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The Minimum Fee for each Lost Book is $50.00. The person charging this material is responsible for its return to the library from which it was withdrawn on or before the Latest Date stamped below. Theft, mutilation, and underlining of books are reasons for discipli- nary action and may result in dismissal from the University. To renew call Telephone Center, 333-8400 UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN FEB 04 499 FEB 91 19 L161—O-1096 Pa EE aa Digitized by the Internet Archive in 2011 with funding from versity of Illinois Urbana-Champaign — 2 ' i Se ‘| Le ae AA Rs -ear' ILLINOIS BIOLOGICAL MONOGRAPHS VOLUME XII PUBLISHED BY THE UNIVERSITY OF ILLINOIS URBANA, ILLINOIS 0 if EDITORIAL COMMITTEE Joun THEODORE BUCHHOLZ FRED WILBUR TANNER CHARLES ZELENY, Chairman ILLINOIS BIOLOGICAL MONOGRAPHS Vol. XII No. 4 EDITORIAL COMMITTEE Joun THEODORE BUCHHOLZ FRED WILBUR TANNER CHARLES ZELENY PUBLISHED BY THE UNIVERSITY OF ILLINOIS UNDER THE AUSPICES OF THE GRADUATE SCHOOL UrsanA, ILLINOIS DISTRIBUTED June 20, 1934 UNIVERSITY OF ILLINOIS 1000-—6-34—-4855 tt PRESS #8 A STUDY OF FRESH-WATER PLANKTON COMMUNITIES WITH NINE FIGURES BY SAMUEL EDDY CONTRIBUTION FROM THE ZOOLOGICAL LABORATORY OF THE UNIVERSITY OF ILLINOIS No. 448 NGS VO arn s ¢ ee ba hy f ‘" ’ ‘ " A Oe? ‘ : n > he ‘ 4’ he AE RTS AOR We P A ‘ CA ee ee Cg ERs Bie A, t . ¢ i \ SON bay a , 4 2] % 7 rt - my) eT } LO , if us ands 4° Oy Ze chan Wutie e he i a r - ; iat) rT i ite y i ‘ : y ' f i 7 pe ay , ce ha " Sean yh h i F ' ‘ Wha Zor Ai Aa 4 uf i ‘ * { ahi a ‘ : ia MG i : pie a) Noe? 4 . A, A weg te f i h yt + ‘a ‘ ] ’ I s " ~ rl » 4 . F 4) f° Hoh ; ‘ : Ay i c . ¢ J bah ote Y y 4 . i ae eale 4 F any : 4 ‘ A Fae “ ? , Y ‘ , : 15 4 i x ; , ’ ah f i i . : 7 ety ‘ ; =; eo] : 4, hen ~ f vrs , ‘ i ee Sot ats ie ; iafiuinte : bys hls a Ay ‘ ler AINE | See | 7 i a: The rene : r har * ’ : yh ys Pal vie: Ne 4 M Saas Ht MR, Waa cee hy Cac ER " aah ay en of ‘a r 7 ‘s i > 4 , \ i ue i : ; ie iw Wes) oN iain ry f : e hires / “t $1 AE ‘ , ; 2 ‘ i » ‘ ! F Fe cf iy - j i ‘ ty 5 - i 4 " wal { ( e ; A \ - r » bs ¢ oa A J , . W % ”, ‘. + en — 7 £2, t i “ es ; x a ; a , x 2 Pe WB = ’ ‘ 4 i A : ah M A : ; ; ‘ } os cy ~ ‘ +5 ‘ t " bar j =e t ~~ ‘ | © a se i j : aT. ; Ps / id ~ \ : ‘ j ; t ‘ e h ’ ’ ‘ ; } 4 iv ‘ . : 4 a ‘ 13 ; : Ly a ‘ . ‘ we ’ ’ 7 w : ii S 4 : va é ‘ i ata , ; 7 i ‘ r 1 : : ae, VPA NHK. « y “ay vr « : USA ey wr ah a wr ll Deity, AAA CONTENTS Introduction. Areas Studied Methods. Constitution of the Plankton in Streams Stable Streams Impounded Streams. Young Streams Constitution of the Plankton in Lakes and Ponds . Shallow Lakes and Perennial Ponds Temporary Ponds Deep Glacial Lakes . Seasonal Communities as Indicated by Plankton Organisms . Stable Streams and Perennial Ponds . Young Streams Temporary Ponds Glacial Lakes. Plankton Development and Related Factors Age of Water. ACME TE Pa lon ition iu) He Ee scaly Piet uae beth ks Velocity and Water Level . Turbidity . Light . Chemical Factors Biological Factors Interrelations of Factors Geographical Distribution and Ecological Classification Summary Check List of Names of Species . Tables Bibliography Index. ACKNOWLEDGMENTS The writer wishes to thank Professor V. E. Shelford, under whose direction this study was made, for his advice and encouragement. The writer is especially indebted also to Professor S. A. Forbes, late chief of the Illinois State Natural History Survey, for the use of the survey collections and data from the Mississippi, Illinois, and Ohio rivers and from the glacial lakes of northern Illinois. Mr. Arthur Hjortland, of the University of Illinois, kindly made collections for the writer from lakes in the vicinity of Ely, Minnesota, and from Lake Superior. Pro- fessor H. J. Van Cleave and many other members of the faculty and graduate students of the Zoology Department of the University of Ilh- nois have made and contributed collections from distant points, many of which, although not directly referred to in this paper, have helped the writer in reaching his conclusions. Professor Van Cleave and Pro- fessor L. A. Adams, both of the Zoology Department, and Mr. H. Carl Oesterling, then editor of the Illinois State Natural History Survey, read and criticised preliminary drafts. To all these the writer wishes to express his appreciation. [6] INTRODUCTION Various ecologists have frequently demonstrated that both plants and animals exist on land in well-defined communities, but have given com- paratively little attention to the existence and character of similar com- munities in water. Land communities show definite development and behavior. Certain species known as “predominants” (Smith, 1928), or “prevalents,” are conspicuous in land communities because of their size or because of their abundance due to a favorable response to the con- ditions and they serve as an index to the community. Some of them have approximately the same abundance throughout the year, while others show seasonal fluctations in abundance and indicate the presence of seasonal societies or ‘“‘socies” (Clements, 1916). Some predominants, known as “euryoecious” species, have such widespread distribution as to mark the boundaries of the formation, which is the community of greatest rank. Others of limited distribution are termed “‘stenoecious” and indicate the boundaries of communities of associational rank. Land communities develop and reach maturity by a long series of successional stages. If comparable fresh-water communities exist, they may be expected to reach maturity through a series of developmental stages. Communities of various ranks comparable to similar aggregations on land should be definitely ascertainable in fresh water. It should be possible to show a series of developmental communities ultimately reaching a permanent stable condition equivalent to that of a terrestrial climax community, as outlined by plant ecologists (Clements, 1916). Most ecologists have assumed that permanent fresh-water commu- nities do not exist and that aquatic communities are but early develop- mental stages of terrestrial ones. The writer believes that permanent fresh-water communities exist, reach maturity, and show aspects com- parable to terrestrial communities (Shelford and Eddy, 1929). Aquatic communities should demonstrate their stability and perma- nence by maintaining a composition in which no further succession takes place. Their rank and status should be determined by the presence of predominant or dominant organisms. It is part of the purpose of this paper, by studying plankton as an index to the pelagic portion of fresh- water communities, to determine the existence, rank, behavior, and status of the plankton element. “Plankton” has been variously defined by many authorities. Hensen (1887) originally defined the term as denoting all that floats in water, “Alles was in Wasser treibt.” Kolkwitz (1912) defined the term as the natural community of those organisms that are normally living in water [7] 8 ILLINOIS BIOLOGICAL MONOGRAPHS [328 and are passively carried along by