; ’ 3 : ; ; ; ' ' ' ; ' : e 4 ' ' ' 1 t r i i 2 0s se cece : 1 ; 7 x ; a cores 0s ee ee : : . : ! oe 8 Mag ae Tl t ‘ 1 Tie , ‘ n « a ' ' ' ' ' ' 1 1 oor": eae - : : Gcroow 00T e ; ' ’ : u 6 8 0 ' ' 7 ; t que ; - ~ esis OSE ; ! ; 3 ’ “ 00g." ; : ositars OSGeeiaee? ' : 7 } ’ iy a ' ' ooe 7" ; ocen can ; ocen : 5 4 : A ; 5 oor esc (6 | "AON ‘LOO "Ldag ‘ony “ATO -“aNAe ‘AV "IIddV “AXXX Pd TX ‘OA “peo Vv “STA ‘SURAL = ; ro , 1 ‘ tony st ‘ i 4 J Ft GS) (Oe, ee ey oe eo cy . i 4 ' é . * é ’ P te ae me 8 - . : ‘ \ AGY a a . bt i { a a On wT . i J , ‘ ee Bs 4 s » ‘ ‘ ” Je 4 . 2 * 7s : oF { prws 7 - ‘ { , ‘ P ? r Ly . . 2 : a % ’ . . i ara | i : Vertical Distribution of the Crustacea—Spring. 881 production of the species, while the presence of a larger number in the bottom water of the lake than in the surface water indi- cates that the species is past its maximum and is already be- ginning to decline in numbers. In both years the numbers of crustacea in the upper water show an increase during April, due to the multiplication of Cyclops. This increase went on, as was shown in the early part of this paper, much more rapidly in 1896 than in 1895. As a result, the population both of the surface water and of the lower levels increased much more rapidly in 1896, and the latter part of April, 1896, represents about the same condition of the ‘development of the crustacea, as does. the first half of May in 1895. In each case more than 40 per cent. of the crustacea were present in the upper stratum, while the 15-18 m. level had not increased greatly in numbers above its condition in winter. In the latter part of April, 1896, the 15-18 m. level contained less than 3 per cent. of the whole number of crustacea present; and in the first part of May, 1895, it contained less than 7 per cent. As the number of Cyclops and Daphnia pulicaria be- came greater, they moved downward into the deeper water, so that it became relatively more populous. In the latter part of May, 1895, the 15-18 m. level contained 10 per cent. of the crustacea, while in 1896 it contained over 40 per cent, This increase in the population of the lower strata goes on after a considerable decline has come in that of the upper strata. The lower water lags behind the upper both in the increase and decrease of its population, and the maximum population of the lower strata comes from two to three weeks after the maximum popu- lation of the lake has passed. These relations become more obvious if we divide the lake some- what arbitrarily into three levels, 0-3 m., 3-9 m., 9-18 m. The distribution of the crustacea among these three regions is shown in Figs. 24 and 25. By reference to these it will be seen that in 1895 the two upper levels increased much more rapidly than did the lower half of the lake from the latter part of April to the middle of May. In the latter part of May the reverse is true; and in early June the population of the lower Water was stationary, while that of the upper half of the lake 382 Birge—The Crustacea of the Plankton. was rapidly declining. In late June the population of alli levels. declines altogether. This relation is even more conspicuous in the diagram for 1896. The population below 9 meters did not increase at all: until the end of April, while that of the upper levels increased: several fold, the 0-3 m. level growing more rapidly than thati below. In the first half of May the lower half of the lake- gained absolutely more than either of the levels above, its gains. per cubic meter being about half as great as those of the upper water. In the last of May the levels below 12 meters continued: to gain, while the 9-12 m. level was approximately stationary,. and the upper strata fell off rapidly and about equally. At this. time the lower half of the lake contained nearly 40 per cent. of the total number of crustacea, nearly equally distributed, while the upper three meters contained only about 28 per cent. In early June all the strata below the 0-3 m. level lost heavily,,. owing to the disappearance of the spring broods of Cyclops and D. pulicaria; while the 0-3 m. level remained approx- imately stationary, the new broods of Chydorus and Diaptomus,. which appeared in that level, compensating for the decline in other species. The result of this decline in the population of the lower water serves to give the 0-3 m. stratum over 50: per cent. of the whole population, and the number in this level continues between 45 and 50 per cent. during the remainder of the summer. Summer — From the middle of June to the middle of September. The change from the late spring to the early summer has just been spoken of. The most important fact influencing the ver- tical distribution at this time is the formation of the thermo- cline, and the accompanying exclusion of the crustacea from the lower waters of the lake, and ultimately from the entire region below the thermocline. The thermocline was observed in each year about the middle of June — June 11th, 1895, June 13th, 1896 — and was present regularly afterward. The depopulation of the lower waters does not coincide with these dates, as will be seen from the tables. This would be expected since the exclusion of the crustacea is due to the chemical condition of the lower water, Vertical Distribution of the Crustacea—Summer. 383. resulting from the temperature conditions. In both years the pop- ulation of the lower half of the lake in the latter part of June is equal to or greater than that in the same region in the early or even the latter part of April. The population of the 9-12 m. level remained substantially stationary until the middle of June, 1895, and the same was true of this level and the level below until the middle of July, 1896. In June, 1895, the population of the bottom level was high, owing to the accumulation there of large numbers of diseased and dying Daphnia hyalina; but as soon as these had died, the numbers rapidly fell off, and the population in the 15-18 m. level was very small in the first half of July. In 1894, observations begun with the 1st of July, and at that. time the population below 9m. was extremely small, far smaller than in either of the succeeding years. At that time the temperature conditions below the surface were not observed, but it is fair to infer that the thermocline was established at a comparatively early date in that year. A second fact which influenced the distribution in 1894 is the unusual preponder- ance of Diaptomus among the crustacea in that year. A very high percentage of this species is found at all times in the up- per water, while Cyclops, whose per cent. in the lower water is greater than that of any other species, was represented by very small numbers. During July the population of the waters below 9 m. declines very rapidly, as will be seen from the table which gives the population of the lower water during the months of June, July and August. TABLE XXIX.—Population per cubic meter. 1895. 1896. 9-12 m. | 12-15 m. | 15-18 m. || 9-12 m. | 12-15 m. | 15-18 m. MP HHG I-15, ee cace sees e- | 45,000 36, 000 46, 600 24,000 12, 600 30, 600 une 16-30 22.05. ......6.200...| 18,300 9, 200 17, 500 24, 200 18, 800 15, 000 ial onion cm aes aas cess | 24,200 4,200 2,200 25, 1C0 20, 900 2,600 MRE AGS) oo. occ wesc ce cecs ssp 10,900 2, 100 1, 400 9, 700 700 300 “EUS a? Ea Ee ee 27,500 4,100 600 11, 200 1,200 20 Beedeust 16-31... eke 32, 700 3, 500 550 49, 600 6, 900 500 384 Birge—The Crustacea of the Plankton. While the absolute population of the lake during the summer months has varied very greatly in the three years of my obser- vation, the vertical distribution of the animals has been almost exactly the same, as may be seen from the following table: TABLE XXX.— Average percentile distribution of crustacea June 15-Sept. 15. (In 1894, July 7-Aug. 23.) | PER CENT. IN EACH 3M. LEVEL. Average No. gt a Ee ee 0-3m., 3-6. 6-9. 9-12. 12-15.‘ 15-18. 1894... .....406, 000 45.5 30.2 16.0 6.7 1.3 0.4 1895... ....707, 000 44.0 24.6 18.4 8.9 2.2 1.9 -1896..... 1, 116, 000 45.1 27.5 14.9 7.7 3.4 1.2 From this it appears that from 44 to 45.5 per cent. of the crustacea were present in the upper three meters of the lake _ from the middle of June to the middle of September, and from 25 to 30 per cent. more between 3 and 6 meters, from 15 to 18 between 6 and 9 meters, leaving from 8.5 to 13 per cent. for the lower half of the lake. The percentile distribution of the crustacea during the summer and its relation to the thermocline are shown in Figs. 26 and 27. In each diagram the depth is computed above which were found ineach half month, respectively 25, 50, 75, 90, and 95 percent. of the erustacea, on the assumption that the crustacea in each of the 3m. levels were equally distributed through it. The points repre- senting the depths for the corresponding percentages were platted on the diagram and then connected by lines. There is added in each diagram the position of the isotherm of 20° which lay in the thermocline in both years, although in 1896 the lake cooled below 20° before the thermocline disappeared. In Fig. 26, the temperature for each date was computed from ‘the average of the week preceding and that following the date. The temperature-line of Fig. 27 is taken from Fig. 4. The diagrams show that 25 per cent. of the crustacea are al- most always found in the upper two meters of the lake. No doubt the position of this line would be higher if it had been i ee “‘gge ‘d 00g “EggT JO AOWIUINS ‘eodeISNIO Jo UOTINGLAYSIP [LOI}AEA O[TJUEdI0g—“9Z “OTL og ee 3 : U0 gr ’ if ‘ ; H ‘ * t . . { Se ° : ' mise ¥ ecr fas SS = Lidar Caoamr ane 5 H aN , ’ . rst ' } ‘ : i i ' is ‘ i ; 000 (¢) : 7 : 5 = uC H ' ‘ ' f 3 H ' 5 ' ' ; ‘ H A » i H 1 eee ae ' ' Ul ZI — OL aT : \ te ee 1 N ' , : \ | ' 4 Ny ag : as jG ' R eee ‘ ees ! ‘ i t i { i ' ; ! ‘ ‘ 1 ; ! : : ‘ ie a ug 1s i : ' ' ‘ ; ! ‘ a 1 ' . i . ry ' ‘ A 1 - nee UW § ; Ul ¢ ee ‘ i Se Weare ’ 1 (232 SSS Saw i u - 1 ; ‘ ‘ : I f 4 ' ; 1 1 = 1 4 : Ul 0. se RE 00% CD 008 0 002 ‘TIdeq. ‘Lads ‘ay ‘KIO "aNOp ‘XV "qydeq “TAXXX > 97%Id TIX ‘JOA “Peoy “SIA\ “Surry, "pee ‘dooG ‘OGRT JO IOMIUINS ‘UOTINGLAYSIP [VOT}AVA O[IJWODIOG—")Z “OI T ' 1 f ; , ' . ' f ‘ ' ' \ t : . ' U , i ' ; ' ' ! -\ ' ree fa fea i ' Vee a ' 1 \ 3 : : Ls \ ' ol 1 ; Ne ' ' ya x ‘ ' ri fj ‘ ' ‘N : « N ‘ ' 4 x 1 : \ 0 1 A é . Ni ‘ \ ' , z ' S ' XN ' : b Z ' “ : , 7 ' x u ‘ ' i 1 C N Lares uw Cee ' = , ' / 4 SS iT wt ST ‘ \ ' ' i “ ; , \ ‘ ‘ ; S ; P N 5 ; ’ y ' ' 4 Ne A u : c a a H ' : oe ' ‘ 5 Seer . ; % ' ; . Nn L amen i 4 a Wo ' Li 4 ' ' ‘ ; ‘ ya ‘ ' are PN. « «0 ' UL W ZT ge : ' i i r : 5 9 3 d . ‘ vee “Ul Ul 6 , G ' : ' n ’ 1 ’ eo. 100 ie) g ae ; : ' ‘ ' ' : ’ ’ S ' 0 ' ‘ ' i] ' ai 7 " mS ’ xX ~ ' ug” S oS - : ul i See : 20G ‘ DS ea eicre ene & ‘ pe U : ra ale ‘ PS < : : ae ' } Sse . ' ZO t ° ae , i sq ; : ‘ SS ee a oa sees eee oe we . 20% ' ‘ ' ‘ ' ' ‘ o ‘ ’ M + « ’ ' * . 1 i i ' ‘ ‘ ‘ , 1 ' ' i WEN) “UO ates 00% . 006 ‘yydog "Ld dg ‘pny wi fol cen ames ‘TNO ‘1VW ‘yydoq TIAXXX [ld 7 "IX ‘TOA GBEON, SIM. “SUBAL = ee Vertical Distribution of the Crustacea—Summer. 385 possible to indicate the real concentration of the crustacea in the upper meter. During May the percentage lines all moved downward, owing to the downward movement of Cyclops during that month, as its numbers rose to their maximum. The move- ment extends into June, 1895; while in the early part of June, 1896, the center of population moved upward more than 3 meters, owing to the earlier death of the spring broods of COy- clops in that year. The center of population then remains close to the three meter line until the middle of August. In late June and early July of both years there is a rapid decrease of numbers in the lower levels of the lake. The 90 and 95 per cent. lines reach the level of the thermocline early in July, and they remain there through July, August, and early September, closely following the thermocline as it moves downward through the water. The center of population, which remains for some time near the 3 m. level, moves downward rapidly in September, and reaches a depth between 7 and 8 meters in October. If the crustacea were uniformly distributed throughout the lake it should lie at 9 meters. The 90 per cent. level was as high as 8m. in July and August, 1896; and between 9 and 10.m in 1895, but moves downward to about 16m. in October. This practical exclusion of plant and animal life from the lower water during summer is a factor of great importance in the life of the lake, as the following considerations show: First, during this period the number of crustacea and the quantity of the plankton is independent of the depth of the water below the level which the thermocline has reached. Second, the ex- clusion from the lower water of species unfavorably affected by warmth prevents their appearance in the plankton or causes them to decline during the summer, while in the other lakes in which the deeper water is inhabitable their numbers may go on multiplying. This is pre-eminently true of Daphnia pulicaria, whose numbers are small in lake Mendota during the summer, while in many of the Oconomowoc lakes it is abundant during the same period and inhabits the entire depth of the lakes below the thermocline. The summer decline of Cyclops brevispinosus may also be due to the same cause. Third, the total number of the crustacea during the summer is far smaller than it would 2 386 Birge—The Crustacea of the Plankton. be if the deeper water could be utilized. It is not impossible also that one factor in determining the small number of the periodic species of crustacea in lake Mendota may be in the fact that the upper water is so completely occupied by the perennial forms as to leave little chance for the development of other species. Fourth, the crustacea are not excluded from the deeper water of the lake by the low temperature of the water, as is proved by the occurrence of the same species in the far colder water of other lakes in the same district. The exclusion is due to the accumulation of the products of decomposition in the lower water, which remains entirely stagnant after the thermo- cline has been formed and is never exposed to the action of sun and air. This water in lake Mendota acquires an offensive smell and a disagreeable taste, though in neither respect does it go as far as certain waters mentioned by the Massachusetts Board of Health (Drown, ’90, p. 553.) It is always clear and bright to the eye. The products of decomposition of the algae and crustacea of winter and spring remain stored in the deeper water, and un- doubtedly the addition of this store of nutritive material to the water of the lake as the thermocline gradually moves downward is one of the factors which occasions the enormous increase of the vegetable plankton in the late summer and autumn. Autumn—October, November, and December. The summer conditions of distribution end with the breaking of the thermocline and the resulting establishment of the fall homothermous period. This occurs at different times in different years. The date depends on: First, The rapidity of cooling of the surface; Second, The summer temperature of the bottom; Third, The amount and direction of the winds, especially of gales. In 1895 and 1896, the “turn over” came in the last week of Sep- tember; in 1894 the distribution of the crustacea shows that it did not come until the first week of October, and it was equally late in 1897. In the year 1894 no observations were made in the first, half of September, but the distribution in the latter part of September of that year closely resembles that in the early part. — —— Vertical Distribution of the Crustacea—Autumn. 387 of the month in 1895, and in the latter part of August, 1896. The distribution in the first half of October, 1894, is not very different from that two or three weeks earlier in the preceding years. The leading general feature of distribution during the late summer and autumn is the progressivé occupation by the crus- tacea of the deeper strata of the lake as the thermocline moves downward through August and September, and the coincident rise in number of the crustacea toward the fall maximum. It is a fact which was wholly unexpected by me that the 0-3 m. level shows little or no increase in the number of its crustacea after the early summer maximum in early June or late July. In 1895 its numbers steadily declined, or at best were stationary, after July 15th. (See Figs. 22,23.) In 1896 there was considerable variation in numbers, but on the whole there was no increase except a sharp temporary rise in late October, due to the occur- rence of great swarms of young Daphnia hyalina at that time. In 1894 the numbers in the upper level rose in the autumn, as would be expected, since they were at an abnormally low level in July, owing to the peculiar condition of the vegetation of _ the lake in that year. The crustacea between 3 and 9 meters show also the same re- lation in their summer and autumn numbers; while those below 9 meters show a great increase, beginning in the 9-12 m. level, as the thermocline moves downward through it in August. The increase steadily proceeds to the the lower levels of the lake. Itis very rapid in September and early October, and continues until the storms of late October, when the popula- tion decreases in all levels of the water. This result is the sum from 5 to 7 species of crustacea, and of course it does not hold accurately for each species. It is also true that since the broods of young appear in the upper level, they may temporarily increase the number of a species there, but this excess of one species is balanced by a deficiency in another, and often for the single species the semi-monthly averages agree pretty well with _ the general law. A good example of the effect of age upon distribution can be } seen from the case of Daphnia hyalina in the latter part of Oc- 388 Birge—The Crustacea of the Plankton. tober, 1896, when great numbers of young appeared on several — occasions, and when the old animals were nearly all full grown, so that there were very few half developed individuals. This is wings Wig Weegee given on p. 398. During November and December the population of the lake — falls off pretty uniformly in all levels, more rapidly in Novem - | ber than later, and at this time the distribution of the animals may be more even than at any other period. If Daphnia pult- caria is present itrises toward the surface in December and in-— creases the population of the upper strata. This occurred in 1895. In all years the distribution in November is more uni- ‘i } form than that of December, in which month the population of © ; the lower levels of the lake seem to decline morerapidly than that of the upper stratum. Taste XXXI.—Average percentile distribution Oct. 1—Dee. 31. a PER CENT. IN EACH 3 M. LEVEL. Gree ; ° Josm.| 3-6. | 6-9. | 9-12. | 12-15.) 15-18. 9 | OGL eee neta? 595, 000 25.8 18.8 16.0 15.7 14.0 9.8 A895 Spee ccunee: 436, 000 29.7 18.3 14.3 14.9 12,2 10.6 TSOG Sell Tee 759, 000 25.9 21.0 15.3 13.9 12.4 11 | Figures 22 and 23 represent the total population of each of the 6 levels into which the lake was divided. The scale is 100,000 crustacea to each vertical interval. If the scale be di- vided by 3 the same diagrams will serve to show the population i of each level per cubic meter. The relations of the increase — and decrease of the population in the several levels are shown : very plainly from these diagrams. For instance in 1895 it will be seen that while the two upper levels began to increase dur- t ing the latter part of April, the population of the lower levels scarcely changed from the winter condition until about the first of May. The population of the three upper levels reached its | maximum in the latter part of May, while in the lower part 0! ¢he lake the population went on increasing, or at least remained stationary, until near the middle of June. The 6-9 m. lev Vertical Distribution of the Crustacea—Autumn. 