currents. Rylov (1922) in discussing the forms which come under the term plankton, uses the terms “obli- goplankton,” referring to true planktonts only, and “facultative plank- tonts,” referring to those forms found in both limnetic and littoral regions, and also states that “pelagic organisms” are not the same as “planktonts,” for planktonts may be both pelagic and littoral. Plankton investigators usually include under the term all forms found in open waters regardless of origin; consequently, they often include many bot- tom and shore forms. As the term is restricted to minute forms and does not include the larger organisms such as fishes, the writer regards the term as applied to an aggregation of organisms constituting all micro- scopic forms found in open waters. Properly speaking, although sel- dom recognized as such, bacteria living in open waters should be in- cluded under the term “plankton.” A common definition includes those forms with little or no resistance to currents, living a free-floating or suspended existence in open or pelagic waters. Kolkwitz and Marsson (1902, 1908, 1909) have attempted to classify plankton from the standpoint of pollution, using plankton organisms as indicators of degrees of pollution. Wesenberg-Lund (1908), Bach- mann (1921), Smith (1920), Naumann (1927), and Krieger (1927) all have attempted to classify plankton in terms of plankton constituents and relations to the habitats. Griffith (1923) has classified plankton algae in terms of the ecological features prevalent in the habitat. From such attempts have arisen such terms as “rheoplankton” (river), “ben- thoplankton” (shallow pond), “limnoplankton” (deep pond), “heleo- plankton” (pond), and many others, all describing the plankton on the basis of habitats. The writer realizes that all these types of plankton are distinct and can be distinguished from each other by their specific composition but believes that they should be classified on the basis of the abundant or conspicuous organisms which act as indicators because of their favorable response to environmental conditions. A true ecological classification of plankton should recognize plankton as a part of a living community which is comparable to an organic unit passing through the stages of youth and maturity. ‘ The animals of any considerable body of water group themselves into two natural communities, the bottom and the pelagic communities, which may be termed societies or socies as they correspond to the ter- restrial stratal societies or socies of Clements (1916). The true com- munity dominants, namely, fishes, do not respect the difference between bottom and open waters. Most fishes are, properly speaking, bottom organisms but their constant forays into open waters place them as im- portant factors in the pelagic community. These socies offer an inter- relationship that is unique and entirely different from that of the cor- 329] FRESH-WATER PLANKTON COMMUNITIES—EDDY 9 responding terrestrial groups. Terrestrial societies all rest on solid substrata, and we know of no group of terrestrial animals which spend their entire existence suspended in the surrounding medium. The or- ganisms of pelagic societies are always suspended in the water. Type of bottom cannot influence the reactions of organisms of the pelagic societies as it does those of the bottom society, which are usually in close contact with the bottom. Furthermore, the pelagic society is constantly shifting with currents or waves, while that of the bottom remains fixed. The development of the pelagic society is not as closely related to, and dependent on, the development of the bottom society, as is the develop- ment of the various terrestrial stratal societies to each other. In newly formed bodies of water, as will be discussed later, the pelagic society develops much faster than the bottom society. The organisms of the plankton constitute an important element in the pelagic community. Over 500 species of organisms have been re- ported from the plankton of the waters of the State of Illinois. Prac- tically the only other organisms existing in this fresh-water community are the pelagic fishes, which are usually dominants; and in both num- bers and species, the plankton organisms greatly out-number these. Truly pelagic or nektonic fishes are rare in fresh waters and are limited to such _ forms as the cisco and other coregonid fishes which occur only in very deep lakes. The plankton serves as a convenient index to pelagic communities. Quantitative collections are easily obtained. The great abundance and number of species give a greater range of data than fishes afford. The organisms serve as a good index to general conditions because of their inability to resist currents and to move into more favorable areas; con- sequently, their abundance may be considered as evidence of favorable reactions to the conditions of the habitat. In land communities certain species are abundant and therefore con- spicuous because of their favorable reactions to environmental condi- tions and are known as “predominants”’ or “prevalents.”’ Others, known as “dominants,”’ usually fishes in fresh water, exercise some control over the community and are responsible, in part at least, for its existence. Cahn (1929) found indications that carp introduced into a lake could destroy the vegetation and change the character of the bottom to such an extent as to alter the fish population. Organisms forming important elements in food chains are known as “influents.” “Characteristic” or- ganisms are those forms which may serve as indicators of conditions. They are not necessarily abundant; indeed, they are often rather scarce, their mere presence or absence being more significant than their abundance. These terms should be applicable also to fresh-water or- ganisms insofar as communities in water are comparable to those on 10 ILLINOIS BIOLOGICAL MONOGRAPHS [330 land. In this paper we are chiefly concerned with the characteristic and abundant or predominant organisms, which include both seasonals and perennials, as our knowledge and data on interrelations between aquatic organisms are insufficient to determine the other types definitely. Ac- cordingly, all abundant or characteristic species are considered either as perennial predominants (“perennials”) or as seasonal predominants (“seasonals”) until more is understood about their relations to other members of the community. It is probable that many plankton organisms act as influents by serving as vital elements in the foods of other forms and possibly even as dominants in controlling the community by this action and by clearing up waste and inorganic materials, thus preparing conditions whereby other forms can exist. Another group of organisms found in the plankton may be termed “incidentals.” These forms often constitute more than half of the species but are sporadic and seldom abundant. Such forms generally occur once or twice a year and frequently originate from foreign sources. Therefore, as they are apparently not significant or important to the community, their presence has been generally disregarded in this study and only considered when describing the entire plankton population. Communities are determined by their organic constitution, in which one or more species stand out and serve as indicators, and by the physiographic state of their habitat. The first analysis of a community should be made from a physiographic study. This is best applied to both permanent and developmental communities. The conception held by most ecologists, that aquatic communities are developmental stages of land communities, is not necessarily true in all cases. Streams are permanent so long as the existing climate endures, and this is the same condition under which land communities reach and maintain a permanent or climax stage. Only the abandoned areas of streams become devel- opmental stages of land communities. Apparently streams contain the only permanent fresh-water communities. Streams show a definite physiographic development, from the small intermittent stream, with its rapid fall, to the large permanent stream approaching a base level and having a uniform current (Shelford, 1913). Their characteristic fluc- tuations of current and level make it difficult to study their communities. All lakes are, at least in part, developmental stages of land commu- nities. Physiographically, the ultimate fate of even the deepest lake is to become a swamp proceeding toward a terrestrial climax. Many lakes are enlarged stream channels, forming large and deep bodies of waters. These are best considered as abandoned parts, or natural duplications of abandoned parts, inasmuch as in their later stages, when they proceed toward land, they contain communities similar to stream portions actually abandoned. 331] FRESH-WATER PLANKTON COMMUNITIES—EDDY 11 AREAS STUDIED In order to secure the data necessary for the study of plankton as an element of aquatic communities, it was essential to select ponds, lakes, and streams representing all stages of physiographic development. This paper is based on data obtained from more than two thousand collections studied from this viewpoint. Advantage was taken of several dams and canals to furnish quasi-experimental evidence on development and ma- turity of plankton communities. The most extensive data were obtained from the Sangamon River, a tributary of the Illinois River, between Mahomet and Decatur, Illinois. Collections on this stream were made semi-monthly or weekly at Decatur from 1923 to 1929, at Monticello from 1927 to 1929, and at Mahomet from 1928 to 1929. Collections also were made at stations ten to fifteen miles apart over the entire Rock River system and the IIlinois-Mississippi Canal under the direction of the Illinois State Natural History Survey during the summers of 1925, 1926, and 1927. Ponds at Decatur and Urbana were studied semi-monthly from 1926 to 1929. Temporary ponds in the vicinity of Urbana and Seymour, Illinois, were studied weekly during their wet periods in 1926 and 1928. Many occasional collections have been made and studied from various parts of the United States whenever opportunity was afforded to the writer. In addition, extensive collateral data have been obtained from collections made by other workers as follows: by Dr. C. A. Kofoid from the Illinois, Mississippi, and Ohio rivers; by Mr. R. E. Richardson from the Illinois, Mississippi, Ohio, and Fox rivers and from the glacial lakes of northern Illinois; by Dr. S. A. Forbes from lakes of Yellowstone Park and Wisconsin; by Dr. H. J. Van Cleave from lakes of New Mexico, Arizona, and California; by Prof. Frank Smith from lakes in Colorado and Michigan; by Dr. R. D. Bird from lakes in San Juan County, Washington; by Mr. A. L. Hjortland from lakes in Minnesota ; by Miss Beth Hefelbauer from lakes in New York; and by Mr. E. E. Wehr from lakes in Montana. METHODS The usual methods of collecting by silk net, decantation, filter paper, and centrifuge were employed. Each set of collections consisted of a silk-net collection with an additional collection by one of the other three methods for the purpose of obtaining data on the nannoplankton. At least two samples of each collection were counted in a Sedgwick-Rafter slide, an entire cubic centimeter being counted instead of the usual ten random counts on a single sample. The 200 fields of the microscope on 12 ILLINOIS BIOLOGICAL MONOGRAPHS [332 the slide were divided into ten groups, and the counts of all these groups were made separately and then averaged. From the averages thus ob- tained on the two samples, the numbers of organisms were finally com- puted per cubic meter, which was the standard unit volume. This method gave a more representative list of species as well as a better average than is usually secured by the common method of basing com- putation on a single count of 0.1 cc. It is well known that the silk net is very inefficient and allows many of the smaller organisms to escape through the fine meshes. However, as larger quantities of water could be used (100 liters), it gave better qualitative data than could be ob- tained by any of the other methods. The quantitative data from the silk net, after repeated tests, showed reasonable accuracy for the larger forms such as the Entomostraca. All the collections referred to in the tables of data in this paper were made with a net of No. 25 bolting silk unless otherwise indicated. The decantation method proved to be a quite accurate and very con- venient method for collecting nannoplankton. A known volume (1000 cc.) of the water was treated with 2 cc. of formalin and allowed to settle for two weeks. The water was carefully siphoned off until only about 20 cc. remained, which was saved with the residue for counting. When floating organisms were present, care was taken not to siphon off any of the surface water. Many of the nannoplankton collections from Lake Decatur and the Sangamon River were of this type. Most of the plankton collections from other waters studied were filter-paper or centrifuge collections. As plankton organisms are known to vary in vertical distribution, collections were made at several levels in waters where there was little current. By means of either a pump or water bottle, collections were made from two or three levels depending on the depth. Averaging the collections from the different levels served to equalize the variations in vertical distribution. Whenever a current was present and repeated tests had showed no vertical variation, the collections were made either by dipping with a bucket or by pumping a known volume of water from two feet below the surface. In an effort to find correlations with the distribution of the organ- isms, the physical conditions of the water were determined as far as possible for each collection taken. At most of the stations tests for dissolved oxygen were made in the various seasons of 1928-29, by the Winkler method (Winkler, 1888) as described by Birge and Juday (1911). The temperature and pH of the water were determined at each collection. Also the level of the water, the turbidity, and the current were noted each time. 333] FRESH-WATER PLANKTON