389 hardly shared in the rise to the early summer maximum until two weeks after the 0-3 m. level, while in the lower part of the lake the population declined, or remained stationary throughout July. In August the crustacea of the 9-12 m. level increased in number as the thermocline moved downward into that level, _while no increase was perceptible in the population of the lake below 12 m. until after the middle of September; after which date the numbers rapidly increased. No increase of population was seen in the upper levels of the lake after the month of July; and if this diagram is compared with Fig. 6 which shows the changes in the total population of the lake, it will be seen that the autumnal maximum, which is clearly indicated, comes entirely from the increase of population in the lower water of the lake. The same general facts appear in the diagram for 1896, but, if possible, in a form evenmore striking. The 0-3 m. and 3-6 m. levels follow each other closely, while the spring increase in population comes later in the lower levels of the lake. In the 9-12 m. level the population remains stationary during May, when that of the upper levels is rapidly falling, and at the ‘same time the crustacea in the water below 12 m. are increas- ing in number; more rapidly in proportion to increased depth. In the 0-3 m. level at the first of June the population was sub- stantially stationary, while that in the water below was falling rapidly. This condition was brought about by the new broods of Chydorus, which nearly made up for the loss in numbers of other species. In 1896 the thermocline moved downward much more rapidly than in the preceding year and as a result of this movement, the crustacea in the lower water began to increase in numbers at an earlier date. (See Figs. 3, 4, 26, 27.) A marked increase occurs in August in the 9-12 m. level and begins about two weeks later in the levels below. As in 1895, so also in 1896, the fall maximum is caused by the increase in the population of the lower water, with the exception that in late October of 1896 ' there was a great increase in the number of the crustacea in the a a = == open! meet 0-3 m. level, due to the appearance of great broods of D. hyalina at this time. These soon disappeared, so that the crustacea in 390 Birge—The Crustacea of the Plankton. this level fell off in number even more rapidly than they had in- creased — so rapidly, indeed, that no effect was produced by these broods upon the population of the water below 3 m., ex- cept perhaps to check in some degree the rate of decrease toward the winter minimum. There was also a small rise in December in the 0-3 m. level, caused by the increase of D. pulicaria. It would seem from these facts that there is a maximum pop- ulation per cubic meter beyond which the crustacea are unable to multiply and which differs in different seasons. It is difficult to see what it is that sets alimit to this population in the autumn. At this time the food is in enormous abundance as compared with the number of the crustacea, and it would be expected that the numbers in all levels of the lake would in- crease together. I am quite unable to give a reason for their failure to do so, but the fact recurred exactly in all three years of my observations, making allowance for the peculiar condi- tions in the early summer of 1894. Fig. 28 represents the average percentile vertical distribu- tion of the crustacea for Oct. 1-15, 1896, March 1-15, 1895, August 1-15, 1896. The corresponding figures are given in Table C, appendix. In the diagram each horizontal space rep- resents 10 per cent. of the crustacea and each vertical space, 3 m. On each 3 m. line is platted the percentage of crustacea found below it, and these points are connected by a line which extends from 100 per cent. at the surface to 0 at the bottom. From the intersection of these curves with the vertical lines can be seen approximately the percentage of the crustacea above and below the depth indicated at the intersection. If the dis- tribution were uniform there would be 16.6 per cent. in each vertical space and the percentile distribution would be marked by a straight line running from corner to corner of the diagram. The curve for October approximates very closely to this, the percentage being larger in the surface stratum and somewhat smaller below 12 m., but, in general, the line lies very closely parallel to the diagonal. The distribution for March is almost equally uniform, but here the bottom level has an excess, due to Cyclops, and the 0-3 m. level is slightly below the average. ee ae ay a Sorat Plate XXXVIII. Trans. Wis. Acad., Vol. XI. = ee wr ese bee wm eer esrumnesce ewan eewnenwreas — ese ee ee ee ee ee ical distribution of crustacea, March 1-15, t Fic. 28.—Percentile ver 1895; August 1-15, 1896; October 1-15, 1896. See p. 390. So OO ee ==> MARCH AUG.... Oct.... Cs v Vertical Distribution of Individual Species. 391 In October the distribution of all of the species of crustacea is approximately equal. In the winter the equality of distribution is brought about by the excess of Daphnia and Diaptomus in the upper strata, nearly balancing the excess of Cyclops near the bottom. (See Fig. 30.) The curve for August shows a very large percentage in the upper 3 meters and a very small number in the lower water. It is a characteristic distribution for middle summer. THE VERTICAL DISTRIBUTION OF THE INDIVIDUAL SPECIES. After this full discussion of the vertical distribution of the total crustacean population I do not intend to describe that of the individual Species in similar detail, but I shall confine my- self to pointing out the individual peculiarities of each species, devoting more space to those which depart in a marked way from the average vertical distribution. One general law holds for nearly all the species, as already stated: the broods of young appear first in the upper water of the lake and the increase of population extends downward, becoming approximately uniform _at all depths as the species reaches its maximum, and later in its life becoming more numerous in the deeper water of the lake. To the first part of this rule the oniy exception is Daph- nia pulicaria during summer. There are, however, several fac- tors which prevent the full carrying out of the latter part of the rule. The most important of these is the formation of the ther- mocline, by which all of the crustacean life is confined to the _ upper waters of the lake during that period when the develop- _ment of several species is going on actively. In the late au- tumn also the numbers of the crustacea decline so rapidly after the fall broods appear that it is not easy to find any accumula- - tion at any low level of the lake. The downward movement of the older forms is shown most clearly by Cyclops and Daphnia _ hyalina during the spring, and by the accumulation of Cyclops in the deeper water of the lake during the winter, by the dis- appearance of D. hyalina and D. retrocurva in autumn. Sim- ilar, though less striking, illustrations can be found in all of the species of limnetic crustacea. 392 Birge—The Crustacea of the Plankton. Each species of crustacea, also, has individual peculiarities of distribution, which recur from year to year with surprising similarity and which are independent of the absolute number present. These peculiarities appear when the average of any species is taken, although of course it is entirely possible that the distribution should depart widely from this average at any single observation. In general it may be said that the summer distribution of the crustacea follows very closely the figures which are given in my former paper (Birge, ’95), and that the variations in the distribution which have been found during the two years and a half succeeding the observations reported in that paper, have been of the same type and in general of the same degree as those which were found during the single month of our first study. It seems to me, therefore, unnecessary to point out again these variations in detail for each species. In order to show the resemblances and differences in the per- centile distribution of the crustacea during the summer months, when their numbers are great and the distribution is most characteristic, I have averaged this distribution for the summers of three years: 1894, 1895, 1896. I have included the three standard representatives of the limnetic crustacea which are regularly present in full numbers during this time; Diap- tomus, Cyclops, D. hyalina. The period included is from the mid- dle of June to the middle of September, in 1895 and 1896; and July and August of 1894. It will be remembered that no ob- servations were taken in 1894 before July or during the first part of September, but as the summer conditions were thor- oughly established at the first of July of that year and contin- ued until the first of October no noteworthy difference would appear in the averages had it been possible to extend the period. — It will be seen from these averages that the distribution of Qy- clops in the three years in question varies surprisingly little; the percentile difference in the 0-3 m. level being less than 1.5. This close correspondence in distribution exists in spite of the fact that the numbers of the genus were very different in the three years. The same general agreement is seen in the tables of semi-monthly distribution. Compare July, 1894 and 1896 in Table C, Appendix. 2 — ube weal ce an) ree ee teas Vertical Distribution of Individual Species. 393. TABLE XXXII — Percentile distribution. Summer—Diaptomus. PER CENT. IN EACH 3M. LEVEL. Average No. 0-3 m 3-6 6-9 9-12 | 12-15 | 15-18 1894 226, 000 49.2 | 29.3 16.6 4.1 0.5 0.3 TESA eee, Aaa 172, 000 42.7 29.0 20.9 6.1 0.7 0.6 1896.........-.-.-. 188, 000 52.6 27 4 12.4 5.9 1.9 0.5 Cyclops. (fo G0 oe 138, 000 | 40.7 23.4 20.1 9.4 1.7 | 0.3 LSE ESSE AP ee ie ee 183, 000 39.3 222 19.0 10.0 Sill Se) 1 OT A eee 290, 000 | 40.2 27.1 15.6 10.1 | 4.8 2.3 Daphnia hyalina. , a a T LE eee | 27,000 41.9 23.8 21.4 6.7 1.0 0.3 ESS IIE eidug rae ais wisuls | 210, 000 2.0 20.8 17.6 6.6 lee: iL 13 CS Ape eee | 145, 000 44.7 22.2 16.1 BIBT) 4.7 1.3 The variations in the distribution of Diaptomus are greater, although its numbers were more nearly constant, but in each year the same characteristics are shown. The percentage of the population found below the middle of the lake is 7.5 or less, while in the case of Cyclops the number ranges from 11.5 to more than 17 per cent. Daphnia hyalina also varies more in the upper strata, but is in general intermediate in its distribu- tion between the other two genera. The older individuals of Daphnia hyalina are much more apt to accumulate in the lower part of the water accessible to them than is the case with Diap- tomus, and consequently the lower levels are apt to contain a larger percentage of this species, On the other hand the spe- cies does not extend to the thermocline in numbers anything like as great proportionately as does Cyclops, so that the lower part of the inhabited water always contains a larger proportion of Uyclops than of any other species. The vertical distribution of Daphnia hyalina, therefore, dif- fers very considerably in different years. If the species is pres- ent in large numbers and the young are constantly appearing, a 394 Birge—The Crustacea of the Plankton. very large percentage of the population is found in the upper level of the lake and even in the upper meter. This was the case during the summer of 1895, when this species was the dominant member of the limnetic crustacea throughout the en- tire summer. Under these circumstances its vertical distribu- tion approximates very closely to that of Diaptomus. On the other hand, if the species is declining and the young appear in small numbers, there is a much larger proportion of the species in the lower levels of the lake. This was the case in 1896. In August of that year the numbers of Daphnia rapidly declined, so that in the latter part of the month there were present less than half as many as in the latter part of July, and in connec- tion with this decline the population of the three upper levels was nearly equal. In this year the vertical distribution of Daphnia hyalina approximated very closely to that of Cyclops. The vertical distribution of D. hyalina illustrates very strik- ingly the dependence of distribution on specific habit rather than on number. The illustration given in my former paper (Birge, 795, plate VIII) fairly illustrates the characteristic differences in the sum- mer distribution of the different genera, and the percentage diagram, Fig. 29, given herewith indicates the difference in dis- tribution during the summer of 1896. Diaptomus Oregonensis Lill}. Figure 29.—Table D, Appendix. In general Diaptomus is more abundant in the upper strata of the lake than in the lower at all seasons of the year. There is rarely less than 70 per cent. of the species in the upper half of the lake even in the winter, and the only times when the average distribution approaches equality are in late fall and at the period of the minimum numbers of the species in the latter part of April, or early in May. The other extreme of distribu- tion is reached when the new broods appear and as their appear- ance is somewhat irregular the distribution is correspondingly variable. The maximum average number in the 0-3 m. level was reached in the latter half of May, 1895, where the average Orie “Qs ts niin itt Plate XXXIX. Trans. Wis. Acad., Vol. XI. 100: 80 70 20. 30 40 10 Pr. ct..0 N Re AEN Oa Sh | Sp pac Pe coms RCN) ar Pe ee ene ae aD t iy Due eas ea : oS \\ fies gett ae NUS NS ll oe Pgs ot oS a ape ht da | er a as AP a |e al Ran [ee ROR aera, ee ee \" Pn ee ee a ee ; [ee iit spate mn Ee x \ ig sees eee Ne oS Ss i io a oe ee iets See ae es eae NAN | \ IN Hee ect ea eee NG So) RE eas ah Re ans Ul eR RES See act Oh et \ NS N\ - \ X. \ SS \\ 26 alb 9 2 1D m1... 18 m Fic. 29—Summer distribution, 1896. Diaptomus, Cyclops, D. hya- D. hyalina..... Cyclops........ Diaptomus... fi Pau, ee ee ere gale ah We Lo eae Vertical Distribution of Individual Species. 395 was 61.5 per cent.; and in June, 1896, where the average for the whole month was 69 per cent. Each of these numbers is higher than the average for July, 1894, which was less than 53 per cent., and higher than the highest average per cent. for any period of July, 1894, which was 63 per cent. in the second period. The variations which are found in the percentile distribution are substantially like those which are recorded in my former paper. (Birge, O3;) p. 455.) In no case do the older individuals of this Species show a tendency to accumulate in the deeper water of the lake but as the broods which appear in the spring, or later, become older and the water becomes more crowded, they migrate progressively into the deeper levels, but appear to prefer to stay near the surface. Marsh (97, p. 194) finds that the vertical distribution of Di- ‘aptomus in Green lake is uniform throughout the year. This is entirely different from the facts as I find them, since the up- per three meters in summer contain more than twice as many of the species as they doin winter. Apstein (’96, p. 80) finds that Diaptomus was chiefly in the deep water from January to April. Here again his observations differ from mine, since there was hardly a trace of a descent of the species in lake Mendota. Apstein thinks that this descent in winter on the part of Diap- tomus and Cyclops may be due to their desire to seek the warmer water at the bottom of the lake. This motive cannot hold in the case of lake Mendota, where the temperature of the water is almost the same at all depths during the winter. The aggregations of Cyclops in the deeper water are apparently com- posed of feeble individuals, which do not rise again to the sur- face. Cyclops. Figures 29, 30.—Table E, Appendix. Of all the limnetic crustacea Cyclops seems to be most inde- pendent of external influences in its vertical distribution. The maximum percentage in the upper levels is reached when the ‘spring or summer broods appear. While the absolute numbers of these brvods in the spring are much greater than in summer, multiplication goes on so rapidly in May that the animals are 396 Birge—The Crustacea of the Plankton. quickly forced to move toward the deeper water of the lake, and, since the entire lake is accessible to them in spring, there rarely occurs as great a percentage in the upper stratum as is the case in summer. The highest average per cent. in the 0-3 m. level, reached in the spring of 1895, was 42.7 in the first part of May; and 35 per cent. was the average in the latter part of April, 1896. In July of each year the percentage in the upper stratum rose to about 50, owing to the coincidence of swarms of young in the upper water while the lower strata con- tained a very scanty population. ‘The fall rise in numbers does not cause any noteworthy increase in the percentage in the upper strata, since at this time the entire lake is accessible to: the animals and food is abundant at all levels, and the autumnal gales aid to distribute the species through the lake. In the winter there is a strong tendency of Cyclops toward the bottom and as many as 50 per cent. may be found in the lower three meters, and as many as 70 per cent. in the lower six meters of the lake. [Illustrations are given on page 379. Since many of the older representatives of the species die during’ the winter and the new individuals appear towards spring in the upper water, the population of the lower levels decreases in the early spring, both absolutely and relatively. Diagram 30: shows the percentile distribution of Cyclops in the first part of March, 1895, and in the latter part of July of the same year, in which the extremes of its distribution were found. The spring broods of Cyclops show exceedingly well the progres- sive occupation of the water of the lake by the increasing num- bers of the species; the way in which the numbers of a declin- ing species disappear first from the upper waters of the lake, where they first appeared; and the equality of distribution during thedecline. The following table shows the spring history of Cyclops during 1896. The story for 1895 would be substan- tially the same. Vertical Distribution of Individual Species. 397 TasLe XXXITI.— Cyclops, 1896. Number per cubic meter stated in thou- sands. Depth, meters. 0-3. 3-6. 6-9. 9-12. 12-15. 15-18. Agrietth)......... 17.2 17 18.9 20.3 12.8 15.0 April 16-30........ 109.4 84.1 52.5 28.8 18.8 9.6 WES ES yaa 190.2 124.9 117.4 $4.5 52.9 42.7 May 16-21......... 37.0 37.3 34.3 35.2 42.1 64.8 June 1-15 ......... 20.5 13.7 7.6 6.7 5.7 14.1 June 16-30 ........ 59.2 32.4 17.9 13.4 6.7 9.5 Marsh (97, p. 204) finds that Cyclops fluviatilis is present in great numbers near the surface. Its distribution, therefore, agrees more nearly with that of Diaptomus than it does with C’. brevispinosus. The latter species is present in Green lake in very small numbers apparently in and below the thermocline in summer. Daphnia hyalina. | Figure 29.