COMMUNITIES—EDDY 13 Where weekly or semi-monthly collections were made, the data were averaged and tabulated on the monthly basis for convenience. The author is well aware of the many fluctuations in abundance occurring within short periods of even a week, but the long tables necessary to in- clude these detailed data are undesirable because of their additional bulk. Such observations are desirable for a detailed study of plankton, but in a general study monthly averages serve very well. For each station where series of seasonal collections were made, the organisms have been tabulated in order of abundance or im- portance and according to their seasonal arrangement, without any regard to their systematic order. This is essential in an ecological paper where distribution is of more importance than systematic relations. As in ter- restrial communities, the abundance of organisms is not always the in- dex to their importance. Some very small organisms are very abundant, running into billions per cubic meter, while others much larger are equally important though running only a few hundred per cubic meter. CONSTITUTION OF THE PLANKTON IN STREAMS In an ecologically “mature” community there is no change in most of the predominant species from year to year. Such communities should be distinguished from “developmental” communities by their relative sta- bility and by the absence of invaders which would establish different sets of conditions. Communities may be distinguished from others of different status by the presence of abundant or predominant organisms serving as indicators of given sets of conditions. STABLE STREAMS Streams for the study of mature communities should be selected first on a physiographical basis. Alterations in the channel, changing the physical or chemical conditions of the water, by an increase in turbidity, by the introduction of water of recent origin, or by an increase in rate of flow, may retard the rate of development or change the ecological age of the water at any given point. The tendency is to wear away the stream bed, creating a more uniform channel. No stream perhaps ever completely reaches the limit of old age, as the upper course is usually in such a physiographic condition as to produce a volume of silt which more or less affects the lower portion. Most of the streams of the United States are in the process of aging, but hardly any two are in the same physiographical stage. Therefore, since the physiographical stages show considerable variation, it is necessary to select those suitable for 14 ILLINOIS BIOLOGICAL MONOGRAPHS [334 the study of the ecological conditions of maturity. The current should be relatively slow and uniform. The Illinois River approaches these conditions very closely. The Mississippi and the Ohio are not as mature as the Illinois. Purdy (1923) states that the Ohio is geologically young, not having reached a base level and being consequently subject to con- siderable channel erosion. The Mississippi channel is subject to erosion and receives much silt from tributaries; consequently the water is very turbid. Summer collections from Rock Island to Cairo, made by C. A. Kofoid in 1902 and R. E. Richardson in 1908 for the Illinois Natural History Survey, have been examined by the writer and show an abun- dance of silt and a scanty plankton (Table 1). The same seems to be true of collections by the same investigators from the Ohio between Paducah and Cairo. Galtsoff (1924) in his investigations on the upper Mississippi found the plankton more abundant upstream. He reports a rather general distribution of the following plankton organisms in the section of the river between Hastings, Minnesota, and Alexandria, Mis- souri: algae, Microcystis aeruginosa, Aphanizomenon flos-aquae, Scene- desmus quadricauda, Pediastrum duplex; protozoans, Eudorina elegans, Codonella cratera; and rotifers, Filinia longiseta, Polyarthra trigla, Brachionus calyciflorus, Brachionus capsuliflorus (varieties), and Kera- tella cochlearis. The writer found the plankton to be rather rare both in species and in abundance in the collections of Kofoid and Richardson from the Ohio and the Mississippi. Purdy (1923) gives no definite in- formation as to the specific content of the plankton of the lower Ohio but states that in the Paducah district the plankton is scarce. The collections made by Richardson from the Ohio and the Missis- sippi were made continuously by pumping a stream of water through a plankton net suspended in a barrel on deck as the boat traveled down the Mississippi and up the Ohio. In the regions listed in Table 1, little difference existed between the collections within a given region, so that an average of the collections gave a typical and representative list of the plankton for that area. Difflugia lobostoma was the most abundant form in the Ohio. The other but less abundant forms were the usual river planktonts as follows: Pediastrum duplex, Keratella cochlearis, Polyarthra trigla, Cyclops viridis, Filinia longiseta, and Brachionus angularis. The Mississippi from Rock Island to Cairo may be divided into two sections as distinguished by plankton production. The upper section, from Rock Island to the mouth of the Missouri, carried a plankton much richer both in species and in numbers than the lower section, between the mouth of the Missouri and the mouth of the Ohio. This is at least in part due to the increase in silt from the waters of the Missouri—an 335] FRESH-WATER PLANKTON COMMUNITIES—EDDY 15 increase which was distinctly noticeable in the collections. The plankton of the lower section was very similar to that of the Ohio, containing the same predominants, or prevalents, with the addition of Daphnia longis- pina, Cyclops bicuspidatus, Pedalia mira, and Brachionus budapestinensis. The upper section not only contained the same species in greater abun- dance but, as may be noted in Table 1, contained many other species which are common river predominants. The Illinois River approaches the physiographic conditions of a mature stream more closely than the Mississippi or the Ohio. According to Kofoid (1903), the Illinois River occupies an ancient and well-worn preglacial channel which causes the stream to have little fall and a slow current. The erosion and turbidity seem to be less than in the Mississippi. Kofoid reports a fall of only 0.13 feet per mile in the Illinois River from Utica, Illinois, to the mouth; while Galtsoff reports a fall of 0.44 feet per mile in the Mississippi from St. Paul to Alexandria. The writer has examined many collections from the Illinois between Utica and the mouth which agree in species and relative abundance with the extensive data collected by Kofoid. In addition to the recent collections made by Mr. R. E. Richardson and the writer, the collections and un- published data of Kofoid for the years 1896 and 1897 have been re- counted and checked by the writer, and tabulated monthly (Table 2). These data, together with those for 1898 published by Kofoid (1908), represent the seasonal distribution of the plankton for perhaps the most mature stream yet studied in North America. Certain species such as Lysigonium granulatum, Polyarthra trigla, Keratella cochlearis, Diffiugia lobostoma, and Codonella cratera are shown to be definitely perennial, existing through all seasons of the years studied. Others are seasonals, appearing regularly at definite periods as discussed later. In general, the plankton is very abundant below the points of pollution. The predomi- nants mentioned as occurring in the Ohio occurred in the Illinois as either perennials or seasonals. An extensive survey of the summer plankton of the Rock River from near the source to the mouth showed an abundance of the same pre- dominants as found in the Illinois. A series of dams throughout its length creates many pools which serve to stabilize the current. Weekly collections were made at 10-15 mile intervals from the Wisconsin line to the mouth of the river. The specific content of the plankton was found to be the same at different points although there was some varia- tion in abundance due to local conditions. The station at Sterling was selected as being very typical of the Rock River, and the predominant, or prevalent, organisms for the months of June, July, and August, 1926, have been averaged (Table 1). Tvrachelomonas volvocina, Codonella 16 ILLINOIS BIOLOGICAL MONOGRAPHS [336 cratera, Polyarthra trigla, Pediastrum duplex, and many other forms ap- pearing as perennials and seasonals in the Illinois River, appeared in the Rock River abundantly. Collections from the Fox River at Algonquin, Illinois, made by Mr. R. E. Richardson in the summer of 1916, were counted, averaged, and are tabulated in Table 1. The plankton of the Fox River was rather abundant in both numbers and species and contained most of the pre- dominants found in the Illinois in summer, in nearly the same relative abundance. A tow made from the Wabash River at Mt. Carmel, Illinois, in the spring of 1927 when the river was at flood stage, contained many of the predominants which occurred at the same season in the Illinois. In all the large streams examined which seem to approach stability, the same plankton predominants occurred, though with some irregularity. The most abundant or conspicuous of the predominant, or prevalent, species of the plankton of a stable, or permanent, river community in the region studied are as follows: Algae Daphnia longispina ‘Microcystis aeruginosa Moina micrura _ Lysigonium granulatum Leptodora kindtii Scenedesmus quadricauda Asterionella gracillima ; Pediastrum duplex Polyarthra trigla Rotatoria Closterium acerosum Protozoa Trachelomonas volvocina Difflugia globulosa Difflugia lobostoma Codonella cratera Synura uvella Stentor coeruleus Phacus longicauda Eudorina elegans Euglena acus Euglena viridis Euglena oxyuris Tintinnidium fluviatilis Ceratium hirundinella Cladocera Chydorus sphaericus Bosmina longirostris Brachionus calyciflorus Brachionus capsuliflorus Brachionus angularis Brachionus budapestinensis Brachionus havanaensis* Keratella cochlearis Keratella quadrata Notholca striata Filinia longiseta Conochiloides natans Pedalia mira Asplanchna brightwellii Synchaeta pectinata Synchaeta stylata Copepoda Cyclops viridis Cyclops bicuspidatus Diaptomus pallidus Diaptomus siciloides These species occurred either as seasonals or as perennials in the plankton of the Illinois River (Table 2). Most of these were abundantly 1This species has been confused with Schizocerca diversicornis by some investigators and erroneously determined as such. 337] FRESH-WATER PLANKTON COMMUNITIES—EDDY 17 distributed in all the summer collections examined from the Rock River and many were found in the other large streams discussed. The same species have been reported for the Mississippi by Galtsoff (1924) and for the San Joaquin in California by Allen (1920). They show some seasonal fluctuation, especially in the winter, and many that are never entirely absent may be termed “fluctuating predominants.” Plankton organisms are comparable to terrestrial insects in their seasonal distri- bution. In winter some species usually persist in the adult form in small numbers. Other species exist only as eggs, resting cells, or en- cysted stages. Ecologically, only those which are in an active stage and influencing the community are important at this season. IMPOUNDED STREAMS Observations on the plankton of streams after damming have showed that the impounded water becomes biologically matured. In the many pools on the Rock River created by power dams, each duplicating the hydrographic conditions of a mature stream, the same species of plank- ton organisms were found to occur as elsewhere in the river, but much more abundantly. The diversion of a small part of the water at Rock Falls, into the Illinois-Mississippi Canal, created a body of water with the ideal conditions of a stable stream. All the disturbing features of stream study, such as fluctuations in current and level, were removed. The water flows at a slow uniform speed and constant gradient, with- out fluctuations of level, emptying partly into the Illinois and partly into the Mississippi. As the water proceeds down the canal, the same plank- ton organisms as are found in the Rock River become much more abundant, especially the cladocerans and copepods. The bottom fauna, on the other hand, according to Dr. D. H. Thompson, of the Illinois State Natural History Survey, while of the same composition as that of the river, is relatively much less abundant. In 1922 a dam was built across the Sangamon River at Decatur, creating a lake one-half mile wide and twelve miles long, commonly known as Lake Decatur. The water averages from six to eight feet in depth. Only at the narrow places where bridges have been constructed can any current be detected. Observations on the plankton just above the present lake showed that only a slight plankton developed when the river was low. As a result of damming the stream, stable conditions have been established and an abundant plankton produced. Since Septem- ber, 1925, collections either weekly or semi-monthly have been made on the pool at Lost Bridge, two miles above the dam and ten miles below the head of the lake. Previous to 1925, collections were made there from June, 1923, until April, 1924, through the assistance of Prof. A. O. 18 ILLINOIS BIOLOGICAL MONOGRAPHS [338 Weese, then of James Millikin University and now of the University of Oklahoma. The temperature of the water ranged practically the same in all the years studied. In January it remained about 1° C. under the ice. Beginning in February it increased gradually and attained an average of about 28° C. in summer. In August the surface temperatures oc- casionally reached 30° C. The water at this point was from 15 to 20 feet deep, and the bottom temperature was always about one degree lower than the surface temperature. The pH value was rather steady throughout the year and was always well within the limits of life, gen- erally remaining at about 7.6 and occasionally falling to 7.0 in mid- winter. Dissolved oxygen determinations were made as follows: Cubic centimeters Date per liter