— Table F, Appendix. There are two facts which give the peculiarities of vertical distribution of Daphnia hyalina and the allied species D. retro- curva. These are: First, a decided tendency of the young animals to accumulate in the superficial strata of the water, frequently in the upper meter. Second, a tendency on the part of the older animals to settle toward the bottom. These species, therefore, show a very high percentage in the upper levels of the lake in periods when they are increasing, and especially at those times when the broods of young appear. On the other hand, when the species is declining in numbers, and in the in- tervals between the appearance of broods, the distribution may be comparatively equal throughout that part of the lake inhab~ ited by the species. As examples, compare the table on page 398, and the detailed figures of Table F, Appendix. The percentage in the upper level rarely falls below 25, even in the winter. In May, when the spring broods appear, the average number in the 0-3m. level ranges from 45 to 55 per cent., and the same ratio is found during the summer when the species is increasing in numbers. On the other hand, when the 398 Birge—The Crustacea of the Plankton. species declines in numbers, as it sometimes does in August, the percentage in the lower levels may be nearly, or quite, as great as in the 0-3 m. level. (See August, 1896.) At the time of the fall maximum great numbers of young often appear at once. At this time the brood sacs of the females contain from five to nine eggs. There are very few half-grown animals, and the eggs may all hatch in the course of a week. At such a time it is not difficult to determine the difference in distribution of the young and old, and the following tables show these relations in the latter part of October, 1896: TABLE XXXIV.—Daphnia hyalina, per cubic meter. OcTOBER 26, Noon. OcTOBER 27, 8 A. M. DEPTH. Young Adult. Young. Adult OE SIN: Gr eee ee eeiete en Nene iee sie cen ciavelcdee eteseiniere 122, 200 0 30, 400 1,200 B= OR persone a tose tein Slaha cesta (acta. Sere ave Aros ee anton e eS 27,500 250 13, 300 760 GeO HN herstaicte dk Mnedciote bite evel aston ae 15, 800 380 1,900 6, 300 CORA AW Sa Ride a DP Bi caiage BONE 1,600 4, 100 2,500 3, 800 Bete rachis icici Sl bua lester auvesaee e mrevere taeda eer ate 0 2,500 2,500 8, 900 TANS is Petenigie aaa RRA eS, SR nal Coe on eter 950 1,300 19, 000 After the production of the young in late October or early November, the old females die off rapidly; some few remaining as late as the first of January. In the latter part of May, or the early part of June, according to the progress of the sea- son, those individuals that have lived over winter become weak, are attacked by various diseases, caused by fungi, bacteria, and microsporidia, settle toward the bottom of the lake and die. This downward movement of the older and weaker individuals causes an increase of the number in the lower part of the lake, which was quite conspicuous in June, 1895, and in the latter part of May, 1896. Shortly after this date the crustacea begin to disappear en- tirely from the lower water, and during the remainder of the summer the life of the species goes on, like that of the other crustacea, in the region above the thermocline. The vertical distribution of this species does not appear to have been carefully studied by other authors. = » : Vertical Distribution of Individual Species. 399 Daphnia pulicaria. Figures 30-32.— Table G, Appendix. The vertical distribution of this species is so peculiar that it demands a somewhat more detailed account than has been given to the other species. The history of the species begins or- dinarily in the early part of July of the odd numbered years. During the first part of July it has been present only in very small numbers, but in the second part of July, 1895, its numbers were so large that it appears in the lists. At that time more than 50 per cent. of the species was found between 6 and 9 meters, in the region of the thermocline, and nearly all of the remainder was found between 9 and 15 meters. In August the Species moved downward, following the downward movement of the thermocline, and continued in this position until the coming on of the autumnal homothermous period in late September and October. During October the species was distributed with approximate uniformity through the water of the lake. In November, as the lake cooled, the animals began to move toward the surface, and in late November and December a period of active reproduction began. The young animals were found in the upper level of the lake, most numerously in the upper meter, and as the result of this distribution, the numbers in the upper level were far greater than those in any other portion of the lake. This relation continued throughout the winter of 1895-96, during which time reproduction also continued, although more slowly, until in March and the early part of April reproduction nearly ceased and the numbers of the species declined somewhat rapidly. At this time the distribution was uniform, or such irregularities as were present seemed to be accidental. In the latter part of April the spring period of reproduction began and an enormous number of young were produced in the upper water. At this time as many as 80-85 per cent. of the species were found in the upper level; a larger proportion than has been found there of any other species except Chydorus. In the early part of May a reproductive pause occurred, during which the animals were pretty evenly distributed through the water; 400 Birge—The Crustacea of the Plankton. the largest number being found in the bottom stratum. A second reproductive period came on in the latter part of May, in which the upper water was again crowded, although the numbers increased so rapidly that the population of all the upper levels of the lake was greatly increased. During the early part of June the distribution became once more equal, with the largest number again in the bottom level, and during the latter part of the month the population rapidly declined, falling off most in the upper levels. At this time more than 60 per cent. of the species was found below the 12 m. level and less than 2 per cent. in the upper level. Late in June the species began to move away from the bot- tom water, or perhaps it would be more correct to say that the individuals at the bottom of the lake died off more rapidly than those in the levels immediately above, so that in the early part of July nearly 60 per cent. of the species was between 12 and 15 meters and only 6.5 between 15 and 18 meters. As the Species declined in numbers the decline took place chiefly in the lower levels of the lake, so that in July and August the few representatives of the species that were left were concentrated in the region of the thermocline, thus occupying the same posi- tion that they had held in the corresponding months of the pre- ceding year. The following table shows the numerical relations. TaBLE XXXV.— D. pulicaria, 1896. Population per cu. m.of each level stated in thousands. Depth, meter. .:|, 028 3-6 6-9 9-12 | 12-15 | 15-18 Aprileiguon, oe 1.0 1.5 3.2 2.5 1.3 0.6 Arpril 16°20 ...;..-2 41.6 5.2 0.4 0.7 0.9 0.2 May 1215,. 2g 8 10.4 12.8 15.5 9.2 | 1383 17.8 May 4623020). 05) 55.4 33.7 37.4 28.8 19.8 23.4 Tune 16h 10.3 5.9 8.8 12.5 5.9. tose June 16-30........- 0.4 15: 2.8 3.7 10.9 4.4 STuly 145 a eal Coord i 3.5 1.3 0.8 July 16-31. :....... | RON | en ics 3.2 1.7 0.1 0.1 — - Fig. 31 shows the movement of D. pulicaria during the late summer and autumn of 1895. Points were established indicat- Se ioe is 2g Trans. Wis. Acad., Vol. XI. Pr ct..0 10 20 30 40 50 60 70 80 90 100 0 m.. 6m.. 12 m.. 15 m... Fic, 30.—Cyclops, March and July, 1896; D. pulicaria, August, 1895, and April, 1896. See pp. 396, 399. Cyclops..... D. pulicaria = nee = | Plate XL. 10 20 ~ 30 40 50 60 70, 000 ——————— en : i] ' 4 ' ‘ ’ ! ' ' ' : ‘ ; 1 1 A ' 4 ; 4 ! ! n ! t ' c ‘ ' 1’ 1 1 ) 1 4 (Os) saat. | ! ‘ ' ' 1 ! t] i] 4 ' i] ; 4 § 1 4 ; U ' ' ‘ ’ 1 : 4 ' Y 4 LAD ta,” : 1 ; , 1 1 v 1 A A ; 1 i] i] 4 ‘ : : ; i ' A ’ ; 1.5 m.. U u ! ' ' i] 1 ' 1 ‘ ' ‘ ; 1 Y : ‘ 1 ' v ) ; 3 ) ! ! : . . : ; ; 1 ‘ ' ' 4 2.0 m..1 0 u : i : : : : ' ' ' : ' ' t ! ' ; \ ' 0 ' ' t ' ' : ' ' t ' U 1 ' 2.5 m..! ! ! ' ! t ' ' ' i} 1 | : 4 i I ' : ; ! ; ; ' ! l : ' t u ' 3.0 m..! ! \ a Egg Fia. 31.—Distribution of crustacea, 0-3 m., Sept. 13, 1896, 2"p."m. (a), and 9 p.m. (b). Scale, 1 horizontal space = 10,000 crustacea per cu. m. ee are interrupted at levels where no observation was made. See p. 413. —~ Trans. Wis. Acad., Vol. XI. Plate Xl. Depth." 15 m.. 18 m... AUG. SEPT, Oct. Nov. DEc. ; : ' 1 ! 1 ' i ‘ ’ = ' 10% : 2e Seeieie ° 6 ' 1 ' i : ' , ' ' U) 5 ' ' 1 1 Tt ; ' ' 1 ' 1 ; : ; ' 1 U ' ' a U ’ 1 ! i ' ‘ ' 4 ' 1 ' ' \ ; J ’ 1 ' i U v ‘ ’ H 1 ’ ' 1 : ; Y ’ U U ’ 1 F ' 1 a ' 1 ' ' ' ! ! ' ‘ ’ ' ' ' ' 1 i) 1 é ' ' ’ ! I ' : ' 1 ' ' 7 ' i] ! 70% Fic. 32.—Percentile vertical distribution of D. pulicaria, August-December, 1895. See p. 401. Vertical Distribution of Individual Species. 401 ing the level below which the respective percentages of the species were found and these were connected by lines. The dis- tribution is based on assumption that the individuals of the Species were uniformly distributed throughout the 3 m. level in which they were found. This assumption is peculiarly incorrect for D. pulicaria, since the species is limited to the region of the thermocline. It is often confined within a space of 1 meter, or even less, yet it often passes beyond these narrow lim- its, aS is indicated by the fact that not inconsiderable numbers may be found in two or even three levels. While, therefore, the diagram spreads out the distribution of the species during the summer more than is correct, the general relations are well enough indicated by its lines. It will be seen that in the latter part of August more than 65 per cent. of the species was found between 9 and 12 meters and that the species moved downward during September as the thermocline moved down. In October, after the breaking up of the thermocline, the distribution was much morenearly equal. The center of popula- tion rose rapidly and regularly from the latter part of September to the middle of November, lying near 14 meters in late Septem- ber and at 4 meters in the first part of November. After a smal]: fluctuation in the latter part of November, it rose once ‘more, and in the latter part of December lay about two meters below the surface, where it remained during the early part of the winter, until the decline in numbers came on in March or April. If this diagram were reversed it would serve fairly well to indi- cate the downward migration of the species in the spring. In Fig. 30 are given curves for the percentile distribution of D. pulicaria for April 16-30, 1896, and August 16-31, 1895, showing the extreme variation of its average distribution. The diagram is similar to that described on p. 384. I have not found any other case recorded of a Daphnia which in summer remains at or below the thermocline. At least one other species of the genus has the same habit in thisregion. A form which I have identified as D. longiremis Sars, belonging to the cristata group, is regularly confined to the region be- low the thermocline in some of the lakes of the Oconomowoc system and in lake Geneva. 26 402 Birge—The Crustacea of the Plankton. Daphnia retrocurva Forbes. Table H, Appendix, This species belongs to the periodic crustacea and is present in the lake from July to December. Its numbers during July are small and the proper history of the species does not begin until the latter part of this month, or the early part of August. In 1896, indeed, the numbers were very small until the decline of D. hyalina in the middle and latter part of August gave an opportunity for the presence of this species. In vertical distribution this species agrees very closely with D. hyalina, as would be expected. In the early part of periods of increase, from 45 to 60 per cent. may be found in the upper level. This was the case in the latter part of July, 1895. It was also truein late September and early October, 1896, although the crustacea moved rapidly downward so that the two-week averages do not disclose the fact. In the old age of the broods, as the numbers are declining, they are found chiefly \in the lower water of the lake. This was especially obvious in late November and in December, 1895, when the species disappeared quite slowly and lingered latest in the lower waters of the lake. In 1896 the formation of the ephippia was nearly simul- taneous on the part of all of the females and the species disap- peared rapidly and completely in the early part of November, so that this phenomenon of the old individuals lingering in the lower water did not appear. Marsh (97, p. 210) finds the distribution of Daphnia Kahl- bergiensis in Green lake very similar to that of D. retrocurva in Mendota. He finds, however, a marked difference between the vertical distribution by day and night, which I have not seen. The fact, however, that D. retrocurva descends to a some- what greater depth during the day than does D. hyalina seems to indicate a greater sensitiveness to light than that of its con- gener, although this sensitiveness does not lead to as great movements as Marsh’s observations would indicate for Green lake. cee lt a een east Te: Ps ae ese a = Vertical Distribution of Individual Species. 403 Diaphanosoma brachyurum Sars. Table I, Appendix. This species belongs to the periodic crustacea, its active de- velopment extending from the first of August to the middle of October. It is provided with very large antenne and is one of the most powerful swimmers among the limnetic crustacea. It is also positive in its relations to light. In both these respects it resembles Diaptomus and its vertical distribution very closely agrees with that of the latter genus, although its numbers are very much smaller. In the early history of the species 50 to 70 per cent. of the whole number are found in the upper stratum of the lake. The distribution becomes more equal dur- ing the decline of the species and at no time is there found any aggregation of individuals in the lower waters of the lake. The distribution of the small numbers present in the decline of the Species is, however, quite irregular and the number in the upper part of the lake becomes smaller than that in the lower water. — Marsh (’97, p. 216) suggests that the vertical distribution of Diaphanosoma is controlled by light rather than temperature. He finds it negative to light and thinks that it prefers cool water. In the laboratory Diaphanosoma moves toward the light along with Diaptomus, so that my observations would indicate that it is positive in its relations tolight. I find also uniformly a larger percentage of adult animals in the upper meter by day than I find of the species of Daphnia. There is, therefore, noth- : ing in my observations to confirm the idea that the species is negative in its relations to light. Since, however, the absence of crustacea from the upper centimeters of the lake when the light is most intense, indicates a certain negative relation on the part of nearly all forms, it may well be that this species finds the light in the clear water of Green lake too strong, and responds to it more definitely than in lake Mendota. 404 Birge—The Crustacea of the Plankton. Chydorus sphaericus. Table J, Appendix. This species belongs properly to the littoral crustacea and its presence in the limnetic region depends apparently on the presence in abundance of Anabaena and allied forms. Since these plants tend to aggregate in the upper water of the lake, Chy- dorus shows an equal tendency in the same direction and the per- centage of this species which may be found in the upper levels exceeds that of any other of the limnetic crustacea. It is true, however, for this species, as for all others, that the largest numbers are found in the upper level at the time when the num- bers are rapidly increasing, and that when the numbers are de- clining the distribution may be more equal, or may vary in an accidental fashion. During the periods of rapid increase from 50-80 per cent. of the individuals are found in the 0-3 m. level. These high percentages have been reached in September, 1894, July, 1895, and June and August, 1896. In October and later the species becomes quite equally dis- tributed through the water, but it showed no marked tendency to aggregate in the lower water at times when it is declining, until the numbers became very small in late winter, 1896. It is very abundant during the day in the upper meter and, like Cyclops, is one of the last forms to disappear at the thermo- cline. The fact that Chydorus is relatively very abundant near the surface is noted by Apstein (’96, p. 80). Leptodora. The number of Leptodora caught is so. small and so variable that it is difficult to give any positive general conclusions regard- ing its vertical distribution. The following table shows the average distribution for the months of July, August, and Septem- ber, 1895, with which that of 1896 closely agrees. The Annual Distribution of the Crustacea, 405 TasBLe XXVI, PER CENT. IN EACH 3M, LEVEL. Total 1895. Number taken -3m 3-6 6-9 9-12. | 12-15. | 15-18 Sos ae 285 33.3 34.4 24.6 7.4 0.3 0.0 August ............. 680 41.0 28.8 19.5 8.5 1.9 0.2 September ......... 156 34.0 28.2 17.3 9.6 9.6 1.3 This table shows that the average agrees very closely with that of the other limnetic crustacea. During this season a consider- able number of observations were made after nightfall, but neither in 1894, nor in this year was there any evidence of a movement of Leptodora toward the surface at night, as meas- ured by the three meter intervals. The species is nearly, or quite absent from the upper meter or so during the day, but comes to the surface again with the other crustacea after night- fall. In August, 1895, the number caught in the 0-3 m. level, ranged from 1 to 43 individuals; in the 3-6 m. level, from 1 to 33; and in the 6-9 m. level, from.0 to 46. Below this level, of course, few, or no individuals were obtained. With this range of variation, the percentages might easily be altered greatly by a single ob- servation. Nauplii. Figure 33. The vertical distribution of the nauplii has been very vari- able, as may be seen from the following facts: On July 17th 50 per cent. of the very large number taken were caught be- tween 6 and 9 meters and only 7 per cent. in the 0-3 meter level. On the 18th the distribution was substantially the same, while on the 20th 38 per cent. were found between 0 and 3 me. ters, and 31.5 per cent. between 6 and 9, and on the 21st 49 per cent. were found in the upper level and only 19 per cent. be- tween 6 and 9 meters. On the 5th of August 90 per cent. were found between 6 and 12 meters, and on the 8th 23 per cent. be- tween 9 and 10 meters, and 50 per cent. between 6 and 10. 406 Birge—The Crustacea of the Plankton. These observations were all made in the day and under substan- tially similar conditions of weather and temperature. During August and September, 1897, numerous observations were made by means of net and pump and in nearly all cases the great ma- jority of the nauplii were found in the lower part of the inhab- ited water, although a considerable number was also found in the surface levels. On the 13th of September a very large num- ber of nauplii were found in the upper half meter, by far the largest number being found at the surface itself. (See Table XXXVIII, J.) The number very rapidly declined from the surface, reaching a minimum at about 1 meter. They began to increase again at about 5 meters and reached a great number in the lower levels, substantially as shown in Fig. 33. The nauplii in the upper water were well developed and apparently about to change into the form of the immature Copepods, while the great number lying between 10 and 13 meters was composed of very young individuals. It seems probable, therefore, that the nauplii dur- ing their younger life dwell in the lower part of the inhabited water and move toward the surface when they are about to leave the nauplius stage. The immature forms, both of Diapto- mus and Cyclops, are present in large numbers in the upper strata of the water and the egg-bearing individuals are present in larger numbers in the Jower strata, although they are never absent from the upper water. In all the lakes which I have examined in summer the great majority of the nauplii have been found in the region of the thermocline; either just above it, or immediately in and below it. I infer, therefore, that this distribution is a common one. In October and later the distribution becomes uniform and so continues until late in the winter. In March, as the larvae begin to change into Cyclops forms, they approach the surface. Apstein (’96, Table IV.) does not appear to have found the nauplii more abundant in the deeper water than near the surface. The Distribution in the Upper Meter. 407 THE DISTRIBUTION IN THE UPPER METER, AND THE DIURNAL MOVE- MENT. Figures 32, 33. The observations recorded in my former paper showed uni- formly that there was no general diurnal movement of the crus- tacea and no movement at all which couid be detected by the use of three-meter intervals. This conclusion has been con- firmed by all of the observations which I have since made. During 1895 and 1896 considerable attention was paid to the distribution of the crustacea in the upper meter, with the de- sign to determining whether or not there was a diurnal move- ment of the limnetic forms within narrower limits than three meters. A large number of observations were made in 1896 in order to determine the relative number of crustacea in the upper meter and the remainder of the 3 m. level. These observations were begun early in August and continued until the last of No- vember; twenty sets of observations being made in all. In some cases the crustacea were taken meter by meter and the numbers compared. In other cases the crustacea of the upper meter were caught and their numbers compared with those ob- tained from the entire depth. A single illustration of the former method is given; partly in order to show the results, partly also to illustrate the amount of agreement and difference between the three catches of one meter each and that made through the entire distance of three meters. ‘TasLe XXXVI.— Number of crustacea caught August 24, 1895. 6 P. M. Depth, meters. Dept Cyclops. [p. hyalina. a neere Dinpeae Chydorus. Oa Me eee aiahs sioleree an 700 36U 2,120 280 140 100 a A ae eae dene 340 360 2,060 200 140 120 ATR Canc ats Siereie 460 370 1,150 160 50 50 Pobed o ois ae Le hae eS ee ee ne eo 0-3. | 1, 780 | 1,050 4,250 AT5 850 315 As would naturally be expected, the ratio between the c: tacea of the upper meter and those of the entire level varies 408 Birge—The Crustacea of the Plankton. very greatly, On some occasions the catch of certain species from the upper meter was larger than that obtained by a second catch from the entire three meters. Such instances were due to the presence of very large numbers of young in the upper meter, with a somewhat irregular distribution, so that the catches varied considerably. Upon the whole, however, the average number derived from these twenty observations agreed surprisingly in all the species. It was found that the upper meter contained an average of 43 per cent. of the entire catch of Diaptomus from the upper three meters; 47 per cent. of Cy- clops; and 50 per cent. of Daphnia hyalina. These catches were made during the day and may be taken as fairly indicating the relative number of crustacea in the upper meter during the daylight hours. It will be seen that these observations fully justify the statement made in my former paper (Birge, 95, p. 479) that “a general movement of the crustacea as much as one: meter would have been detected,” and indicates that at no time is the population of the upper meter of the lake notably de- ficient. The minimum percentages were very irregularly dis- tributed and depended more upon the presence or absence of young individuals than upon any influence of light, weather, or wind. These observations also indicate the extent to which the lines. of Figs. 29 and 30 should be altered in the upper three meters. in order to express the average distribution within that level. During 1897 observations were made with a view of deter- mining the exact distribution of the crustacea in the upper ‘meter. They were made by two methods: First, a net with an opening ten centimeters in diameter was supported so that it. could be drawn horizontally through the water for a known dis- tance at an uniform rate of speed. The crustacea so obtained - were counted and the number present at a given level was thus. determined. Second, a pump was taken out in the boat, by whose aid the water of the lake was pumped through a hose and. strained by the plankton net, the mouth of the suction hose be- ing placed at the successive levels. Water was taken from the surface at a depth varying from two or five centimeters in calm weather, to ten when the lake was agitated by the wind; at one- The Distribution in the Upper Meter. 409: half meter; at one, two, and three meters, and sometimes deeper. The results of these two methods were the same and can be stated in general as follows: 1. On calm sunny days the upper ten centimeters of the lake may be almost devoid of crustacea, as was the case on August Ist, 2d, and 25th. At a depth of half a meter, however, the numbers become considerable and may be very great. On August 25th the total population of the water at this depth was at the rate of nearly 70,000 crustacea per cubic meter, without including the nauplii, which numbered 18,000 more. At one meter the population was nearly 200,000 per cubic meter and below that depth the numbers rapidly declined. A large num- ber of similar observations were made on other days, and in one of the cases where the observations with the pump were ex- tended throughout the inhabited water the results have been diagramed and are shown in Fig. 33. 2. The population of the upper meter is largely composed of immature crustacea, the percentage of young varying in dif- ferent species. It is most marked in Diaptomus, Daphnia hya- lina, and D. retrocurva. Great numbers of young are found in the upper meter, as was the case on August 25th, and especially on September 8th, and the adults may be entirely absent. At the depth of a half meter a very few half-grown individuals. are present, while they are fairly numerous at one meter and at the same depth the adults begin to appear. Below one meter by far the most conspicuous part of the pcpulation consists of adults, although the young may be present in numbers as great as the comparatively few adults. A similar relation of distri- bution holds for Daphnia retrocurva, although the proportion of this species in the upper meter by day seems to besmaller than that of its congener. The adults of Diaphanosoma approach nearer the surface when the sun is bright, than those of Daph- nia, but at least 75 per cent. of the individuals found between the half meter level and the surface are immature. The same state- ment is true for Diaptomus. Cyclops shows the least difference; females carrying eggs being regularly found in considerable numbers at half a meter, or even above that level, coming to __ the surface on cloudy days and occasionally in sunshine, Yet 410 Birge—The Crustacea of the Plankton. while it is not easy to determine the exact proportions of young, it is very obvious that the majority of the immature Cyclops are near the surface. . 3. A far larger proportion of Cyclops is usually obtained from the upper five or ten centimeters than comes from any of the other forms of limnetic crustacea, and it may be present at the very surface on hot, calm, sunny days, as on Sept. 13. 4, The nauplii are found in considerable numbers in the up- per water during the day and frequently extend to the very sur- face, yet ordinarily the number at the surface is only a third, or even a smaller fraction of that found at one-half meter. Older nauplii may be found in large numbers at the surface and confined to the upper one-half meter. 5. In windy and cloudy weather the crustacea approach nearer to the surface, the numbers of Diaptomus and Cyclops being es- pecially increased by the change in the condition of the sky. Daphnia hyalina also may come nearer the surface. But the num- bers of these species during the day in the upper ten centimeters are always decidedly smaller than at one-half meter, so far as my observations extend. 6. At night the population of the upper meter changes in character. The young, instead of being concentrated in swarms in this layer, become more evenly distributed, and the adults which were found below the one-meter level rise toward the surface. Leptodora and larval Corethra have been regularly taken at the surface in considerable numbers at night. During the day these animals are rarely, if ever, found close to the sur- face, although they may be abundant enough above the three meter line. It would appear, therefore, that these animals move toward the surface at night, together with the crustacea on which they feed. Hpischura seems to have the same habit. The Distribution in the Upper Meter. 411 TaBLE XXXVIIL—Typical catches from the upper water giving the rate of population in thousands per cu. m. at the depth specified. d a ee ea obs kas ;| 3] 2 D 3 pes pe beaks be lise he |S = q S) = a 5 See Sele dee te tetas, | eel lakes a Bi Seen ee dre ke eee Ne am | ees Pee pate yaya | alvaelazalaAls)s A. “Ane; Noon. 0-0.1 tee 26 Gul ight nort Breeze. Net O01 22.51) 25 1... Blk. Sor IN ao ee rawn orizon- tally 20 mot-| | 1-0-22| 43.8 | 5.7] 11.7) 2.2 esd 912 |). ai ee ers. Bip totes | 14:00|° 224) sag) 1.6: 024 198.2) aah doe. Dowie bl Ta gatos CA a a aes fel ee Rtg Me ai ees ray aa Aug.2.. Sp. || 05-06] 9.2] 7.8] 12.0) 14)...... 30:4, | 40.6) bes) WOH co oats ig lomdee) Calm.4 | 10-11 | 11.6 | 9.2) 13.4) 2.2 |...... 41.4} 3.6| 0.04] 0.04} 0.08 Net drawn 15 meters. || 20-21} 96] 10.4] 9.6) 2.8]... Boe af eee 0.04) 0.08 ip 20-371 | 20.0| -8.6| 9.0), 2.4 |.....- AON BHO | > 0.02| 0.02 2 ( 0:05] (5... 0:4) 0:6) 0.6 |...+.: 1.6; 402 Wie & ein |. Aug.6, 2 p.m. | 0.5 |22.5| 4.11 0.75 | 0.4|...... 80) GBR we 8.6) lina light clouds, 4 light $2.) tht 422 al 9.0 1 BO BYON2 S20) | Taleo dante sauce breeze. Pump. L 2| 22.5 | 15.0 | 23.7 | 12.2 |:..... Te aie Ota s | 0.2 O05). 2. iOS pad a all oe 15 60) Trewin eee D. Pies ier dale Vato |.) 67.2)|48.04; 00 | 1045 ee Aug, 25, noon, Al eee 1.0 | 61.5 | 20.7 | 51.7 | 63.5 |...... (08:4) 52h ae Sahat iwi Pump. AKU cB toy ale eM 2 B1.3;| Gul) ios, 2.0| 4.5 ! 2610.0) 6.0-).6.5 1 45,1 2.0/'20.0) 15 |... 2.0| 3.0 0.1] 6.5 | 12.0/ 13.0 |...... A2Oul 43058 | 45 |, Sasleie ae heen ey BEPC 0.5| 9.0] 19.7 | 50.3| 1.8 | 13.0 | 90.0|...... OV: [ea tte eas p. m. clear, | fresh 1.0} 8.0] 17.5} 17.0|...... Gu hvsOnce= « Ot leece breeze. nay 2.0| 9.0 | 24.0) 12.0 |...... 11.5 | 56.5 |...... 6.0) phe jae | Gb we (1028 | 8.24 148 18s. EVI erg tema Uae elie Osi 10.8 140 | 27-6 | 12.8)... Eee Uy eid pit Sy | a | On iGo | yal 6 8 | 12 ce. 92.6 | AT .AT. cooly eee ae wp. Clouattrosh 2.0 | 12.0 | 24.6 | 17.8 | 13.8 |...... tar Bh ec eae a Smid eos | 608 (Ge | 1,8 | 7226 | 1206) |... cheese aeleeans ; 4.0| 7.6| 20.0] 5.6| 6.4|...... 30.6| tok ee Senta ( pi) G4 (ato hk B2"| 16}... rae a ime 0 seer aes] 0 412 Birge—The Crustacea of the Plankton. TABLE XXX VII.—Continued. : a ; a Z Sle ere : (<>) E (2) oS . 3 2 se) 8) 8) 8) ee g g a Be a ) st ate } 2 ra a Bla |e iS | 8) 8). 8 Se o SS BS Bea ies Neils os | 8 | 8 fal Aalto Tra |) @ tel) Se eee ( O44 1-315 5018 4S AO ee 17-0 1 205 G. 5 10) (Ga eee de ; Seno NGpE. 0 1.0} 6.0] 6.0| 6.5 19.5 | 29.0 ‘ Clear, fresh. 8.4 1.0] 6.0| 14.5] 10.5] 5.0]...... 36.0: | 25.0 |.) 6. cele ieee W. breeze. Pump. 2.0! 6.0} 12.0 | 10.5 | 4.0.) 5.0 | 87.5: |Q5em | 2, ase) aoe L 310) 15.4 | 1046] eae ON: 21.21 13.2 |.) eee The preceding tables show the results of some of the more important observations of this kind made in 1897. The fig- ures of these tables express the rate per cubic meter found at the given depths, not the actual population between certain depths as is done in the tables based on the vertical net. In most of these lists, the preponderance of Cyclops in the upper stratum is striking. In A, all of the Diaptomi at 0.5 and 1m. were young. The same was true of D. hyalina at 0.5 m., and above. In all catches 85-95 per cent. were young at1 m. on sunny days. The effect of cloud is plainly visible in B, C, and : F, and of wind in E and G. The tendency of Gloiotrichia to aggregate at the surface is well seen in D. , In the following tables the record for two more complete ob- servations is given, together with one illustration of a night distribution. In the latter there were almost no nauplii, an exception to what has usually been found at night. The popu- lation for the given depths in the catch of September 8th has been platted in Fig. 33, and Fig. 32 shows the upper three meters of the two sets of observations on September 13. Rai als . Ge Trans. Wis. Acad., Vol. XI. Plate XLIT. 20 840 60 80 100 120 140 160 180 200 220 Nauplii. Pepi 10 20 30 40 50 60 70 980 90 100 110 Crustacea. my a T Hi Fi 7 v 1 1 ies : dameeee Zs : Dae. | . 55m 10 m | : : 0 na 12 m : Bee 3 : ' a -—_ — 1 (Ora ee ose eee a 1? 152 16° 17° 18° 79°] pe Zia as 14:m...! i UES ey eam ! ; : ; a , ' ' r A ' 1 ‘ ‘ : ' : 4 ' 1 ' 1 ! ' 1 ’ ' ' ' ' ' ‘ ee 15 m E y ' 1 ' 1 D i / ' ' ‘ ; H } H : 0 ; 5 : 5 ‘ ! ! ’ ' ’ ' ' 1 ' OU bee oa 5 I ul ' 1 i) ' ry ! : : ; ' ; ‘ ‘ ' ’ ' ' 1 1 ’ ' 1 1 ry ' ' 1 ’ 1 ) ’ ' t e ! n 1 1 ' 1 ‘ ‘ ‘ i 1 ’ 1 : . aaa ' 1 : A ; ‘ H 1 ' » 1 ‘ ' : \ i A ' ' ' ; —_— ; i} , i 5 ‘ 4 H 6 : ) ' i] ’ ' ‘ ° t) . ' nl ' ' . 1 : i ; r : H t _.20 m Fic. 33.—Vertical distribution of crustacea, nauplii, and temperature, Sept. 8, 1896, noon. Scale, crustacea (full line), 1 horizontal space = 10,000 per cu. m.; nauplii, 1 space = 20,000; temperature, 1 space = 1 degree. See p. 413. The Distribution in the Upper Meter. 413 TaBLeE XXXVIII.— Typical catches with the pump from the entire depth. The numbers are stated in thousands per cu. m., and give the rate of the population at the depth specified. a: a (<0) | 3 a = ie. fh} on. | 22.22 | OED 23-2 a 1 21.8 | 2 21.6 ae | 4 21.5 6 21.4 H. Sept. §, noon, 8 20.5 alee light S. w.4 reeze ; pump. See || 10 20.1 Saal fe “| 1948 | 12 19.7 PAcDlieceeyes | 13 19.4 | 13.5 | 16.6 | 15 14.3 L| 23 11.8 {! 0.02} 26.5 if | 0.10). |] o.25l...... oat ar iseacer | 1.0 | 24.9 eae. | 2 23.8 eas: 22.2 Sept. 13, ee m. | 5 21.8 Soo hig’s galt are aay eae 9 20.2 | li 19.3 | 13 18.7 | 15 17.8 | 16 | 15.6 |} 18 | 13.2 . L! 20 12.4 Diaptomus. 1.5 6.0 case ee eeeeee eoeeee eesoee 6.6 0.4 ras) A < a eee 78.0 7.5 | 44.5 5.3] 8.3 SOR entea 4.3 1.0 Hf) 4) 55S 2.2| 2.5 5 yA ee: 0.9) 1.3 LAN Poor 2.3 she 1.4 Sareiete.« 0.05 sera 0.5 diel es 0.6 i pere tae 17.5 ae An 25.0 Zeb) A all 235 4.0} 7.0 MDa ose On| eeaercs CNA Val teecaioe ORS eens 2.0 1.0 Oe Ooo aexss (1) Bl ae O#O5 ener | D. retrocurva. e | Diaphanosoma. eees ee seee es eeee ee eoceoes Ergasilus. ev cece eooees eceece £ 5 is 16.6 | 15.5 106.5 | 27.0 93.3 | 19.5 51.9 | 55.5 58.0 | 40.5 22.5 | 21.0 34.0 | 50.2 16.9 | 55.0 11.2 | 135.0 8.0 | 143.0 10.9 | 225.0 10.2 | 108.0 2.1) 22.5 DA fecicies 9.55) 141.0 2:9.) 95.0 4.8 | 65.0 31.5 | 19.5 40.0 | 16.0 41.5 | 19.5 39.0} 18.5 40.5 | 22.5 28.0 | 33.5 36.5 | 51.0 18.5 | 112.0 13.0 | 122.0 14.8 | 259.0 11.7 } 246.0 13.0 | 52.0 0.4 2.0 O05 i cers ss 414 Birge—The Crustacea of the Plankton. TABLE XXX VIII—Continued. ton ; = = ci e fs ws % 2 ® © rs] : > g 8 Bil Boe ee 8 ~~ Ss i . (>) (o) wn m (0) Q . _ ont (>) om a 5 = o | ea | We ® = oS a wo) ~ 2, = aq aq fan Q wo 3 a s | 2] 21,255.55 | "| 2 |i i aA RO A ce a Oo | <4 > A A <3 = z, raat fran eee 10.5] 22.0] 3.5'| 4.5] 25°) dae 85-5 leaks is i | Oo 7.5) 13.5 | 5.5] 12.5|10.0| 11.5 0.5 We Sept. 13.9p.m. 4| 1.0]...... 6.0) 20.5 | 7.0| 3.5) 110) J4/5).20) sag eee 2:0 |......|| 12.5} 11.5 | 6.0) 3/5’! 5.0) 33.5).e ee) eee B10] ci’ 7.5 11.0] 6.0] 4.5] 6.0| 32.0)......| 67.0 |..... A These observations (and I could adduce many more) show that there is a clearly marked diurnal movement of the crustacea in lake Mendota but that it is confined within the narrow limits of the upper meter, or meter and a half. The day population of the upper centimeters, especially in bright, calm weather, is very small, but the number at one-half meter, even under such conditions, is nearly or quite as large as that at any greater depth, and may be the maximum number. The day population of the upper meter consists chiefly of young and immature crus- tacea; most of the older individuals of all species being found at greater depths. This relation of age to distribution is most marked in the Daphnias and Diaptomus and least marked in Cy- clops. At night the population of the upper meter agrees in general character with that of the water below, the older indi- viduals ascending, and the younger descending. I have found no evidence of an aggregation of adult crustacea close to the surface at night, but my observations have been confined to the hours before midnight. In general, these conclusions regarding the diurnal movement of the crustacea agree with those of Francé, (94, p. 35), with the important difference that while the movements described by him are measured by meters, those which I have observed take place within the narrow limits of the upper meter, or even within a smaller distance. There are, however, some note- worthy exceptions to the agreement. I do not find that the The Distribution at the Thermocline. 415. Cladocera aggregate at the surface at night, but find that the upper water, in the early part of the night at any rate, is ten- anted by a larger proportion of Copepoda than of Cladocera and that a smaller fraction of adult Cladocera is found among those present at this level than at the depth of half a meter, or more. I do not find that a strong wind brings about an even distribu- tion of the crustacea, although it assists in doing so. In moderate winds the crustacea approach somewhat nearer the sur- face than in quiet, sunny weather, and during violent winds the distribution in the upper three meters is more uniform than in cloudy weather, but in case large numbers of young are present, there is always a high percentage in the upper meter. THE DISTRIBUTION AT THE THERMOCLINE. During the latter part of the summer of 1896 observations were made with the net, in order to determine more exactly the distribution of the crustacea at the thermocline. The net was raised from the bottom of the lake to the bottom of the ther- mocline and then closed and drawn to the surface. After wash- ing out the collection it was lowered to the depth at which it was closed, opened, raised through one meter and closed again. In this way the population was determined by single meters for the two or more meters including the thermocline and the water immediately above. Great care was taken that the move- ment of the net should be regular, and the messenger was sent. down the line in such a way as to close the net immediately on its reaching the upper level of the meter under investigation. The results show that the crustacean population usually passes into the thermocline and often toward its lower part, but that here it ends often with great abruptness. If the temperature conditions. are such that the thermocline is spread out over two or three meters the population ends less abruptly than when the thermo- cline is concentrated into a meter or a half meter. The obser- vations showed a population per cubic meter of only a few hun- dred below the thermocline, while in it and above it the popu- lation might range from 40,000 to 60,000 per cubic meter. As these observations agree in general with the more exact results reached by the pump in 1897, the details will not be given. 