Aassuist U6 VIOZR teeta Ae taal ce Ale metan toe Ck a es 3.25 November: 20) VOZ8 6 204 wis sac epacsiicss Gcstavere stata tale Caters SI ee 9.27 Pebrdary 22, O20 eo yeaah Saka « ern Geld valence 6 aoe ana 11.26 Petia ZBO ODO via, iis ais Pacts Ss es ee hr a a 4.10 This showed a high content of dissolved oxygen in winter and a lower content in summer, but always an abundant supply to meet the require- ments of the plankton organisms. Birge and Juday (1911) found plankton flourishing in water with 0.5 cc. of dissolved oxygen per liter and found many planktonts living very well in water with only 0.25 ce. The plankton is abundant during the warmer months, but rather scanty during the colder months. The predominant, or prevalent, organ- isms (Tables 4-7) are the same as those found in the stable rivers mentioned. The greater part of the volume of the summer plankton is composed of Diaptomus siciloides, Cyclops viridis, Cyclops bicuspidatus, Diaphanosoma brachyurum, Brachionus calyciflorus, Difflugia lobostoma, and Codonella cratera. Other predominants, such as the common roto- fers, Polyarthra trigla and Keratella cochlearis, although conspicuous in all the collections, do not occupy much of the volume. The lake was one year old when the plankton was first studied. At this period there was an abundance of chlorophyl-bearing flagellates, partic- ularly Pleodorina illinoisensis. Each year there was an apparent tend- ency toward stability (Table 19). Blue-green algae did not appear until the summer of 1926 and reappeared in the summer of 1928. The number of species of planktonts increased from year to year. The out- standing perennials, such as Keratella cochlearis and Polyarthra trigla, have persisted at all times in the collections. Others, such as Lysigonium granulatum, Diffiugia lobostoma, and Bosmina longirostris, first appeared as seasonals and then became established as perennials. Brachionus calyciflorus and Microcystis aeruginosa, which appeared as perennials 339] FRESH-WATER PLANKTON COMMUNITIES—EDDY 19 in the Illinois River in 1896, 1897, and 1898, have occurred only as sum- mer forms in Lake Decatur. Some species, such as Asterionella gracil- lima, have not yet appeared. From the fact that new species are appear- ing each year, it seems that the plankton has not yet reached a climax, or state of maturity, and that probably a community similar to that of a mature river is being established. No species found in the plankton since 1923 has ever completely dropped out. Even Pleodorina illinoisensis, which appeared abundantly in 1923, has since occurred in small numbers, although not abundant enough to be called predominant or prevalent. This continuous pro- gression of species is not “succession” in the same sense as this term is applied to the development of terrestrial communities, but, as will be discussed later, this progression is more comparable to invasion and colonization of a barren terrestrial area. All the predominants so far found in the plankton of Lake Decatur have been found as predomi- nants in the Illinois River. In Lake Decatur, the absence or seasonal variation in distribution of some of the Illinois River predominants may possibly be explained by the difference in the stage of stability. Galtsoff (1924) in his plankton survey of the upper Mississippi found that there was a great increase in the plankton of the river when it entered Lake Pepin and the lake above the Keokuk dam. In each of these places the waters were impounded over large areas, and the species were the same as elsewhere in the river, but much more abundant. Coker (1929) found that the Entomostraca of the plankton of the Mississippi River increased as the water approached the Keokuk dam. The plankton element of mature streams may be reproduced by im- pounding the waters of immature streams so that they may age under more stable conditions. The waters are retained and aged under relatively stable conditions of level and reduced current—all of which are conducive to plankton development and are similar to the conditions of a large and stable stream. The impounded water, being relatively free from the disturbing fluctuations of floods, permits studies to be made that reveal seasonal trends comparable to those in a stable stream. The establish- ment of the plankton community in impounded waters occupies some time. Many of the species predominant in Lake Decatur in 1928 were present the second year after filling and probably were present the first. The number and abundance of species increased gradually each year until the plankton became similar to that of a stable stream. Younc STREAMS In an attempt to trace the early development of plankton in young stream communities, streams ranging from 20 to 60 feet in width and 20 ILLINOIS BIOLOGICAL MONOGRAPHS [340 averaging from one to three feet in depth were selected. Their current- speeds ranged from two to five miles per hour, because of the relatively great fall, and were subject to sudden fluctuations. Small rapids and pools characterized their course, because of the irregular erosion of the bed. Plankton collections were made at stations from 20 to 40 miles from the source of the stream; consequently, the water was seldom over 48 hours old and generally less than 24 hours old. During 1928, semi-monthly collections were made from the Sangamon River at Mahomet, 34 miles from its source (Table 3). Water levels at this station were relative to those recorded at Monticello (Table 9). The normal width of the stream was 50 feet, and the depth averaged 2 feet. The temperature ranged from an average of 9 degrees C. in March to 27.5 degrees C. in August. No collections were made in winter when the river was frozen over. The pH was always 7.6. The dissolved oxygen content, which was higher in summer than that of any of the other stations, ran as follows: Cubic centimeters Date per liter AIT OTISt, 2 MOZB sels Rieke SAMs Sacasi ae ectela heed arene eats yaot ye ake eee 11.20 November 21) TO2B na" aie cies shia ye tic siete ekes oko eieeiae Eee eee 9.32 Hebruary 20) (O20 0 os cots o cues os been Clee eet cane 8.50 Jie ZEAMOZO Monit ih ce Jee es KR Whee oe ©, ae ee ee 4.85 Few true plankton organisms occurred in any of the collections. Occasionally a few diatoms or protozoans belonging to the bottom com- munity were found in the collections. During the extreme low water in summer when the current was greatly reduced, a green bloom of Euglena viridis formed on a few pools and constituted the only evidence of abundant pelagic organisms. During part of 1927 and all of 1928, collections were made from the Sangamon River at Monticello, 23 miles below the station at Mahomet and 57 miles below the source (Tables 8 and 9). The river at this point, though practically the same in width, was considerably deeper than at Mahomet, averaging 4 feet, but no differences could be found in the rate of flow. Water levels (Fig. 8) were averaged monthly from readings made by the U. S. Geological Survey from a gage maintained about one mile below the station. In 1927 the river was high in March, April, May, and