416 Birge—The Crustacea of the Plankton. In 1897 similar observations were made by the aid of the pump; 40 liters of water being ordinarily pumped from each level. The results were substantially the same, al- though the number of crustacea found in and above the thermo- cline was smaller, since the population of the lake was smaller in 1897 than in the preceding year. The following table shows the results of some of the observations. It will be noticed that the abruptness with which the crustacea stop is evidence that the pump did not draw water from any considerable distance from the mouth of the suction hose. ‘TaBLe XXXIX.— Typical catches from the thermocline stated in thousands per cubic meter. See also Table XX XVIII. wn Se) 8 8 5 oy e E 3 = 8 ® a) s w ss I 3 e s 2 2 A s g a 2 5 o = 3 9 q 3a s = © 3 a a = o = 3 ° = a rs = rf ° > > e a Q = = roy re) r= jo} oC a 3 Oo e|/ es) 2) 2) 2) 2) es) os) eee rie Q = Q Oo =) A A fan) ca | a Zz 1) (| 9 ZAC BTW O16 16 .S a OD nee 4.8} 0.8 | 0.05 | 50.5] 4.1; 0.05 a | 10 20S 60) SiS VaeT tae eo 5.3} 1.8] 0.05/ 58.1] 3.2]...... face ‘tompera:| ra ote | eae AS! ales 4) DIBA ua O13) 4mm 0.02 | 24.7] 11.8] 0.1 Jee alec. | 12 | 15.9 4.0 8.7 28.6| 5.81 0.15 ass 146-4 Keil) 0.2 B. f 11 | 20.6 |] 1.1 5.0| 5.0 £3 let 15.31 92 ke ck free heed 12 all AG, 10h. 6-8 oi iv | eee 12.1 | 10.0 |. ...2 - ° tare, TN een 01 Gite ee 0.08 11 | 21.3 || 10.2 | 8.4] 3.0 3.0 4.6 (4a. 2 F oe $200) PA 24) 8) 19.82 Wing ell soo. 3.9 29.8 Wada os ug. . uUr- qate wey 12.5 | 20.4 2.4 129.8] 1.6]. saesecl 20h S45) IGS Wk. cee ure CIs i | 1S) a aGUB UN O01) Od [oe c| el... | ean celse ae oof nr Lj 14 | 15.0 || 0.05] 0.05) 0.05)......1 2.2. 00] cece est oc ct 2c loa eae 0.05 f 10; 205 8 le BM | ee 16.7 | 1966 fs...2 ee O02 120.8 al) 225} OB B22 tees Ae. colibeee sl a 22.4 | 42.5 | 0.05 Pingo Sur- | | 12 Gomes ASO.) PONG ., 10.0 | one ce) sede ist wcllh oe een peeameemen eae empera- ture 21.2°, 130 | 1526 | 0.05) O.0) © 0.02) secuc. oc. colocecaclene cl 0.15 44. 1 15.0} 0.05] 0.1) 0.1 |...0..|c205..1.....)0.2 H L} 15 | 13.0 : a ee ee ee ae The Distribution at the Thermocline. 417 The distribution of the nauplii at the thermocline is especially noteworthy. During the period of the observations there were frequently found enormous numbers of larval Copepods in the lower water. The numbers began to increase at ten or even eight meters, at a point several meters above the level at which the temperature began to fall, so that this distribution does not seem to depend on temperature. The number of nau- plii rose to a maximum rate of more than 300,000 per cubic meter in and above the thermocline, but ended with very great abruptness. This termination of the population often took place within the space of half a meter. The number of algae also declines very rapidly at the thermo- celine and those which are obtained below this level are dead or dying. The amount of algae thus obtained is, however, far greater than the number of crustacea; indeed the algae below the thermocline are many times more abundant in rela- tion to the number of crustacea present than is the case in lakes like those of the Oconomowoc system, in which there is a large crustacean population in the lower waters. It is obvious, there- fore, that the exclusion of the crustacea from these deeper waters is not due to the absence of food. The algae at times appear to accumulate above the thermo- cline, and to pass it, as they settle, only after considerable delay. I have attempted to discover whether this delay was due to the greater density of the water, occasioned by the dim- inution in temperature. A large glass tube, six centimeters in internal diameter and about two meters long, was filled with water and the lower half meter placed in a vessel of ice-water. After a few hours a very marked thermocline was formed, the temperature falling some 6° C. in the space of about 10 cm. Water containing algae, chiefly diatoms, was introduced at the top of the tube and the algae gradually sank through the water. On reaching the artificial thermocline they paused for a few minutes, but rapidly acquired the temperature of the water, as would be expected, and then sank to the bottom of the vessel. The delay at the thermocline could not have amounted to more five minutes for an individual alga. It seems probable from. 27 ; 418 Birge—The Crustacea of the Plankton. these experiments that temperature does not cause the accum- mulation of algae often found above the thermocline. Their death and consequent rapid sinking in the deeper water account for their small numbers below the thermocline. In this region Cyclops is the least sensitive of the limnetic crustacea to the influences which exclude them from the lower water. Chydorus is close to it in this respect when present in large numbers. A larger proportion of these species than of any others is found in the water immediately above the ther- mocline, and of the few crustacea which are found below that level by far the greater portion is composed of these genera. When Chydorus is extremely abundant more individuals of this species than of any other may be found below the thermocline. At one time nearly 70 individuals were taken by the net between eleven meters and eighteen, more than four times as many as all the other crustacea together. An examination showed that all, or nearly all of these individuals were in the process of moulting and had apparently become in some way entangled in the shell, so that their presence in this deeper water was an evidence of injury or weakness. The crustacea below the thermocline are, however, not dead or dying when brougnt to the surface. | The larvae of Corethra are found in considerable numbers be- low the thermocline and seem to be the only limnetic animal which normally inhabits these waters. Not infrequently the numbers of Corethra are far greater than the total number of the crustacea obtained. Indeed this is regularly the case when Vore-. thra is present in any considerable numbers. Since Corethra can carry a stock of air in its breathing tubes it is easy to understand the possibility of its living in the water below the thermocline. It is less easy to see why it should go there unless it retains in lake Mendota the habits which it has in the far more numerous lakes whose lower waters are habitable by crustacea. Factors Determining Vertical Distribution. 419 FACTORS DETERMINING VERTICAL DISTRIBUTION, The following factors contribute to determine the vertical distribution of the limnetic crustacea, 1. Food. 2. Temperature. 3. Condition of the water in respect to dissolved oxygen and other substances. Light. Wind. Gravity. . The age of the members of any given species. Orr OT Specific peculiarities. Food. Food influences the distribution of the crustacea both by its amount and its quality. As a general proposition, the crus- tacea should be most numerous where food is most abundant and least numerous where food is least plentiful. Since, therefore, the reproduction of the limnetic algae goes on most rapidly in the upper strata of the lake, it is natural that the crustacea which feed upon these algae should also be most numerous there. Yet this simple relation of food and eater does not at all cover the facts of vertical distribution. The amount of the algae in lake Mendota is in general so great in proportion to the num- ber of crustacea that the quantity of food is rarely the pre- dominant factor in vertical distribution. In early spring the erustacea, and especially Cyclops, increase more rapidly than does the food. But after the opening of summer the food appears to be almost always in excess of the crustacea, and their distribution, therefore, does not follow variations in its distribution. i For example, it is well known that the limnetic algae appear in what may be called successive waves of development. A sin- gle species rises to a maximum, predominates for a short time, then declines and nearly disappears, and its place is taken by another species. During the period of decline, especially in the case of diatoms, there is a time when the algae are sinking and 420 Birge—The Crustacea of the Plankton. when they are more abundant in the deeper strata of the water than near the surface. At such times the crustacea do not fol- low the food downward, but retain their normal summer distri- bution. Again, in the autumn there is a period, beginning a little before the first of October and extending to the freezing of the lake, when the algae are present in immense quantities, and are distributed with approximate equality through the whole mass of the water. Yet the crustacea are not by any means as uniform in their distribution, and at times some species are as closely aggregated near the surface as in summer. Their position depends on age and other factors rather than on food. The position of Daphnia pulicaria, also, cannot be determined by the food. It may be added that the crustacea in the deeper strata of the water are usually less numerous in comparison to the food present than they are in the upper strata. On the whole, while the quantity of food accounts for many of the larger facts of vertical distribution, it leaves wholly un- explained most of the details of the distribution of all of the species. It entirely fails to account for the position of Daph- nia pulicarta, or for the absence of crustacea from the deeper water in summer. The quality of the food at different depths is of some importance in the distribution of the crustacea. Anabaena, Aphanizomenon, and allied genera of algae are found in larger numbers in the upper strata of the water, while the diatoms, with their siliceous shells, tend to be more evenly distributed and never accumulate | at the surface. Anabaena and allied forms, also, being small in size and devoid of skeleton, are more readily eaten by the young crustacea than the diatoms, while the diatoms in turn can be very readily eaten by the older and larger crustacea. There is, therefore, a tendency for the young of nearly all species of limnetic crustacea to seek the algae in the surface strata of the lake, and the difference in the distribution of the algae is no doubt one of the factors which keep so high a percentage of the young near the surface. The fact that the crustacea in the 0-3 m. level do not rise above a certain number (p. 387) shows that food is not the only J Se a Sie gene oe ee 7 a Sa a, pis Factors Determining Vertical Distribution. 421 regulating factor, since the amount of food in that level in au- tumn is more than sufficient to support the total crustacean population. Temperature. Temperature may be considered under three heads: (1) the rise and fall of the average temperature of the water from spring to late autumn, (2) the diurnal variation of temperature, (3) the vertical distribution of temperature. I have not been able to discover that the warming or cooling of the water in spring or fall affects directly the vertical dis- tribution of any species except Daphnia pulicaria. The move- ments of this species are undoubtedly determined by the rise or fall of the general temperature of the water. It is a sub-ther- moclinal species in plankton-poor lakes and in summer it keeps as near as possible to the cool water in lake Mendota. The diurnal variation of temperature has no noticeable direct effect on vertical distribution. The most striking fact in the vertical distribution of temper- ature is the formation in the lake during summer of the thermo- cline which forms the lower limit of the crustacea from July on. The crustacea follow accurately the position of the thermo- cline. This layer has a vertical-oscillation of two or even three meters, being affected by the direction of the wind. In every case the lower limit of the crustacea oscillates with the posi- tion of the thermocline and follows it downward as it gradually descends during the summer. ‘The statement made in my former paper (Birge, ’95, p. 481) that “during July, only the upper twelve meters are tenanted by crustacea, and over ninety per cent. are in the upper nine meters” should be modified so as to read, that ninety-five per cent. or more of the crustacea are found above the thermocline, which in July is situated from nine to twelve meters below the sur- face. Yet, close as is this correspondence between crustacea and thermocline, the temperature is not the fact which limits their downward extension. This will be shown under the next head. | I have no doubt, however, that the thermocline is always an 422 Birge—The Crustacea of the Plankton. important factor in determining the position of the crustacea. Diaphanosoma is pre-eminently a summer form and flourishes only when the temperature of the water is at or above 20° C. It would hardly extend its range into the cold bottom water. In Pine lake and Oconomowoc lake, in both of which many crustacea extend freely through the thermocline, Diaphanosoma is confined to the region above it. Marsh states that Hpischura occupies the same position in Green lake, in which lake also most of the crustacea extend far below the thermocline. In all small iakes whose deeper water is habitable it will probably be found that the limnetic crustacea (and the rotifers also) can be divided into three sets: 1. Those permanently above the thermocline, including Dz- aphanosoma, Epischura (Marsh, ’97, p. 195), and probably some forms of Daphnia hyalina and Ceriodaphnia. 2. Those below the thermocline, including D. pulicaria and longiremis and Limnocalanus (Marsh, ’97, p. 201). 3. Those which are found on both sides of the thermocline, in- cluding Diaptomus, Cyclops, and others. These forms are named on small evidence in most cases, and the list must be regarded as suggestive only. The thermocline and the upper meter or two are certainly the two important strata in vertical distribu- tion. Above the thermocline there are no differences in temperature which could determine the distribution of the crustacea. There is rarely a difference exceeding two degrees between the top of the thermocline and the surface of the lake, and the variations in the vertical distribution of the crustacea above this layer must depend on other causes than temperature. After the first of October, lake Mendota is nearly homother- mous. Differences exceeding one degree are rarely found, and only in the warmer parts of bright and calm days. This condition is assumed while the temperature is fairly high —16° to 18°—and so early in the autumn that the development of the crustacea goes on actively for a month or more. During this period, therefore, other factors than temperature or food must determine the vertical distribution. Uniformity of distribution, however, is not attained until the decline in numbers of the Gf eg OG +s Factors Determining Vertical Distribution. 423 several species of crustacea. So long as the crustacea are mul- tiplying, the higher strata may contain as high a percentage as they do in summer. (Cf. p. 398.) One indirect effect of temperature should be noticed. A higher temperature increases the sensitiveness of the limnetic erustacea to light, and thus aids in driving from the upper strata those species which are negatively affected by light, es pecially Daphnia hyalina. Chemical relations of the water. The abrupt Jimitation of the downward extension of the crustacea in lake Mendota by the thermocline is not due to the change in temperature. This is shown by the fact that in lakes which are poor in plankton the crustacea extend far below the thermocline and in many cases the colder water is the more densely populated part of the lake. The crustacea are excluded from the lower water by the accumulation in it of products of the decomposition of the plankton plants and animals. Thes accumulate in the stagnant water below the thermocline and their decomposition finally, and in lake Mendota rapidly, fills the water with decomposition products and exhausts the oxygen. The State Board of Health of Massachusetts in 1889 and 1890 made elaborate examinations of the condition of the deeper water of numerous ponds in that state. It was found (Drown, ’90, p. 554) that in the deep water there was “an accumulation of in- termediate products of decomposition of nitrogenous organic matter, the hydrogen compounds of carbon, sulphur, phosphorus, and nitrogen, which, owing to the exhaustion of the supply of free oxygen, cannot be further oxidized.” It was found also that “in foul water of this character the varieties of animal and vegetable life which we find in water nearer the surface are almost, if not altogether, absent.” In 1891 investigations were made of the amount of oxygen in the bottom water, showing (Drown, ’91, p. 373) a rapid decline in the dissolved oxygen below the thermocline and its total disappearance from the bottom water of the ponds. It is not possible to state positively whether it is the absence of the oxygen or the presence of the decomposition products which excludes the crustacea from the 424 Birge—The Crustacea of the Plankton. lower water, in the absence of more exact investigations on the subject. In lake Mendota the lower water is always clear, but the whole region below the thermocline rapidly becomes unfit to support life, so that the life in the lower waters ceases very shortly after the formation of the thermocline. In lakes with a smaller amount of plankton the bottom water may become unfit to support life in late summer, although the plants and animals extend far below the thermocline. In Pine lake on September 5, 1896, Cyclops was by far the most abundant crustacean in the cold water, and numbered 21,000 per cubic meter between 12 and 15 meters, and 3,000 between 15 and18m. It was practically wholly absent between 18 and 24m., only 8 individuals being taken by the net within that distance, and no other forms of crustacea were taken. In Okau- chee lake the crustacea are numerous to a depth of 24m. in Sep- tember, but between 24 and 27.5 m. they were very few. In lake Geneva, Wisconsin, the crustacea in September extend to the bottom at a depth of more than 42 meters. This lake is ex- tremely poor in plankton. The statistics given by Marsh for Cyclops and Diaptomus (’97, p. 191, 204) may indicate a partial exclusion of the crustacea from the lower water of Green lake in late summer and autumn. While the plants and animals of the upper water are excluded by this means from the lower part of the lake, animal life is by no means entirely wanting. Worms are found in the mud at the bottom, as also is Cyclas, in considerable numbers. There must, therefore, be oxygen enough in the water to support some life. Cyclops and Chydorus are the least sensitive of the limnetic crustacea to these injurious influences. As shown by the tables on page 416, they always predominate in the lower strata of the inhabited water and form almost the entire population of the water below the thermocline. It is possible that the exhaustion of the oxygen from the lower strata of the water is the cause of the death of Cyclops and Daphnia hyalina at the bottom in spring and early summer. I have, however, no positive evidence on this point and in the Factors Determining Vertical Distribution. 425. case of the latter species a great majority of the old animals are so affected by various diseases as to need no other explan- ation of their death. Undoubtedly the condition of the water in summer causes the rise of the survivors of the spring broods of D. pulicaria from the bottom to the region of the thermocline. Tight. In lake Mendota the direct effect of light is confined to the upper meter or two, within which distance it has a powerful influence in determining the position of the crustacea. Laboratory study shows that the relation of the crustacea to. light differs in different species. DapAnia in all of the limnetic species has a strongly negative movement. Diaptomus, Dia- phanosoma, and Chydorus are strongly positive while Cyclops is, on the whole, positive, but is not very strongly affected either way. Yet the vertical distribution of these species is not very different when studied in the lake by three-meter inter- vals. Compare Fig. 30, and the percentage tables on p. 392 Diaptomus and Daphnia show an especially close correspond- ence in spite of their opposite relation to light. These species, placed in a glass vessel near a window, will segregate, Diapto- mus collecting near the surface and toward the light, while Daphnia goes to the bottom and to the side furthest from the light. This movement away from the light is not shared by every Daphnia present; some may move toward the light, usually not more than one per cent. of the adult or half-grown individuals. Young Daphnias, especially the newly hatched, are attracted by the light. The adult individuals of Diaptomus are found in a higher level of the lake than those of Daphnia. The young crustacea have a monopoly of the upper half-meter, or thereabouts, during the day. It is easy to see the advant- age of this arrangement to the species. In the upper meter, plant-life is most abundant, and is represented chiefly by small forms like Anabaena which are especially adapted as food to the small crustacea. On the other ‘hand, the adult crustacea. find an abundance of food suited to their size and masticatory 426 Birge—The Crustacea of the Plankton. organs, in the diatoms, which are more uniformly distributed in the water. The young, therefore, are freed in part during the daytime, by the action of light, from the competition of most of the older forms of the same species for the food which is especially adapted to the young. On August 26th, 1895, there was an alternation of cloud and sun, which made the day especially favorable for the study of the relation of light and the vertical distribution of Daphnia. It was found by numerous observations that the adult and half- grown Daphnias were approximately one meter below the sur- face during the sunny periods, but rose to about one-half meter during the cloudy intervals. The rise immediately followed the obscuring of the sun and the return was as prompt when the sun again shone. It was as though the Daphnias were depressed by a force against which they were contending, and they rose when the sun disappeared with the promptness of a compressed spring when relieved of weight. In laboratory experiments Diaptomus and young Daphnias move quite to the light end of the box in which they are placed. If sunlight is reflected by a mirror, they still move toward it and find no light too strong which can thus be sent to them. It would seem, however, that the direct sunlight of the open lake is too strong for them, or they would be present in larger numbers in the upper centimeters of the lake. If the warmth of the water repelled them we should expect this stratum to be tenanted as the lake cools in the fall, and should also expect that the young crustacea would gradually withdraw during the day as the surface warms. Neither in autumn nor in early morn- ing, however, do we find the crustacea close to the surface. The withdrawal from the upper quarter meter or so continues at least until the first of November, and the crustacea descend from the surface very promptly after sunrise. As already stated, the old nauplii are the only crustacea which I have found in large numbers immediately at the surface on calm, bright days. A high temperature, however, increases the neg- ative action of light and a low temperature lessens or reverses it. In early winter when the ice is transparent, D. pulicaria and D. hyalina may often be seen in large numbers immediately Factors Determining Vertical Distribution. 427 below the ice. This is especially noticeable in the case of the former species. The position of D. pulicaria must be controlled by tempera- ture. I have never been able to detect any noteworthy differ- ence between Daphnia pulicaria and Daphnia hyalina in their relation to light, by means of laboratory experiments. Nor have I as yet been able to find any difference in sensitiveness to light between Daphnias brought from a depth of three meters and those from a depth of twelve or more meters. The conclusion is, therefore, that in the upper meter and per- haps within a range not exceeding two meters from the surface, light is an extremely important factor in determining the vertical position of the crustacea. Below this depth, however, there are no effects which can be definitely ascribed to light. Iam not at all inclined to deny that, in lakes whose water is more transparent than that of Mendota, light may influence the crus- tacea to a greater depth. During the summer the water of lake Mendota is always turbid with vegetation, which cuts off the light very rapidly. My brass-topped dredge can rarely be seen to a depth greater than two meters, and frequently disap- pears between one-half and one meter. Vegetation, also, is especially effective in cutting off the violet and blue rays, on which the action of the light chiefly depends. In lakes whose water transmits these rays more freely, light may be a far more important factor in controlling distribution. The diurnal movement of the crustacea, which is clearly present during summer within the narrow limits of the upper meter, is chiefly due to light. Wind or calm alter the condi- tions of movement but during summer can hardly be considered factors in causing it. Wind. On the whole, wind has only a small influence on the vertical distribution of the crustacea, although its effect varies greatly with the season and with the condition of the several species of erustacea. The action of the waves prevents the formation of the dense swarms of young crustacea which are apt to be near the surface during calm weather. These young crustacea seek the 428 Birge—The Crustacea of the Plankton. algae which on calm days accumulate near the surface. When the lake is rough the algae are distributed to a greater depth, and the crustacea follow them to some.extent; although, even when the wind blows with considerable force, the young crus- tacea still form the chief population of the upper meter of the water. Ihave not been able to discover any descent of the crustacea during windy weather, but, on the contrary, have always found the upper meter fully occupied by them even when the lake was so rough as to make it very difficult to go out with a row-boat. The wind may affect the vertical distribution, also, by creating” currents in the water. ‘These are either lateral or vertical; we are concerned only with the latter. During the summer the: vertical currents can penetrate no deeper into the water than the thermocline; that is, from six to fifteen meters, according to the time of year. These currents, however, seem to produce very little effect on the distribution of the crustacea— at any rate, at a distance of 850 m. from the shore, where my observa- tions have been made. In the next section it will be shown that crustacea must be able to move through a distance of at. least 100 meters vertically per day, and that the larger individ- uals move through four or five times that distance. There is, therefore, no difficulty in their maintaining any position in the: water they may choose to occupy, against the somewhat slow vertical currents produced by the wind. Indeed, the wind affects the vertical distribution of the limnetic algae much less. than would be expected. I have frequently collected after severe gales, and, in summer, have never failed to find the algae of the upper three meters far more numerous than those from. lower levels. I have never been able to detect vertical currents,. produced either by wind or sun, which were capable of dis- tributing the algae uniformly through the mass of water in summer, and of course the active crustacea are far more inde- pendent of these currents than are the algae. In the autumn the entire mass of water in the lakes is put into somewhat active circulation by the autumnal gales. The algae are at a maximum and are pretty uniformly distributed through the water. Neither the quantity nor the quality of the Factors Determining Vertical Distribution. 429 food, therefore, give any reason to the crustacea for moving to any particular level. The effect of light, also, is lessened by the declining temperature of the water. Hence the crustacea are far more apt to yield to the action of wind and gravity than they do in summer, and become more evenly distributed through all levels of the water. In the spring a similar distribution occurs immediately after the breaking up of the ice, when the lake is homothermous, and the crustacea and the algae have not yet started their spring development. Very soon, however, the surface strata contain much more food material than those below, and the young crus- tacea tend to remain near the surface until crowded down by the swarms of newly hatched forms. The lake, too, rapidly be- comes heterothermous and the circulation of the water in late April and early May is by no means as complete as it is during the long homothermous period of the autumn. A slight effect is also produced by the wind on the vertical distribution of the crustacea, since it causes the thermocline to oscillate through one or more meters, In general, it may be said that the on-shore wind tends to depress the thermocline, piling up the warm water on top of it; while the off-shore wind tends to raise it by stripping off the warm water of thesurface. This general law, however, is subject to many modifications owing to the irregularities in the outline of the lake and in the confor- mation of its bottom. Whatever effect however, the wind pro- duces on the thermocline it also exerts, of course, on the lower limit to which the crustacea extend. Gravity. The action of gravity has more influence on the position of erustacea than I had supposed on beginning this investigation. Its effects are most plainly seen in Daphnia, and least in Diap- tomus. Gravity does not act as an accelerating force upon the movements of the crustacea, and yet their ordinary movements are adjusted with some reference toit. If Daphnias are watched in an aquarium, it will be seen that they usually remain at about the same level, permitting themselves to sink and then with a few 430 Birge—The Crustacea of the Plankton. strokes of the antenne resuming their former position. In this way they pass up and down through the water utilizing the material available for food. After a time the animal mayswim off to a new place, but soon begins to repeat these alternate movements. The movements of Diaptomus are far less regular,. yet it, too, keeps at about the same level, unless some at- traction causes it to move up or down. Cyclops, which hunts for food of all sorts, and is decidedly a more predacious ani- mal than either of the first two named, is far less regular in its movements, and Leptodora, as a true carnivore, Swims ac- tively in all directions. The amount of energy required of the crustacea in order to maintain their position in the water is not inconsiderable, and is doubtless the main muscular labor demanded of them. They are all of them heavier than water, and sink at arather rapid rate, which very quickly becomes uniform. The full-grown Daphnia, 3to 4 millimeters long, sinks at the rate of 20-30 centimeters per minute even with expanded antenne. Small, newly-hatched individuals, one millimeter or less in length, have arate less than one-third as great, from 5 to 10 centimeters per minute, The specimens experimented upon almost always fell edgewise through the water, with the head down, if the an- tenne were folded, and with the head up, if the antennz were expanded. Diaptomus sinks at about the rate of about 12 cm. per minute, and medium-sized adult Cyclops without eggs at a rate of 9.5 cm. per minute. Live Daphnias sink at the same rate as those freshly poisoned, as far as the eye can determine. This is easily determined in the case of half-grown and adult individuals, but young speci- mens are so active that it is hard to be accurate. At the rate given, an adult Daphnia would sink through as many as 250-400 meters in a day, and must, therefore, maintain itself against the force which would cause it to fall through this distance. Of course the weight to be lifted is very small, being the excess of the weight of the animal over that of an equal bulk of water. It seems im- possible that the animal should ever sleep. As the creatures be- come older and larger the exertion becomes greater than in the case of young individuals, and the older and, especially, the a ee Factors Determining Vertical Distribution. 431 feebler animals, tend gradually to sink and accumulate in the deeper waters of the lake. Such aggregations of Cyclops are often found at the bottom of the lake in winter. In March, 1895, for example, from fifty to seventy per cent. of this species were in the lower three meters. Daphnia hyalina shows a similar downward movement in late May and early June on the part of those individuals which have lived over winter. In late autumn, also, the adult. members of this species are far more numerous in the lower strata than they are at higher levels. Since, at this time, there is a superabundance of food at all depth of the water, and, since the crustacea are relatively few in number, this distribu- tion can hardly be due to any other cause than gravity. (See p. 398.) Diaptomus and Diaphanosoma with their very powerful swim ming organs, rarely show this tendency to sink. Perhaps the large amount of fat usually present in Diaptomus also aids in preventing sinking. Age. It is a general rule that the young individuals of a species appear near the surface, When the crustacea begin to multiply in the spring, the increase appears first in the 0—3-meter level. All very exceptionally large numbers of any species obtained during the summer have been caught in the upper three meters, and usually consisted of young and half-grown animals. Nosim- ilar aggregations have been found in the deeper water, except as noted for Cyclops in the last section. When a species is declining in numbers, the distribution is more uniform, and as the decline goes on, the lower levels may contain a larger number than the upper. If the crustacea obeyed this law with mathematical accuracy, there would be a sort of progress of the members of a brood from the top to the bottom of the lake, the successive broods of the young contin- ually displacing the older in the upper strata. Good illustrations of the distribution of the young and adult individuals can be obtained from the fall broods of Daphnia hyalina, as stated on page 398 432 Birge—The Crustacea of the Plankton. The nauplii of the Copepods seem to form an exception to this rule of age. During the period when the thermocline is present, the maximum numbers of nauplii usually occur in the neighborhood of this layer, although not confined to it. In Pine lake, also, the thermocline and the level immediately below it contained more than sixty per cent. of the nauplii present. In Mendota they cannot go below the thermocline, but they congregate in and above it as shown in Fig. 33. The young Cyclops and Diaptomus, however, congregate near the surface by day, yet are by no means so closely confined to the surface as is the case with Daphnia. In autumn and winter the nauplii are pretty uniformly distributed. The causes of this distribution by age are to be found in the different relations of old and young to light, food, and gravity. Light and food are probably the most important factors. Cer- tainly it is true that Cyclops, which, of all the limnetic crus- tacea, is least affected by light and most omnivorous in diet, never shows as complete a separation of old and young as do the other genera. Yet even in this case there are more egg- bearing females, in proportion to the total number, in the deeper strata than near the surface. This is possibly due to gravity, which would have a greater effect on females laden with eggs. Specific peculiarities. It must be remembered that these various factors affect highly organized animals, which therefore do not respond with the mechanical uniformity of bacteria or of swarm-spores. Yet, in looking over my lists for catches which would illustrate ex- ceptions to the principles given and to the averages of the tables, I have had difficulty in finding them. A few exceptional catches of Diaptomus occurred in all summers, where the 6—9 m. level in perhaps half a dozen cases contained more than the 0-3 m. But even such cases are very rare and in general the several species of crustacea follow their law of distribution with the range of variation already noted. It is in the nature of the response of the species to these factors that the specific differences usually appear, rather than in aber- rations from the general law. It has been very interesting to Bibliography. 433 see how these specific differences regularly presented themselves in my averages in spite of great variations in absolute numbers. Hiven those so small that they were at first supposed to be merely accidental recurred with great uniformity. In conclusion I would repeat what I said in my introduction, that this discussion of general causes is to be regarded as sug- gestive. I shall be quite satisfied if it indicates lines of invest- igation to students of the fresh water plankton. LITERATURE TO WHICH REFERENCE HAS BEEN MADE. Apstein, 96. Das Sitisswasserplankton. Methode und Resul- tate der quantitativen Untersuchungen. Carl Apstein. Kiel, 1896. Bitte, 9d: Plankton Studies on Lake Mendota. Il. EK. A. Birge. Trans. Wis. Acad. Sci., Arts and Letters. Vol. X., pp. 421-484. Birge, 97. The Vertical Distribution of the Limnetic Crustacea of Lake Mendota. E. A. Birge. Biol. Centralblatt, Vol. XVII., pp. 371-374. 1897. Drown, 90. Interpretation of the Chemical Analysis of Water. T. M. Drown, Ph. D. Mass. State Board of Health, 22d Report, pp. 533-578. 1890. Drown, 791. Dissolved Oxygen in Waters of Ponds and Reser- voirs at Different Depths. T. M. Drown, Ph. D. Mass. State Board of Health. 23d Report, pp. 353-373. 1891. EKigenmann, 95. Turkey Lake as a Unit of Environment and the. Variation of its Inhabitants. C. H. Higenmann. Proc. Indiana Acad. Sci., Vol. V., pp. 204-296. 1895. FitzGerald, ’95. The Temperature of Lakes. Desmond Fitz- Gerald. Trans. Am. Soc. Civil Eng. Vol. XXXIV., pp. 67- 114. 