June, reaching a low stage in August. In 1928 a low stage was reached in May and remained until December except for several small rises. The temperature showed the usual gradual rise from 2° C. in February to about 26° C. in August. The pH seldom varied from 7.6. The dissolved ‘oxygen determinations were as follows: 341] FRESH-WATER PLANKTON COMMUNITIES—EDDY 21 Cubic centimeters Date per liter BE NSN oe. G aug: dla e tco ale: slave avGiaela ms dislamid anstagel Ral aia’ e a renal elevate 325 MOVeImDeE re OZ Uh. «Ps 2a 'ic aa Mas otek Abia cae alatereinareraradcleterets orgie 9.27 Pipes licen el ea. Ae 20s Sevat Ae sa ci Rahat dc, clelgard a ae Caled <8Ren Reree — Teer de Ty | | lil Basa AEHEAEAI Boa : Beat rr = Ficure 1 Graphs showing relative volume of predominant organisms in the plankton of Lake Decatur, 1926, 1927, 1928. (For graphical method, scale, and reduction, see text, page 37.) A key to the symbols at the left is supplied on the opposite page. 359] FRESH-WATER PLANKTON COMMUNITIES—EDDY 39 The hiemal socies usually started the last part of December and continued until April, when the vernal socies was well established. The plankton organisms of the hiemal socies were general scanty. Diatoms were common. Minute flagellates and a few ciliates, such as Stentor, were conspicuous. Perennials such as Polyarthra trigla, Keratella coch- learis, and Codonella cratera were scarce. Synura uvella was character- istic, often appearing late in January and remaining until April or May. Occasionally Notholca striata and Cyclops bicuspidatus, which properly belonged to the vernal socies, appeared in January, but they were never abundant or conspicuous until later in the spring. The plankton seasonals probably appear much earlier in the south than in the north, although no evidence has been published on this point. From data collected by the writer for the Illinois State Natural History Survey from Horseshoe Lake near Cairo, Illinois, 250 miles south of most of the areas reported in this paper, there is very little evidence of a hiemal socies. Hiemal forms such as Stentor spp. and Synura uvella are present during the month of January only, and the vernal socies has begun to be established by the first of February. Temperature is undoubtedly the most important factor limiting the hiemal socies. The temperature of all the waters studied in winter remained at 0.5° C. when covered with ice during January and February. Tests made through the ice always showed that there was sufficient dissolved oxygen to support plankton. The turbidity was usually low, and the pH value never fell more than slightly in winter. Light is well recognized as one important factor which is limited in winter, not only by the shorter days but also by the greater angle of incidence. With the increase in temperature in the latter part of February, the perennials increased in abundance, and in March the seasonals of the vernal socies usually appeared. In the park ponds at Decatur and Ur- bana, the vernal community was well established by the end of February. Key to Sympots UseEp IN Fic. 1 POY 0:2 sos ates ae Polyarthra trigla Ehr. SEO sce ste tarot Scenedesmus quadricauda RA COCH ier. Pe Keratella cochlearis (Gosse) (Turp.) Bréb. COD Oy) scieee nicer Codonella cratera (Leidy) Vorce PH. LONG....... Phacus longicauda (Ehr.)Duj. ee Rn eae oe Cyclops bicuspidatus Claus BY ANG). ciate facts Brachionus angularis Gosse RRS So vdtera'b «10% Difflugia lobostoma Leidy DIAPER of ujen etece Diaphanosoma brachyurum 2 Pe See Synura uvella Ehr. (Liéven) INOUE 2 Gs aise ales Notholca striata (O.F.M.) GERAT a. 6 paleriies Ceratium hirundinella O.F.M. KCI AD . 5560: s\eve Keratella quadrata (O.F.M.) BIDUNIA SS ois. a Filinia longiseta (Ehr.) POAT oo cs snis Brachionus calyciflorus Pallas ANS Ee ee sk hrera re Asplanchna spp. SS Cs Aa Synchaeta spp. BOS wis oc eseieree Bosmina longirostris O.F.M. ee GRAIN isis ete: ays Lysigonium (Melosira) granu- DIAPT.......... Diaptomus siciloides Lillje. latum (Ehr.) Kuntze WHOM VEG Halas ates Euglena oxyuris Schmarda DLS SR rere Pediastrum duplex Meyen APMIS Jer es stores asatala Tintinnidium fluviatilis Stein CARN cis. cierae «cas Cyclops viridis Jurine PEDAL 3. oa sient Pedalia mira (Hudson) EAC. Gio. oscars Closterium acerosum (Schrk.) TRACH. 22 okee8 Trachelomonas volvocina Ehr. Ehr. 1D 2d en a ee Leptodora kindtii (Focke) RDED essa 5. oo 4 aieve Eudorina elegans Ehr. 40 ILLINOIS BIOLOGICAL MONOGRAPHS [360 The characteristic rotifers Keratella quadrata and Notholca striata often appeared early and remained until May or June. Many of the hiemal forms persisted in small numbers through this period. Cyclops bicus- Bees vernals [ORS are WU FIGURE 2 Graphs showing relative volume of principal plankton groups in Lake Deca- tur, 1926, 1927, and 1928. (For graphical method, scale, and reduction, see text, page 37.) pidatus became the characteristic and abundant copepod. Daphmia longispina, Synchaeta pectinata, Conochiloides natans, Filima longiseta, and Pediastrum duplex usually appeared at this time and persisted through the summer. 361] FRESH-WATER PLANKTON COMMUNITIES—EDDY 41 WUITT LL Lore: zz flier: SS ANS SSS SSS ANAS aaa eee TRANS ea casceuascsucaees Z 7 , ‘ ? , Vernals eos i incidentals FIicureE 3 Graphs showing relative volume of principal plankton groups in the Park Pond at Decatur, 1926, 1927, and 1928. The blank spaces for January represent periods when the ice prevented collecting. (For graphical method, scale, and reduction, see text, page 37.) [362 es vos S ese SS SPE RENVMAL > ROR SLR REDD < ees oe ee 5 % <> see te Me ox esata OS SSK eenate OK IR See tess % RKO oan <5 RS ox oS > o CORRS eaten SRS 42 inois perennial he Ill dental organ- in t he volume of the inci ps that of the sp 1 plankton grou ible to add id). Ol lon, see text, page 37.) incipa Ficure 4 ive volume of pr imposs . , scale, and reduct it was and 1898 (data from Kof 1 method 1ca , 1896, 1897, anisms was so great that Graphs showing relat . (For graph iver org 1sms R 363] FRESH-WATER PLANKTON COMMUNITIES—EDDY 43 In May and June other plankton organisms appeared in abundance and formed an indefinite estival socies generally characterized by species of Brachionus. Diaphanosoma brachyurum, Diaptomus siciloides, Diap- tomus pallidus, and many chlorophyl-bearing flagellates appeared at this time. Many of the forms present then persisted until late in the autumn. “Uli ang \ \ \ \ N Le SS es7ivae verve La newwewrar E53 rena FiGcure 5 FIGurE 6 FIGurRE 7 Graphs showing relative volume of principal plankton groups in three bodies of water: Fic. 5, the Sangamon River at Monticello, Illinois, 1928; Fic. 6, a temporary pond near Seymour, Illinois, 1928; Fic. 7, a temporary ox-bow pond north of Urbana, Illinois, 1927 and 1928. (For graphical method, scale, and re- duction, see text, page 37.) In July and August the plankton is usually characterized by the ap- pearance of blue-green algae and the large cladoceran Leptodora kindtu. Increasing in abundance, these forms make up a serotinal community. There is no autumnal community such as has been demonstrated for land