1895. | Francé, ’94. Zur Biologie des Planktons. R. H. Francé. Bio- log. Centralblatt, Vol. XIV., pp. 33-38. 1894. Frié and Vavra, 94. Die Thierwelt des Unterpocernitzer und Gatterschlager Teiches. Dr. Ant. Fri¢ und Dr. V. Vavra. Unters. ii. d. Fauna der Gewiis. Bdhmens, IV. 1894. 28 434 Birge—The Crustacea of the Plankton. Hensen, ’87. Ueber die Bestimmung des Planktons. Dr. V. Hensen. Fiinfter Ber. der Kom. zur Wiss. Unters. der Deutschen Meere. Kiel. 1887. Hensen, 795. Methodik der Untersuchungen bei der Plankton- Expedition. Dr. V. Hensen. Kiel u. Leipzig. 1895. Kofoid, ’97. Plankton Studies. I. Methods and Apparatus. C. A. Kofoid, Ph. D. Bull. Ill. State. Lab. Nat. Hist., Vol. V.) pp: 1-25) L8o7. Marsh, ’97. On the Limnetic Crustacea of Green Lake. OC. Dwight Marsh. Trans. Wis. Acad. Sci., Arts and Lett. Vol. XI., pp. 179-224. 1897. Reighard, ’94. A Biological Examination of Lake St. Clair. J. E. Reighard. Bull. Mich. Fish Commission, No. 4. 1894. Richter, ’91. Temperturverhiltnisse der Alpenseen. H, Richter. ‘Verh. d. 9ten Deutsch. Geographentages zu Wien. 1891. Wesenberg—Lund, ’96. Biologiske Undersoegelser over Fersk- vandsorganismer. C. Wesenberg-Lund, Vid. med. natur. For. pp. 105-168. Kjébenhavn, 1896. Whipple, 95. Some Observations on the Temperature of Surface Waters and the Effect of Temperatures on the Growth of Micro-Organisms. G.C. Whipple. Journal N. E. Water Works Ass’n., Vol. IX., pp. 202-222. 1895. Zacharias, ’96. Forschungsberichte aus der Biologischen Station zu Ploen. Theil 4. Dr. O. Zacharias. 1896. ee Se eee = Diagrams—Errata. 435 LIST OF DIAGRAMS. Plate. Page. Fig. 1. Temperature, surface and bottom, 1895........ XV 286 Fig. 2. Temperature, surface and bottom, 1896........ xvi 286 Fig. 3. Summer temperatures, 1895...... SB ee yin fans XVii 296 Pie.) 4, Summer temperatures, 1896 ........2.. ..00 eee XViii 296 mis.) o. Temperatures, August, 1896%......0.0 000 boc eee xix 296 Pies os, Cotal crustacea, 1894-1896... ek le eee xx 302 rei) Wueadine= crustacea, 1804........605 6.0. cee e wees Xxiii 312 Peo eadine crustacea, 1890... 0.6.65 eee eee ee mk 308 mismo Mmeadime erustacea, 1896... 6... e bk eek ee os Xxii 308 Fig. 10. Total crustacea, 1894-6, deducting Chydorus... xxiii alZ Figs. 11-13. Crustacea, Sept. 16-30, 1894, 1895, 1896 .... xxiv 316 Pega Oiaptomus, 1894-1896)... eww ees we cee ce XXV 320 Fig. 15. Cyclops, 1894-1896 ..... Dect staal ren Sean teeta xxi 328 Fig. 16. Daphnia hyalina, 1894-1896 .................. XXVii 336 Pies 47. Daphnia pulicaria, 1894—-1896.................- Xxili 341 Fig. 18. Daphnia retrocurva, 1894-1896................. XxXix 344 Bie os), Oraphanosoma, 1894-1896 .... 0.0... 0.6.0 205 eee XxXix 344 Peo Omydorus, 1894-1896. oe kk ek cae ce eee ee TO 348 Bien) Cyclops, single catches, 1895. ..5..0.....0000000 XXxi 370 Fig. 22. Vertical distribution, by 3 m. intervals, 1895.... xxxii 378 Fig. 23. Vertical distribution, by 3 m. intervals, 1896.... xxxiii 378 Fig. 24. Distribution, 0-3 m., 3-9 m., 9-18 m., 1895..... XXXIV 380 Fig. 25. Distribution, 0-3 m., 3-9 m., 9-18 m., 1896..... XXXV 380 Fig. 26. Percentile vertical distribution, 1895............ XXXvi 384 Fig. 27. Percentile vertical distribution, 1896........... XXXVli 384 Fig. 28. Percentile vertical distribution, March, 1895, Meas October, IS9G 6 bcc Poe's es we a evista XXXVIii 390 hie2).) summer distribution, 1896. .......2.0. 0 eese eee RX Xie 394 Fig. 30. Cyclops and Daphnia pulicaria................ xl 400 Fig. 31. Night and day distribution, Sept. 13........... xl 400 Fig. 32. Percentile vertical distribution, D. pulicaria.... xli 400 Fig. 33. Distribution of crustacea, etc., Sept. 8........ xii 412 ERRATA. Page 289, line 18, for April 28th, read April 2nd. Page 289, line 21, for Dec. 29th, read Dec. 19th. Page 400, line 2 from bottom, for Fig. 31, read Fig. 32. . Page 412, line 2 from bottom, for Fig. 32, read Fig. 31. Also in Table XXXVIII, I. Page 425, line 17, for Fig. 30, read Fig. 29. In Fig. 13, for D. pulicaria 34, read D. pulicaria 3.4. Birge—The Crustacea of the Plankton. 436 PPT ger «FG (8G ‘TZ "GT ‘et “TT ‘6 “T “OE ‘F2°22 ‘6T ‘LT oh ERTL 6.9-6 75 "82 ‘12 ‘92 ‘#2 ‘22 ‘04 ‘OT ‘aT ‘OL 9 oP S¢ “8 i T "82 “G2 “PS ‘22 ‘0% ‘SI “LT “PE ‘Zt ‘TT ‘Or ‘6 ‘8 ‘% "TE “62 ‘L2 “92 ‘Ve ‘6T ‘LT “SE ‘OL ‘8 ‘9 ‘G FZ 7 “te "en ‘he ‘82 FZ ‘TZ ‘02 ‘RE ‘LT ‘OF OL IE 16 i 7a "6G ‘LG ‘9% “FS “C2 “6T ‘LT “GT '€E ‘OF ‘9 ‘9 ‘¢ “T "62 ‘92 ‘Pe ‘TZ ‘0% ‘ST ‘LT "GT “IE ‘6 ‘9 ‘2 0 *L2 ‘22 ‘02 ‘ST WL TL 'P 8Z ‘IZ ‘OL ai FL ‘SF 08 ‘OT ‘OF PEE = ri Seniesa SSien) SS ce OOOO re 291616 OOeoidiadsr=ret= OO Ce fOr 191s HOD Hrs THN OD St ‘suo | , -earos | SAP "soqeq ao op OO ON “96ST L8T Seen OS ee 1G ‘SI P core cose eens al I 6 I 7 ee ee 02 ST g Se ee eb Obese g ae - OGE0G &@ ‘08 LT g Tt FL TI 8 a, ie) G = OG he eG “CG ‘ST oT 9 ee pera ON 6‘L ‘FS G ee 62 Le &@ Te GT LT 8 “SE 7) 2b. 6 6-5 °C 8 "TT" "8 “FB °8s “OL WA G Bee eM ST ‘TE “OF 6 ‘9 g ie & 1 ST "66 LZ VG Ge 02 ST LT tT __.,.. PE at O19 € T ol ""08 L662 0G 8T OT Lie ee OE L‘t 6 ee Ea GC Ob &T Cf SEE an PLS Tg ZL G eee OL 91 8 SAE ee eR iat L 9 IL SSIES OS eS Saat gt 61 CT OT CO RNa ae a ee 3) g eee ee 91 ge Scie sree 5 sense 9 Z I val *suoly -BAIOS *soveq *S68T Ort “""*Te-9F 109 Ba oS | "00d “"*** 08-91 “AON noon GT-T “AON sees TOOT "490 reves @Toy "190 "71" Qg-9T '3d0g eeece CI-1 *qde9 re98* Te-oF ‘sny I 6 ‘sny “7 [p-97 AT OE [ ict or-7 Ame ong noun Seng rin th Te-o7 KOT SENT Cy | ACW "tr Tg-oy cady era Cy a 8 ‘Idy "1° 5*8-9T "TOT" coucig ey alge oT 8-bT QOq pee eae este a sie eeeeve GI-I ‘uel? Hest rs 0 SON SH HCO KO DOM SED 8 23 aD aD CY) SH OD ‘sep “qo "on | 3° ON 60T oF ‘61 Z T "*"** Te-9T ‘00 oy eal att OL g "ce" GT-T '00q nove taaees|oretaces|+es9 sq 97 “AON “PL ‘9 ‘9 ‘Go Li G G es 6) "AON "ITS ‘08 ‘1 Banat ; “82 ‘22 ‘02 “LE “OT | TE 8 "\* T8-9T (990 eT ‘TT ‘6 ‘9 L i ""**" GT-T °990 "62 “LG eeee ¢ 2G Ie 0¢= GE Sh a L 08-91 ‘deg “'"QT-T deg "2 ‘8 | £ g | 11°" 1s-9F “Say A ee po ST-T “Bny ‘66 ‘8G e@eaeee “12 ‘6I ‘SE ‘LT ‘or | 8e L [171 18-97 Atop ea a) a i qI-T 4qoe ‘suo |. -earos | “S4®P ‘soqeq EMSS | 70 'ON *v68L ‘QPDUL aLaM §U012097109 Yorym UO saz0qg—'V ZIAVE— XIGNAdd V Se See Appendix—Statistical Tables. 437 TaBLE B.— Average number of crustacea per cubic meter in each three meter level, 1895, 1896. See Figs. 22, 23. 1895. 1896. Depth . PU RAN OM NT Ter HINA re, inthe Mae 0-3 | 3-6} 6-9 | 9-12 /12-15 |15-18 0-3} 3-6 | 6-9 | 9-12 |12-15 |15-18 Jan. 1-15... 41.7 | 20.6 | 17.4) 8.4 | 11.9 | 18. 6.9') 0.0) 4.7 |. 5.2) 2.6 | 2.6 ae Jan. 16-31... 23.8} 9.2} 9.38] 5.6] 5.5 | 11.8 Feb. 1-14.... 29.9 | 11.5) 8.38) 9.1] 8.9} 12.9 5.2) 8.7} 6.6) 5.3 | 14.1 | 14.1 Feb. 15-28 .. 20.5} 10.8) 7.7} 7.8] 7.8] 9.1 March1-15. ..| 5.7] 8.4] 6.1] 5.0] 3.7] 10.8 March 16-31 ..| 12.4 | 11.7 | 5.2} 5.2 | 10.1 | 10.1 || 23.9 | 12.3 | 18.3 | 11.5 | 8.0] 5.7 April 1-15 .....| 10.3} 5.8] 3.7) 8.1] 4.1 | 4.2 || 26.5 | 19.6 | 33.9 | 35.5 | 21.2 | 21.0 April 16-30....) 36.1 | 21.7 | 12.6 | 7.6] 6.1] 9.0 |/159.2 | 97.1 | 58.5 | 43.5 | 22.1 | 11.4 May 1-15...... 134.0 | 76.8 | 41.1 | 22.4 | 14.1 | 20.7 |/254.4 |179.6 |154.3 | 98.2 | 69.3") 62.2 May 16-31..... 188.0 | 90.7 | 64.4 | 38.4 | 88.8 | 47.1 |/197.8 [118.7 |117.2 | 94.4 | 81.1 | 99.6 June 1-15, ..../148.4 | 83.3 | 59.9 | 45.0 | 36.0 | 46.6 |/188.1 | 69.0 | 30.4 | 24.0 | June 16-30 ....} 84.4 | 41.3 | 30.3 | 18.3 | 9.2 | 17.5 |/252.2 |106.8 | 43.8 | 24.2 July 1-15. ..../142.1 | 61.4 | 46.0 | 14.2 | 4.2 | 2.2 |/2380.4 [185.2 | 56.8 | 25.1 / 20.9] 2.6 July 16-31 ..../119.0 | 75.8 | 67.8 | 10.9 Aa 14 1027.2) | 76.91 -49-5°! 9.71) 0.8 } Od Aus, 1-45. .... 101.1 | 61.8 | 34.7 | 27.5 | 4.1 | 0.5 [168.0 | 82.7 | 42.6] 11.2) 1.2) 0.2 Aug. 16-31 ....| 87.1 | 45.0 | 88.4 | 32.7] 3.5 | 0.6 |/119.9 |118.6 | 62.9 | 49.8) 6.9] 0.5 Sept. 1-15. ....; 89.4 | 60.4 | 44.8 | 21.4 | 6.5 1.7 {|150.6 |111.9 | 84.8 | 70.5 | 47.0 | 14.4 Sept. 16-30..../ 93.7 | 62.6 | 45.8 | 34.6 | 42.0 | 24.1 |/107.0 | 61.1 | 50.1 | 51.9 | 53.2 | 47.1 83.9 ||105.7 | 84.1 | 81.3 | 63.6 | 61.3 | 59.8 Oct. 16-31. ....| 41.6 | 23.9 | 23.8 | 25.4 | 20.8 | 21.1 |[192.0 | 66.9 | 51.2 | 41.9 442.5 | 43.6 Nov. 1-15.-....| 29.1 | 22.9} 17.1 | 21.2 | 17.5 Nov. 16-30....| 23.9 | 19.3 | 17.3 | 18.5 | 14.1 Dec. 1-15. ....| 33.2 | 28.2 | 17.0 | 10.6 | 8.5 Dec. 16-31..... 45.6 | 16.3 | 10.0 | 15.4 | 9.8 SS SS SSS 56.1 | 47.1 | 30.9 | 36.8 | 28.9 | 28.2 52.2 | 29.8 | 24.7 | 27.9 | 22.7 | 19.6 15.9 | 24.6 | 22 0 | 23.2 22.0 | 10.3 | 10.3 | 10.4 14.0 13.2 6.5 |{ 26.5 | 38.2 7.2 ee 27.5 438 Birge—The Crustacea of the Plankton. TaBLE C.—Average number and percentile vertical distribution of the crustaced. Av. No. 1894. July 1-15 .... 306.2 July 16-31.... 472.3 Aug. 1-15.... 401.1 Aug. 16-31. 382.2 POP Os AO ile ee wean Sept. 16-30 691.6 Oct aaa su.' 820.8 Oct. 16-31 Rotel Nov. 1-15..... 571.4 ney alge Lace Many Meat l a ec, 1-15. Dec. 16-31 a8 Jan 15 an. 1-15 5. Jan. 16-31. GEL eb. 1-14.. Feb. 15-28 ee Mch. 1-15 118.7 Mech. 16-31 164.5 Apl. 1-15. 94.3 Apl. 16-30. 229 .4 May 1-15..... 940.2 May 16-31 1,419.5 June 1-15. 1, 256.6 June 16-30.. 610.7 July 1-15.. 817.6 July 16-31.. 837.9 Aug. 1-15.. 689.1 Aug. 16-81.. 622.8 Sept. 1-15.. 669.7 Sept. 16-30... 928.1 Oct: 1-15... 767.8 Oct. 16-31.... 478.5 Novy. 1-15: ).. 391.5 Novy. 16-30.... 331.8 Dec. 1-15.... 320.8 Dec. 16-31... 313.1 1896. Jan. 1-15 294.1 Jan. 16-31 240.9 Feb. 1-14 219.0 Feb. 15-29 191.7 Meh dati eee Mch., 16-31 281.6 Apis) tala... 480.4 Apl. 16-30....| 1,184.3 May 1-15.... 2, 398 .2 PER CENT. IN EACH 3 M. LEVEL. 3-6. 6-9. 29.5 16.9 30.6 233 AF alee} 21.6 29.0 21.1 observations. 26.0 Thebes 20.8 16.2 19.5 176 18.6 15.8 observa|tions. 16.2!) 14.4 18.5 17.4 16.3 11.9 21.1 15.6 A ae: 9.5 18.4 i a Pa} 13.5 24.8 iS 3453 19.4 13.8 20.0 14.3 21.6 a LS es) 22.8 17.0 2t 4 24.4 26.9 15.0 Dag 18.4 26.9 20.0 DA Wr | Gaye b 15.1 14.6 ye, 152 18.8 14.0 18.2 16.3 oe lose 15:6 9.6 18.0 152 1404) 14.1 abs) 10.0 a brie 8 12.1 observa!tions. 15.4 DOT 12.4 21.5 24.6 14.9 21.9 18.9 9-12. jt ed ONO AAUP OT OOK O MMO jt — ie.) 19.2 a SOoOon~w © OO oO — a Jt ed ek ee pe ped pep bo bt HD AT ATO? C9 Ht CO OTD GY OT? O OD “1.00 CONT HWOF ANWENARDWDOOWOINMNW DeDH br ON ee Oe a eg ke ieee een | as see ase epost pa vgn peesio see es eSirae- meg nT nara eee 12-15. |15-18. ee pet et pk bo ARDHW GDOOrH i WIRED HME OC iw) oo © no e ® et et BARRO HHOTM RENWWOODRDINUNNONUDUE! G2 CO eat fet feet Pad fk eo SD NOMS] OWOBDWKHKWEWNHHHOHRODORO ei QO C100 ek ek fee ek [o_o olie oan) oooo @Moow OFF 12.4 et oo PADD AWW WWASDCSSSDODOOMLO ADH WOHD ONRTNMNOCODWNUMNDROOUID — =~] bo G2 00 Appendix—Statistical Tables. 439 TaBLE C.—Continued. ————SS——————e—e———————————————————————— PER CENT. IN EACH 3 M. LEVEL. 0-3. 0-6. 6-9, 9-12. | 12-15. |15-18. eS ee a < — ~ a ee ene May 16-31....| 1,901.3; 27.9 16.7 16.5 13.3 11.4) 14.1 June 1-15....) 844.8 53.0 19.4 8.5 6.7 3.6) 8.6 June 16-30....| 1,265.0 54 7 23.1 9.5 5.3 4.1) 3.2 July 1-15....| 1,314.2 48.9 28.7 12.1 53 4.4) 0.6 July 16-31....| 776.5 48.4 29.2 ihoiaal 3.7 0.3) 0.12 Aug. 1-15....| 960.4 54.2 27.4 14.2 3.7 0.4) 0.0 _Aug.16-31....{ 1,073.3 33.4 33.1 17.5 13.9 iS OE Sept. 1-15....| 1,440.9 31.3 23.3 BET 14.7 9.8} 3.0 Sept.16-30 ...| 1,112.3 28.6 16.2 13.4 13.9 15.3) 12.4 Oct. 1-15....| 1,368.4 23.0 18.4 17.8 14.0 13.4} 13.1 Oct. 16-31....| 1,314.8 43.9 15.2 a are 9.6 9.71 9.9 Nov. 1-15....| 684.8 24.6 20.7 13.5 foun 12.7, 12.4 INov.16-30....) 537.7 29.5 16.8 14.0 15.8 12.8} 11.1 Dec. 1-15....| 365.8 18.0 25.7 11.0 16.6 14,9} 15.8 Dec. 16-31....| 285.0 15.4 29.0 23.1 10.8 10.8} 11.0 TaBLE D.— Diaptomus. Average, maximum, and minimum numbers. Percentile vertical distribution. PER CENT. IN EACH 3 M. LEVEL. 0-3. | 3-6. | 6-9. | 9-12. |12-16./15-18. | | | | | LN, fF ES, | Se 1894. July 1-15 ..| 242.2) 290.6) 178.0)) 48.9) 31.6) 15.6) 3.1) 0.4] 0.4 July 16-31 ..| 298.9} 553.3] 155.8), 53.6) 31.2) 13.0) 2.1) 0.07; 0.06 Aug. 1-15 ..{ 218.7; 394.3; 126.5), 45.5) 26.8) 20.9) 6.5) 0.2; O.1 Aug. 16-31 ..| 87.4| 117.9) 43.8)) 49.7) 27.6) 17.4) 5.0; 0.2) O.1 Sree ee aes e Pat eee ebacill ia cle eel viewers [io eae slew nese) adie Cate wate Sept.16-30 ..| 54.6) 84.5} 10.8]; 58.1; 20.4, 12.2) 8.0; 1.0}; 0.3 Mere ata 2) 67.2) 92.8) 38.9) 38.5) 23.2! 13.1) 11.4) 10:1) 3.4 Oct. 16-30 ..| 38.3) 72.0; 3.6]| 25.4) 20.3) 17.2) 14.7) 15.9] 6.3 _ Nov. 1-15...) 44.0) 95.4) 26.0) 28.6) 17.6 16.1) 16.9) 13.0/ 7.8 eee se 167. eee 440 Birge—The Crustacea of the Plankton. TaBLE D.— Continued. PER CENT. IN EACH 3 M. LEVEL. 0-3. | 3-6. | 6-9. | 9-12. |12-15.|15-18. ———— | | | J SS ES | EE | Jan. 1-15..| 17.5] 28.9] 8.0 Jan, 16-31 ..| 15.9 22/9 13/3} 28-2| 23-1) 20.2) 17.4) 5.5 | 5.5 Feb. 1-14 Feb. 15.98 | 28.0} 47.7) 16.5)| 22.6) 28.1) 21.3) 12.0) 10.5 | 10.5 Meh. 1-15 2.1). 28.3) O5.6)) 23 Bic ccc lek oe ale aie Shall c ole at ene Mch. 16-31 ..| 34.7| 70.5} 27.1|| 22.7] 95.¢) 20.0] 14.7) 9.31 7.7 Apl. 1-15..| 14.0] 23.5] 10.8]| 28.1] 19.0] 13.5] 13.5] 12.9 | 12.9 Apl. 16-30 ..| 20.6] 52.7] 0.2/| 39.7/ 32.1) 13.3! 9.6 5.51] 6.8 May 1-15..| 34.4] 45.1] 17.9]/| 58.9] 22.6] 12.0) 3.3| 2.0] 1.9 May 16-31 ..| 207.9] 284.2} 49.6]| 61.5} 921.7/ 11.3] 3.6] 1.1] 0.8 June 1-15 ..| 285.0) 459.8] 178.1/| 57.3} 24.1) 9.6] 3.3] 2.6! 3.1 June 16-30 ..| 190.6] 396.9] 95.4/| 51.1] 24.9] 14.9) 5.7| 1.8] 2.1 July 1-15*..| 187.4] 397.5] 105.5|| 41.4) 30.1) 22.6 4.8) 0.8] 0.3 July 16-31 ..| 217.8] 366.3] 127.8]] 31.1] 24.9 36.5} 6.7] 0.5] 0.3 Aug. 1-15 ..| 110.5] 169.8] 61.7|| 47.2) 29.9] 13.8] 8.3] 0.5] 0.2 Aug. 16-31 ..| 101.3] 264.5) 45.9/| 45.3) 27.7; 19.2] 7.1] 0.5] 0.2 Sept. 1-15 ..| 224.6] 311.6! 69.3]/| 40.41 36.4] 18.41 3.8] 0.8] 0.3 Sept. 16-30 ..| 331.5] 586.3) 152.0// 40.3) 26.1] 15.8] 10.1] 5.41 2.3 Oct. 1-15 ..| 148.4! 323.1] 101.7|| 27.4 15 4] 15.6] 13.4] 17.2 | 11.8 Oct. 16-31...) 79.7) 115.1] 42.6]| 99.1) 17.7) 14.8) 17.1) 125 ee Nov. 1-15...) 55.8] 71.8! 49.6] 13.9] 14.7) 19.3! 20.5] 20)0)aeum Nov. 16-30..| 46.0] 54.1) 43.8/| 12.8 19.8; 16.6] 17.9] 17.5 | 15.3 Dec, 1-15..| 33.6] 47.1) 22.8|| 13.1) 20.0] 23.3) 20.8) 13¢0 aes Dec. 16-31 ..| 58.0} 67.4] 22.8/| 25.2} 21.4] 183.7] 20.8! 12.11 6.8 1896. Jan. 1-15..| 48.6] 62.9] 40.0)| 26.9] 24.0] 21.9 9.9 9:3) 89 Jan. 16-31...) 23.3} 34.3) 22.8] 25.4) 16.2} 20.7] 20.7) 14.0| 3.0 Feb. 1-14..| 38.9] 57.5] 27.3]| 28.0] 24.4] 11.9] 14.0'14.6] 7.0 Mab 15-29) 0) aa el a 33.01 16.3! 16.41 14:4) 10%e aaa Meh Paar OO st 24.11 16.0] 20.0] 18.3} 11.7 | 10.0 Mch. 16-30 ..| 33.3} 38.8} 26.7/| 19 0] 14.6] 23.4! 30.71 6.0! 6.3 Apl. 1-15..| 35.2] 43.4] 21.6)! 26.2} 24.0) 21.9] 9.9] 9.8] 8.7 Apl. 16-30..| 29.9] 66.7| 9.5// 16.3] 28.3] 19.6] 19.6] 10.9 | 5.3 May 1-15 ..| 102.3] 388.5) 38.2/| 48.2) 31.31 17.7| 1.8! 0.9| 0.1 May 16-31 ..| 360.2; 645.5] 297.6]| 38.2} 22.41 18.5] 10.6] 6.1] 4.0 June 1-15 ..| 343.5] 740.9] 152.6)| 67.9 a 5.6] 1.8] 0.4] 0.6 June 16-30 ..| 386.2] 725.6) 103.0] 69.9] 22.9} 5.0] 1.6] 0.2| 0.3 July 1-15 ..| 202.9] 319.2! 178.7)| 49.0] 28.0! 16.0} 5.4/ 1.3] 0.3 July 16-31 ..} 152.1) 222.6) 93.4|] 62.6 23.2! 96] 4.0) Olan Nae, 1 1 POLST os hae: 65.0; 24.8) 8.8] 1.0) 0.2] 0.0 Avie 16°31 COR Oe eon cl: 31.81 37.4) 13.6) 15.9) 91.2 Sept. Pb) Moa lON ee 37.2) 98.2} 21.6 7.51 3.1] 2.3 Sone, 162804 | Meare Whe 30.1) 22.8) 13.0} 9.3] 11.2 | 13.5 Ogi 1215 1 oo Se: 25.2' 19.8] 12.6] 10.8] 22.6] 9.0 Oct 16S SANS its 39.01 20.6] 15.6] 14.4/ 7.8] 2.6 Noy (1215 Ut pOOVSh iia de, 98.9| 18.5} 23.6] 11.5/ 17.5] 0.0 Nov, 46230:)(0)) Rb) ae 15.6) 18.7| 6.3] 21.9] 15.6) 218 Dee: 1295). Soha a 19.3, 30.0| 3.9] 19.2) 14g Dee, 16-36 41) 24 a 93.1] 27.0) 20.7| 11.6) 9120 ae Appendixa—Statistical Tables. 441 TasLeE E.— Cyclops.— Average, maximum, and minimum numbers. Percentile vertical distribution. PER CENT. IN EACH 3 M. LEVEL. Av. | Max.| Min. 0-3. | 3-6. | 6-9. 9-12, |12-15.|15-18. a ne | ee |e | eee | Re | ee | | | 1894. July 1-15...} 39.8} 63.6) 11.2]| 37.4) 24.6) 21.9 July 16-31...) 151.0) 347.2) 53.2)) 44.4) 31.2] 13.8 Aug. 1-15..} 161.0) 297.6) 85.2]; 41.0) 28.8) 22.5 ars Sept. 1-15 Sept. 16-30..} 190.1) 272.2) 129.7]; 42.3) 25. Oct. 1-15...) 347.1) 421.6) 251.8), 33.5] 21. Wer too). ..)) 261.3) 383.4) 173.0)| 15.7) 20. Nov. 1-15 ..; 246.4) 440.1; 108.8), 12.9) 17. OE FT, TCSREN TORUS) A AMNPANAIThGD) CRRA] aU aa | HOP te| MeN nr Dec. 1-15...| 75.0) 243.5) 44.5)) 22.7) 15. Dec. 16-31...) 44.5) 46.1) 42.6)} 29.6; 14. 1895. bk OD je Ww on Gem). .|) 91.5] 48.3) 13.3] 24.81 17.2 16.0 Jan. 16-31...) 40.0) 60.9] 82.1) 5.1) 8.8) 25.8 eb. 1-14 Feb, 15-08 | 82.7| 112.6 55.3] 5.1) 9.1) 7.1 Mch. 1-15..| 55.7| 104.9| 39.4// 9.3] 13.2] 8.5 Mch, 16-31..| 66.9] 143.1] 49.6|| 15.2] 17.0; 9.9 Pi as. || 53.9) 63.6, 38.9] 29.9] 17.1) 13.3 Apl. 16-30...| 242.5] 604.8} 82.0] 39.3] 92.1) 13.7 May 1-15...| 864.9|1252.8| 759.0|| 42.7] 23.8] 14.2 May 16-31...| 944.4|1234.9| 715.3|| 30.5] 17.6] 15.3 elds). | Gi6.9| 966.7| 281.5} 21.3] 17.8| 17.5 June 16-30 ..| 262.6] 361.8| 197.7|| 33.0] 22.4] 13.9 July 1-15...| 323.6] 388.0| 148.2] 52.5| 19.3) 16.2 July 16-31...| 131.4] 218.4] 85.2]| 32.8) 31.8] 25.4 Aug. 1-15 ..| 107.6] 189.1| 64.8 Aug. 16-31 ..| 129.6] 343.7/ 108.1]| 36.4) 27.7| 20.7 Sept. 1-15..| 142.0] 237.2} 169.8|| 34.7) 23.1| 24.0 Sept. 16-30 ..| 226.0] 308.4] 169.8] 24.5) 20.5) 18.3 au (op) (=) bo =I bo je Ts (=) Oct, te-oh)_. | 219.7| 242.3) 202.2)| 23.9) 16.2) 15.7 Nov. 1-15...| 144.7) 157.7). 1388.6]|. 18.2) 15.8) 14.0 Nov. 16-30...) 146.3} 158.3] 186.1); 16.4; 14.0) 16.2 Dec. 1-15...| 90.2) 100.4) 76.3); 14.3) 20.0) 15.7 Dec) 16-31...) 89.1) 104.3) 52.1), 11.2) 11.9) 14.7 1896. dart toe!) Ott 0) 131-6) | 78.8 Jan. 16-31...| 151.0) 237.8/ 105.5 Feb, 1-14...) 91.6) 108.1) 75.6 Hea VG—08) | SAO) oo isl ea ces Mch. 16-31..) 212.5) 239.4; 74.4 Apl. 1-15...) 400.7) 763.2) 183.1 Apl. 16-30...|1011.2/1607.8) 543.7 May 1-15...|1858.4/2359.6)1071.6 May 16-31...) 705.9/1294.8) 176.8 Ned e bop co oD H OURS © jt (oy) ise) jt bo op) 09 C9 HOO ROR NS C0 > CO ST bo fon) CO — oO co OHO: poled petits bo MANS: wo ; CO STDOe Oct 1-15. . 3) 327.5] 338.5! 313.5)| 26.6) 15.3) 14.9 00 1 CO €© H 00 OO (2) Ke) C9 00 ee bo bo fob ek MAWWOMBANWONANCHOO © OY RAIMI WOE WOMHOORORMMNONWODO W OD BO DD RE Et tt et et Od 1S 60 CO SCoorFF- H> FE! 00 O1 F 00 00 to bo bo Ol axe’) ke bo OTH eH Wr D OO ADE HF OD OT OR OVO ODO A100 WH OVW Et HE OD O1 00 CO! © eS SS ee ee ee et jet See ay aryen HABA jd bede ted DD Oite: BROCjOr OMOowo: AMDpNw ©? 00 oooo eee 09 pet oOo © Q0 OC Op) ke bo re bo O 10.2 fot fet ped COOHUORA fod bet ed et et C0 O10) O1 © CO CO = OT CO 1 CO TE G9 DD C0 CO 1 CO OI © bo bo bo bo H> CO 00 J owowhy 442 June 1-15.. June 16-30 .. July 1-15... July 16-31. Birge—The Crustacea of the Plankton. TasLE E.— Continued. 189.5/ 297.6} 139.2 358.7} 716.1) 223.2 371.0} 442.0) 341.5 Js BL7 col 412.1) 138.0 Aug. 1-15.. Aug. 16-31 .. Sept. 1-15.. Sept. 16-30.. Oct. A=t575. Oct. 16-31... INow. 115 2 Noy. 16-30 .. Dec. 1-15... Dec. 16-31... Kho DOE DO WW RR WR OD AE AODADSWASMOINS AWHHUIWNWODWUNORO at fed ft et Ft ed dt DOD WHOOD RODE RHE COC COWNAWERODLRORD® —— eee PER CENT. IN EACH 3 M. LEVEL, eee | | | | | | | | | bo ADNWHFNFEFONOOC Or OO ODED wWWNwOO WNW Od CORONA WOOF OO Ol DD ee tt Appendix—Statistical Tables. 443 Taste F.—D. hyalina. Average, maximum, and minimum numbers. Percentile vertical distribution. PER CENT. IN EACH 3 M. LEVEL. 6-9. 9-12. |12-15.)15-15. —_—_—_— | — | | | —_ _ ) f | |) —— _ ——_ - Jot fet Pet Pe Av. | Max.| Min. 0-3. 1894. Pog -15....| 19.8) 32.8) 8.5)| 38.1 July 1G-31...; 13.3) 34.4, 2.7) 43.4 Auedits,..; 16.6} 33.3): 9.311 43.5 Aug. 16-31..| 60.7) 82.7) 47.8]| 42.7 Sept. 1-15...;No obl/servatlions. Sept. 16-30..| 148.4] 212.7| 74.4!| 30.6 ‘Oct. 1-15....| 207.6} 461.7/ 117.3}| 32.0 Oct. 16-31 ..} 252.5; 531.0) 96.3)]| 31.7 Nov. 1-15....} 183.1) 462.6} 92.2|| 37.1 Nov. 16-30.. No ob|servat |jions. Dec. 1-15.. ) | 121.5) 154.2) 78.2)) 41.1 Dec. 16-31 § | (48.8)} 56.9} 40.7 1895. Jan, 1-15....| 40.8) 65.4) 36.7]; 24.1 se oy ..| 55.9} 61.0) 53.41) 30.4 eb, 1- Fea eas ; 65.8} 109.4] 41.9]! 18.1 Mch. 1-15 ..} 34.7) 69.3) 25.6]| 22.9 Mch. 16-31..| 638.6) 102.3) 39.1]; 28.1 Apl. 1-15....| 26.4) 24.2; 12.7)| 42.7 Apl. 16-30 ..| 16.3 3.8 B22 3t.5 May i150. 2) 28.9} 81.4) .7.9]| 67.0 May 16-31 ..|} 250.7) 349.8) 71.2)| 59.1 June 1-15 ..| 319.2) 564.8) 183.1)| 42.2 June 16-30. .| 135.6) 327.5) 31.8]/ 51.0 Sulya—ts....\| 139.9) 263.9) 21:0) 56.1 July 16-31 ..| 275.3] 464.3) 129.7|| 58.3 Aug. 1-15 ..| 273.0) 417.2) 78.2)) 47.6 Aug. 16-31 ..| 252.8} 428.6) 143.1) 51.1 Sept. 1-15 ..| 202.8) 349.1) 169.8 49.5 Sept. 16-30..; 201.6) 248.0} 148.1]| 37.0 Oct. 1-15....) 180.5] 253.1) 123.3) 36.9 Oct. 16-31 (ose dats! 954.0) 37.9 Nov. 1-15 56.2} 72.5) 38.8]; 32.8 Nov. 16-30 48.2) 60.4) 36.2)]| 31.3 Dee. 1-15 ..| 35.0) 41.9) 26.4)| 33.9 Dec. 16-31..| 44.6, 52.7) 11.4|| 38.5 1896. Janis -15....| 36:2) 57.8) 15.2|| 31.7 Jan. tG-3)...| 17:3; 20:3) 10.8)| 41.0 Feb. 1-14 ..| 19.6) 29.6) 13.3)/' 26.9 Pew 29.) 2T0) oes ee ee at .6 Tle Tit Bh STR Ree cee | ny Ea Mech. 16-31..! 13.5! 27.3) 6.9 32.1 Jt tek DD NHOG KHObD- Oo PANwW NpNHoOrH i oe be 00 Re-100b9 © a HK WONTOMBRADAORAMNORMRWOHEA O DY bo ptt NARRWNHRNOCORDH OW! fe ped ft pet ek fd bed 4 je oOo ODEOOo ooow a OMwNWNOo GoOow Do Hh Oo wo pan fot bet fet OPRNWOSDOCOHOHOORONHUODLS MDI WORAWORHEAEDOWHaWWHWoH O ~0 oo Hono ooce oO OWHO ONOSO fot e H= DD CO DO CO “100 CO DO CD OOF CLO RRO Hr Ol eeoeceoel|eoeeeeertseuv eevee 444 rs me | eee | oe 1896. Apl. 1-15.... Apl. 16-30°.. May 1-15.... May 16-31 .. June 1-15 .. June 16-30. July 1-15 .. July 16-31.. Aug. 1-15 .. Aug. 16-31.. Sept. 1-15 .. Sept. 16-30.. Oct, 1-154; Oct. 16-31 .. Nov. 1-1ld .. Nov. 16-30.. Dec. 1-15 .. Dec. 16-31 .. 2h 1) 496.7 Birge—The Crustacea of the Plankton. TaBLEeE F'.— Continued. PER CENT. IN EACH 3 M. LEVEL. Av. | Max. | Min. 0-3 14.6} 18.4 15.2} 27.9 124.6} 360.0 270.8} 427.3 50.6) 156.4 319.0} 783.4 104.6 moO _ sil CON ®O ab TW bo CO OO 02 CO! GO 69.5 eeeceeetlescees oe eoeeeertooee eo DO re Co G9 ©? OUD DO CO OT} OF OT C1 > > DO SO OFA SD © CO G2 GUO Ss OVO HE OUR Hm De W OLOle COLO WRPOR OO ONO 3-6 mee NNW NMWNMWNFE Why AWRNMOOKrRE NOK OFPWROOE C2 bo Ft bO 6-9 —— 24.0 DDH et oe SED OO OWE WOE 9-12 |12-14 /15-18 r= bo a QO OF OU H> FH OS) © HE 1 CO CO 09 OLDS DO Fb Kh eer COUNMAWOERWUOODRWROH MOON i) bo bt a je EOOOrFONOINOSCCOFONS WMNOVWOWOH WHE OR WO DMNAI et et Jat ADOODUIPRWOCOHMNTIARID Appendix—Statistical Tables. 445 Tasie G.—D. pulicaria. Average, maximum, and minimum num- bers. Percentile vertical distribution. Per Cent. In Eacu 3 om. LEVEL. 1895-96. Av. | Max. | Min i _ O23.) 3-6. | 6-9: | 9-12.) 119-15 115-187, July 16-31...) 11.6) 17.1 0.2 0.0 O20 bay Lie TB 388 1.6 Aug. 1-15...| 19.9) 42.3 8.0 0.0 O20) eo Or, G5001 2220 1.0 Aug. 16-31..; 38.1) 164.7 5.4 0.0 1.6 23) SOLS. 14-8 1.0 mept. 1-15...) 33.8) 57.2 8.3 0.0 22 4.5) 68.8] 22.6 1.8 Sept. 16-30..| 98.2) 125.9) 10.5 0.0 15 ese Brel) BSS) (sant Wen tt...) 26.9| 49.6) .19.7)| 14.1 13.8| PAW ATO. Dh ET A) See Gertie). ..|, 23.5| 46.4 DOAN ee at ho OO UE CR PORT 9:6) ) 1b-0 Neve dy...) 49.6) 102.3} 17.8]| . 49.7 273 S. 9.2 5.5 6.6 Mew 1G-30..| 58.3; 82.0) 39.5)|° 25.2) 29.2) 19.6) 15:1 Dad. 5.2 Dec tis.) 141.1) 991.9) 25.01) 51.6) 35.5 7.9 2.8 eg 0.2 Meco -sh. 4) 99.8) 57.2). 24.8) 387.8) SL1.1) 11.4) te x ae 0.9 ae te 889) 137.3) 40.0||) 68.3) 14.5) 19.7 es) 0.5 0.5 San iG-3i. 2) 24:8) 31.8) 13.31 77.9 8.4 8.4 eae all pig Feb. 1-14....| 64.1) 81.4/ 29.8]| . 75.8 9.1 5.0 2.8 2.4 4.8 i ee 4. Ol es eles eee 43.4) 20.3 Satp kbs 8.7 tio ersureieoimmey ne ts es ohne Laie Ck ee Pe De eT ae Py Men 16-31.) 20.9). 50.9) 10.1). 34.0) 18.9]. 27.0 9.4 7.6 wee Pineal) 28.0) 47.0) 11.41; 10.4) 14.6) 32.2) 25.0) 12.5 6.3 Apl. 16-30...| 118.2) 251.8} 12.1); 84.9} 10.6 0.8 £5 LF 0.5 May 1-15....|.284.9| 683.2) 85.8); 13.1) 16.2) 19.5) 12.0} 16,7}. 22.4 May 16-31.. | 533.6) 763.4) 291.2); 28.0) 16.9} 18.8) 14.5} 10.0} 11.7 June 1-15...| 168.6) 260.7) 56.6]) 17.5} 10.0) 15.0) 21.3 10.1 25.8 Same 16-505.) 2 78.2!) 157:'7| 18.3 1.9 623) Th 16.9 45:8) 18.5 daly 1-15...) 39.3) 52.1) 19.7 0.0 0.9 8:6) 20.0 57 .0| 6.5 July 16-31...; 11.8) 38.2 0.6 0.0 0.0} 62.8) 33.4 on 128 Aug. 1-15... eT erst Uber ees v VOLO £O20F 270) TL.0 2.0 0.0 Mine deesi | 5.9.1... |... 0.0 0 0.0} 80.0 a 0.0 446 Birge—The Crustacea of the Plankton. TaBLE H.—D. retrocurva. Average, maximum, and minimum num- 1895. July 1-15.... July 16-31... Aug. 1-15... Aug. 16-31.. Sept. 1-15... Sept. 16-30.. Oct. 1-15.... 1896 July 16-31... Aug. 1-15... Aug. 16-81.. Sept. 1-15... Sept. 16-30.. Oct, Ila s.. Oct. 16-31... Nov. 1-15.... bers. Av. | Max. Deal Min. Bet > CD O19 F DO OF ONWORKFOOWO DOHAIORAUNMOMNOS eoeoeeetloeooeeee eocereecelooee ee eeoeceeloeeeee Percentile vertical distribution. PER CENT. IN EACH 3M. LEVEL. 6-9. | 9-12, |12-15 |15-18. — | | |. | bt DD > Heo G0 GO Ht bs fk ek = On a — ah OOOrRDOH ANH H OS NAONDF=100rH WOO siete 1) S et) oo DONE ROWOHE i (op) tN DS DD HE ND DS bb DO DO SCOCOCURWE OE OO bet i) GOH ©) O10) H ST OUD 41.3 27.3 CONNMONMHEOWHON jt =] No} cD et aa ON jad ek tt fal pk ed DD DOHODHROSOOSS DWAIOWANTONHHOO surfaice. 0.4 14.0 19.4; 1 4). 1623) 2 3 1028 an 8 1 0 near 8.0 Irreg|ular 59.0} 32.4 23.2} 24.0 LT 3h V9VS 18.4} 15. 20.0} 14. PSs Stns BOs Gh HOE et et 10.8 0.0 OOH OO bo Appendix—Statistical Tables. 447 TaBLE I.—Diaphanosoma. Average, maximum, and minimum num- bers. Percentile vertical distribution. PER CENT. IN EACH 3 M. LEVEL. 0-3. | 3-6. | 6-9. | 9-12. |12-15.|15-18. Sept. 1-15... Sept. 16-30.. Oct. 1-16.... 1896. Aug. 1-16... Aug. 16-31.. Sept. 1-15... Sept. 16-30.. Oct. 1-15.... HSS) 68.41) 6.9) 50.0); 29.4) 8.1) 7.9 4.6) 0.0 DA 1.8) OT) S£.0) 24.4) 20.7) > 4.9) TO) 4:9 a O17 Db. 4)°) 0.0 0.0; 25.0} 43.8) 0.0} 25.0) 6.2 Sealy ai-0) 2o.0)° 52.9). 29.6) 13.5). 3.8) 0.2] °0.0 32.2} 56.1); 17.8)| 39.3) 28.4) 24.6) 6.8] 0.6) 0.3 Zt Ga.6) 19.2)" 30.6) 30.7|) 19.7) 6.0 1.7) 0.2 Miz at .o) 2.9) AL Th ADO) 14 OF ATT TT 0.0 3.4! 10.8] 1.21) 11.3] 0.0] 34.0] 32.1] 22.6] 0.0 Sea sie Pawe vies TED 2B OP BB OL 6) O08 OO Je 1 ee aa ea ate £05028 oi tO sth Oto etk Os. Oud MS ra. ce nae ih BOL a) Se sOn ab. OM G24 TOT OL BIO Mes oe rasa de 31.2) 20.2) 11.5) 18.3) 11.5) 12.3 448 Birge—The Crustacea of the Plankton. ‘TaBLe J.—Chydorus.— Average, maximum, and minimum numbers. Percentile vertical distribution. PER CENT. IN EACH 3 M. LEVEL. 0-3. | 3-6. | 6-9. | 9-12. |12-15./15-18. | | | | | | | EC} | 1894. Sept. 16-30..} 278.9} 440.7; 96.6]} 55.0] 26.9; 10.6) 5.6} 2.0) 0.0 Oct. 1-15....| 193.3) 251.2! 92.8)) 12.3) 13.3) 16.4) 21:6) 2G) ae Oct. 16-31...| 202.0) 304.6} 82.0) 14.2) 17.8) 18.0) 18.7) 19.9} 11.3 Noy. 1-15....| 97.9) 261.3} 138.3), 7.3) 15.5) 18.6) 28/43) (tee) es Dec. 1-15.... 9.5) 15.9) 3.8]) 16.7) 20.6) 13.3) IGse) ieee 1895. June 1-15...| 36.7) 92.8} 11.1]| 41.9) 24.9} 13.0: 9.41 5.0) 5.6 dune 16-30 ..7 21.9), 45.77... 6.9,| 83.1) 13.4) 2.4 oes Bn ie Be) July 1-15....| 156.8) 271.5) 13.3]) 61.1) 25.3) 11.0) (2:2 ae July 16-31...| 163.4) 283.6) 89.0}} 42.8) 35.2; 20.3) 1.2) 0.2) 0.2 Aug. 1-15...) 78.6) 157.7) 16.8]| 48.2) 30.4 18.41 7.8)" O23) Ge Aug. 16-31...) 18.7| 48.7} 5.0}) 32.7) 33.3! 21.8) 11,9) ieee Sept. 1-15...} 15.6; 39.4) 8.9]! 45.5) 23.6) 22.8) 4.8! 3.4 0.0 sept. 16-30. .1.. \.)... Scattelring |lonly. Oct. 1-15....| 8.6) 14.3) 5.0}! 17.6) 17.6) 6.6) 125) Dies Oct. 16-31 8.1) 12.0) 3.8]| 46.8) 14.1) 7.8) 23:5) aie Nov. 1-15 25.9] 46.4! 10.8)| 11.4) 15.3) 14.3) 24.3)) Bice Noy. 16-30...) 19.7| 29.8) 13.9]| 9.2} 18.8) 24.0) 21.6) 16.0) ea Dec. 1-15....| 15.9} 19.7) 12.0)} 26.0] 6.6) 18.2! 10.5) Te°ei zee rece ..|. 20.9] 36.8] 8.9]| 23.0, 18.6) 11.8) 22/6) ie aie 96. May 1-15....| 28.0) 48.3) 19.0)} 15.9) 23.4) 23.8} 16.2) 8.9) 11.8 May 16-31...} 30.8} 68.6) 13.3]| 34.0) 28.6) 15.9] 19.9) 4.3)) 923 Junel—-15....| 87.6) 279.8] 4.41 77.6] 15.8] 4:7) TO) MOpaaoes June 16-30..} 230.8} 346.0} 145.6)|} 65.0) 24.4, 7.0} 2.6) 0.5) 0.3 July 1-15....} 382.0) 661.4] 169.8]| 57.0) 33.6) 6.9) 2.0) 0.3) 0.0 July 16-31...| 245.1) 465.5) 129.1]| 43.2) 34.1; 20.0) 2.7; 0.2) 0.0 Aas. ba AG LON a ed say sik 54.8] 32.0) 10.41 2.2) Olena Aug 16-31) | 426.0). 0.) ees ees 52.4! 36.2) 21.0| 8.5) aoe ew Sepe. LM GAS Gl eee chi daeeeis 34.9), 26.7| 17.0) 15.2) aah sept. 16-30) ./'263.0).0..0.1...4 2 25.0} 14.8) 14.9] 12.2) 18.1) 14.9 Ost. TAs Cie Dic cie tellus eiaien, 10.3} 15.0) 25.8} 21.0) 14.5) 1374 Oct, 1651 Veron as 16.9} 18.1} 17.1] 12°8)> dete) ee Noy. 1-15 A ( NECR Vna es 18.7} 22.41 13.0) 13°47 tee Nov. 16-30 Gevalia Le 21.3) 12.0) 26.7| 17.8) Ogee Dec. 1-15. Pec ad Wee Lenard Ice 15.0| 16.2) 13.4) 19,1) 9262232008 Dec. 16-31 BO De TNs ., 12.8} 25.0) 15-0) 17.6) )0S°6) ae INDEX. Age, effect on vertical distribution, 391, 398, | Cyclas, 424. 406, 423, 431. Cyclops and C, brevispinosus, annual distri- Algae, in autumn, 311, 357. bution 395. and Chydorus, 304, 350. in autumn, 311, 331. irregular appearance of, 317. date of development, 316. relation of numbers to crustacea, 308, 353. decline in summer, 329, 385. in spring, 308, 356. disadvantage of in summer, 331, 3&5. in summer, 353. eaten by Leptodora, 351. at thermocline, 417. food of, 355, 430. in winter, 307. horizontal variation, 368. See Food. maximum numbers, 309, 319, 329. Anabaena, 304, 346, 349, 353, 420, 425. number caught in single haul and series, Aphanizomenon, 307, 310, 31%, 353, 364, 420. 284. Apstein, 290, 304, 326, 331, 340, 347, 348, 350, numbers, table of, 441. 352, 367, 372, 395, 404, 406, rate of sinking, 480. As erionella, 318. relation to total crustacea, 331. Autumn, algae in, 318, 357. relation to light, 425; to temperature, 330, erustacea in, 311, 317. 359. temperatures, 299, 324. reproduction, 291, 306, 309, 318, 329. vertical distribution i in, 386. species of, 326. Bacteria, 333, 398. in spring, 308, 316, 329, 337, 381. Bibliography, 433. in summer, 309, 317. Birge, 274, 375, 392, 394, 421. at thermocline, 418, 424, Bosmina, 301, 304. in upper meter, 408. Ceratium, 304, 310, 318, 346, 355. variation in numbers, 368, 370 Ceriodaphnia, 4 492). vertical distribution, 337, "379, 381, 393, 396, Chemical condition of water, effect on ver- 407, 413, 416, 422, 441. tical distribution, 423. in winter, 327, Chroococcaceae, 304, 350. Cyclops fluviatilis, 332. Chydorus, annual distribution, 304, 348. Cyclops Leuckartii, 310, 326. 330. in autumn, 311. Cyclops oithonoides, 331 dependent | on algae, 312, 364. Cyclops pulchellus, 326. maximum and minimum numbers, 350, | Daphnia hyalina, annual distribution, 335. 448. in autumn, 311, 312, 317. relation to light, 425, to temperature, 364. diseases, 338. reproduction, 364, eaten by Leptodora, 351. in summer, 310. food, 353. at thermocline, 418, 424. growth in winter, 307. vertical distribution, 382, 389, 407, 448. length of life, 336. Cladocera, see the various genera. males, 338, 340. Clathrocystis, 317, 346, 353. maximum and minimum numbers, 319, Clouds, effect on vertical distribution, 410, 339. 443, 378. 426. number of eggs, 361. Cochituate lake, 293. relations of competition, 365; to bouncer a: Coefficient of net, 278. ture, 337, 361; to light, 372 Coelosphaerium, 353. reproduction, 310, 336, 337, 361. Collections, dates of, 486. sinking, rate of, 430. Competition, effect on numbers, 321, 365. in spring, 308, 316, 337. Computing, methods of, 275. in summer, 309, 339 Conochilus, 411. swarms, 372. Copepoda, see the various genera. variation in numbers, 368, 369 Corethra, 310, 410, 418. vertical distribution, 383, "ae5. 387, 393, 397, Crustacea, in autumn, 311. 407, 413, 416, 422, 426, 443. average number, 313. ae winter, 305, 336. dominant, 316, 365. D. Kahlbergiensis, 301, 346. diurnal movement. 407. D. longiremis, 401, 422. horizontal distribution, 366. D. pulicaria, annual distribution, 340. order of development, 316. in autumn, 313, 317. maximum numbers, 302. food of, 352. number per cubic meter, 318, 437. horizontal distribution, 371. periodic, disappearance of, 312; appear- limitation of numbers, 342, 385. ance of, 311, 358 ; number of, 304. maximum and minimum numbers, 319, in spring, 307. 445. in summer, 309. number of eggs, 362. in upper meter, 409. ronreduenon, 342, 362. variation of in different years, 315. in spring, 316 variation of numbers, 368, 370. vertical distribution, 379, 385, 388, 399, 413, vertical distribution, 375, 438, 415, 421, 427, 445. in winter, 305 D. retrocurva, annual distribution, 345. food of; Peon to light, etc. See Food, in autumn, 312, 317. etc., also, Cyclops, and other genera. affected by competition, 366. and Tables of Appendix. maximum and minimum numbers, 445, 450 Birge—The Crustacea of the Plankton. D. retrocurva: porations to light, 402; to temperature, sexual period, 346, 359 vertical distribution, 402, 407, 445. Dates of freezing and opening of lake, 289. of observations, 436. Diao) method of making, 276, 384, 388, Diaphanosoma, annual distribution, 347. in autumn, 313 maximum and minimum number, 445. relation to light, 425, to temperature, 358. in upper meter, 409. vertical distribution, 403, 407, 409, 422, 446, Diaptomus, annual distribution, 319. appearance in spring 308, 316, 322, 328. autumn numbers, 313, 324. food, 353. maximum and minimum numbers, 319, 325, 439. number of eggs, 360. rate of sinking, 480. relation to light, 425; to temperature, 316, 323, 360. summer numbers, 323. in upper meter, 408, variation in numbers, 368, 369, vertical distribution, 384, 393, 394, 407-416, 418, 422, 439. winter numbers, 320. Diatoma, 307, 318, 353. Diatoms, sinking of, 417. Diseases, D. hyalina, 338, 398. Dinobryon, 357. Distribution, annual, periods, 202. factors affecting, 352. horizontal, 366. vertical, ( See Vertical Distribution). Diurnal movement, 407. po verederer er 326, 331. Drown, 386, Duration of ifs, D. hyalina, 336. Eggs, numbers of, 332, 360, 361, 362. Eigenmann, 277. Hinfelder Soe, 352. Epischura, 332, 412, 422, Ergasilus, 332, "413, "416. Eudorina, 357. Fitzgerald, 290, 298, 300. Food, in autumn, 311. effect on numbers, 352; on vertical dis- tribution, 419. of Cyclops, 355, 430. of Daphnia, 346, 353. of Diaptomus, 353. of Leptodora, 351, 430. in winter, 307, 321. Fragillazia, 307, 318, 353. Freezing of lake, dates, 289. Francé, 374, 414. Fric and Vavra, 348. Geneva lake, 293, 402, 424. Gloiotrichia, 352, 357, 410. Gravitation, effect on vertical distribu- tio Green lake, 293, 300, 326, 352, 895, 397, 422. Hensen, 278, 282. Horizontal distribution, 366. of crustacea, 371. of Cyclops, 368, 370. of D. hyalina, 368, 369, 372. of D. pulicaria, 343, 372. of Diaptomus, 368, 369, of Leptodora, 351, 368. Hydrachnids, 302. Ice, thickness of, 289. Introduction, 274. Leptodora, annual distribution, 313, 350. diurnal movement, 410. food, 351. horizontal distribution, 368. relation to temperature, 359, reproduction, 351, 359. Leptodora: size, 352 swarms, 351. vertical distribution, 404. Life, duration of, 336. Hight, eer on vertical cise 409, Limnocalanus, 422. Lyngbya, 318, 339, 349, 353, 354. Marsh, 300, 326, 332, '346, "348, 371, 375, 395, 307, 402, 403, 422, 494, Maximum number, cubic meter, 319. fall, 311, 387. spring, °308 . summer, 310. See Vable LV, D. 313, and Appendix. Melosira, 318. 353 Mendota, Date of freezing, 289. size, 276 transparency, 427. Meter, cubic, population of, 319, 437. upper, population of, 409. Methods, of computing, 2715. of determining net-coefficient, 279. of diagraming, 276, of dredging, 277. in temperature observations, 286. in vertical distribution, 376. Microsporidia, 338 Minimum numbers, early summer, 309. late summer, 310. winter, 306. See Zable IV p. 313, and Appendix. Movement, diurnal, 407. Mud, temperature of, 290. Nauplii, annual distribution, 334. in autumn, 313. maximum number, 335. relations to temperature, 307, 360. relation of numbers of, to adults, 835. in summer, 360. at thermocline, 417. variations of numbers, 335. vertical distribution, 405, 410-416, 432. in winter, 306, 360. Net, coefficient’ of, 278. N ight, vertical distribution at, 400. Notholea, 3 ee Number, see factors determining, s maximum, 302, 319. minimum, 303. See ee also Appendix, pp. 436- Number of eggs, 332, 360, 361, 362. Observations in winter, OTT. number of, 274, 288, 377, 436. Oconomowoc lakes, 203, 345, 352, 417, 422. Oecistes, 307. Okauchee lake, 424. Ostracoda, 302. Percentile vertical distribution, 384; See also the various Tables of Appendix. kere erustaedss 304, 33538. See the Pine takes 331, 422, 424, 432. Ploen, Dake, 826, 331, 348, 357. Reighard, 2 Re production ‘affected by temperature, 359. of Chydorus, 349, 364. of Cyclops, 291, 327, 329, 359. of D. hyalina, 335, 361. of D. pulicaria, 342, 362. of D. retrocurva, 346, 358. of Diaphanosoma, 347, 358. of Diaptomus, 321, 324, 360. of Leptodora, 351, "359, in summer, 350, 361. in were ’306, 807, 309, 321, 329, 335, Richter, 295. Rotifers in winter, 291. Schizophyceae, 304, 310, 318, 350. Sinking, rate of, 430, Index. Spring, algae in, 356. erustacea in, 307. temperatures, 291, 323. vertical distribution in, 380. Sprungschicht, see thermocline. Stations, position of, 276. Summer, crustacea in, 309. temperatures, 293. vertical distribution i in, 382, 393. Surface, vertical distribution near, 407. swarms at, 371. Swarms, 366, 371. Syncheta, 307. Tables, statistical, 437-448. 2 eae 286 in April, 3 in ee 299, at bottom, 290, 292, 299. diagrams, 288. effect on alge, 307; on appearance of crus- tacea, 323; on “erustacea, 358; on Cy- clops, 306, "308 ; on Diaptomus, 324; on Nauplii, 307, "360; on reproduction, 306, 358; on the sinking of alge, 417; on vertical distribution, 421, effect of wind on, 292. errors in, 287, maximum, 294, methods of study, 286. - of mud, 290. number of observations, 288. relation of, to effect of light, 423. in September, 324. in spring, 291. in summer, 293. at thermocline, 295. in winter, 289. winter rise of, 290. Thermocline, 295. effect of wind on, 299. effect on vertical distribution, 383, 422. movement of, 298. variation of temperature at, 298. vertical distribution at, 415. Thermophone, 286. Triarthra, 307. Variations in number, 281, 306, 315, 335, 343, 351, 368 ff., 405. in vertical distribution, 392, 422. See Tables of Appendix. 451 Vertical distribution, 375. affected by age, 398, 431; by chemical con- dition of water, 423 - by clouds, 410, 426; by food, 419; by gravitation, 429; by light, 409, 425 : by temperature, 421; by wind, 427. in autumn, 386. of Chydorus, 382, 389, 404, 407, 446. of Cyclops, 379, 381, '393, 395, 407, 413, 416, 418, 422, 441, of Daphnia hyalina, 383, 387, 393, 397, 407, 413, 416, 422, 426, 443. of Daphnia pulicaria, 379, 381, 385, 388, 399, 413, 415, 421, 427, 445. of Daphnia retrocurva, 402, 407,413,416,446. of Foro unaneanis 403, 407, 409, 413, ‘416, of Diaptomus, ry 393, 394, 407-410, 411, 416, 418, 422, 4 of Epischura, 412, of Ergasilus, 333, 413, 416. independent of numbers, 384, 392, 394. of Leptodora, 404, 407, 411. methods of study, 376, 408, 417. of Nauplii, 405, 416, 432. at night, 410. Percentile, 384, 390 and Appendix. specific peculiarities, 432. in spring, 380. near surface, 407. in summer, 392, tables, statistical, 437-448. at thermocline, 415, in upper meter, 407. variations in, 392, 410. 432. in winter, 378, 426. Wesenberg-Lund, 307. Whipple, 282, 295. Wind, effect of, on temperatures, 291,295,299. on thermocline, 297, 298. on vertical distribution, 410, 415, 427. Winter, crustacea in, 305. reproduction in, 291, 306, 309, 321, 329, 335. 359, 361, 364. temperatures, 289. variation of numbers in, 368. vertical distribution, 378. Young crustacea, number of, in upper meter, 409. Pacharing : 331, 347, 356. Na PAW re Ht AEMt ih ah Aree 7 i LIBRARIES SMITHSONIAN INSTITUTION 3 722