by Weese (1924), Smith (1928), and others. The organisms present in the estival and serotinal community usually persist through the autumn 44 ILLINOIS BIOLOGICAL MONOGRAPHS [364 and gradually decline as the water cools, until by the end of December, when the ice forms, the hiemal community develops. The seasonal communities for the most stable river community studied would be represented in the annual cycle of the plankton of the Illinois River. A study of Kofoid’s collections and data for the years 1896, 1897, and 1898 shows that the plankton was composed of various elements (Fig. 4). The perennials, although always present, were not constant in abundance. This group formed the greater bulk of the plankton. Season- al aspect was caused by the regular periodical appearance of certain organisms which were conspicuous enough to be termed seasonals. The remainder of the plankton was composed of organisms which appeared sporadically (incidentals) ; because of lack of space they are omitted from the figure. As this represents the most stable aquatic community studied, the most conspicuous perennials and seasonals are listed here: Perennial Predominants Cyclops bicuspidatus Lysigonium granulatum Asterionella gracillima Microcystis aeruginosa ; ' Pediastrum duplex Estival Predominants Difflugia lobostoma (March to December) Difflugia globulosa Closterium acerosum Trachelomonas volvocina Euglena oxyuris Phacus longicauda Euglena viridis Codonella cratera Brachionus angularis Polyarthra trigla Filinia longiseta Brachionus calyciflorus Asplanchna brightwellii Brachionus capsuliflorus Eudorina elegans Keratella cochlearis Tintinnidium fluviatilis Synchaeta stylata Conochiloides natans Synchaeta pectinata Daphnia longispina Bosmina longirostris Diaptomus pallidus Fiwal Peoinane Diaptomus siciloides (December to April) Synura uvella Stentor coeruleus Serotinal Predominants (July to September) Pedalia mira Vernal Predominants Diaphanosoma.brachyurum (February to June) Moina micrura Keratella quadrata Leptodora kindtii Notholca striata Euglena acus Many plankton species are not constant in their seasonal appearance. The late vernal and the estival are difficult to distinguish, for there is a continual arrival of species during the whole period. Often one very abundant species will drop out the following year or will change place 365] FRESH-WATER PLANKTON COMMUNITIES—EDDY 45 with some other form, so that the limits of the estival are doubtful and need further investigation. Some of the vernal predominants of the park pond at Decatur, such as Synchaeta pectinata, Brachionus calyciflorus, Filinia longiseta, and Conochiloides natans, were distinctly estival predominants in Lake Decatur. Species of Diaptomus, because of their erratic distribution, are very unreliable as seasonal indicators. In the Illinois River and in the park pond at Decatur, Diaptomus pallidus appeared regularly as an estival form, but no species of this genus was found in the park pond at Ur- bana. Diaptomus siciloides appeared as an estival form occasionally in the pool of the Sangamon at Decatur and regularly in the collections of Kofoid on the Illinois River, but it never appeared during the four years observations on the park pond at Decatur. As most of our waters are not stable, considerable shifting is to be expected. Also, immaturity of the communities causes many species to arrive later than they would in a body of older water. The plankton organisms in the writer’s collections from the park ponds at Decatur and Urbana and in Kofoid’s collections from the Illinois River showed much more regular distribution than the forms in Lake Decatur. The hiemal, early vernal, and serotinal communities are clearly defined by the characteristic organisms mentioned. Bennin (1926) found the annual cycie of the plankton in the Warthe to be divided into four seasonal groups as follows: February to April, May to June, July to September, October to January. These seasonal groups practically coincide with the vernal, estival, serotinal, and hiemal societies described in this paper. YouNnG STREAMS ” In young streams, where plankton does not begin to develop until the water is more than a week old, the only seasonal socies discovered in the plankton is an estival socies. For example, a scanty plankton first ap- peared in the course of the Sangamon River at Monticello, where the age of the water was about 9 days. This plankton was present only from May until October, and was composed of many organisms which are perennial under more stable conditions farther downstream. TEMPORARY PONDS Seasonal socies were hard to distinguish in the plankton element of the temporary ponds, where much of the plankton was composed of adventitious forms. No winter plankton existed, because the ponds were generally frozen solid during those months. When the ponds thawed in 46 ILLINOIS BIOLOGICAL MONOGRAPHS [366 February or March, many of the characteristic forms appeared and per- sisted until the ponds dried up in mid-summer. At this period a vernal socies can be determined by the presence of Notholca striata. Often other characteristic forms appeared, such as Diaptomus sanguineus and various species of Eubranchipus. Some hiemal species such as Synura uvella, which persist through the vernal socies in streams and perennial ponds, always occurred in the vernal socies of the temporary ponds showing that a hiemal socies would be present if it were not for the ice. The vernal forms usually disappeared in May, and except for the sum- mer appearance of chlorophyl-bearing flagellates there were no regular or characteristic species to designate estival or serotinal socies. The ponds were usually dry in August and September, filling again in October. Within two weeks after filling, most of the forms present before drying up were again abundant and remained until the ponds froze in December. In a strict sense, there are no true perennials in these ponds, as all or- ganisms are inactive during winter and mid-summer. The only organ- isms comparable to perennials are those which are present at all times when the ponds are not dry or frozen. GLACIAL LAKES The lack of seasonal data on glacial lakes makes it impossible to describe their seasonal communities definitely. Very deep lakes because of their lower and more uniform temperature throughout the year would not be expected to show much seasonal differentiation. Lake Michigan, because of its relative stability throughout the year, did not show any great seasonal differences in the plankton (Eddy, 1927). There was some indication that seasonal communities may exist in Lake Michigan from the collections made throughout the years 1887-1888 by the Illinois State Laboratory of Natural History. The occurrence of Daphnia long- ispina, Diaptomus sicilis, Dinobryon sertularia, Synchaeta stylata, and Diaphanosoma brachyurum, all in great abundance in summer, may indi- cate an estival or serotinal socies. A study of the seasonal data of a moderately deep glacial lake as presented by Birge and Juday (1922) shows very few seasonals among the conspicuous or predominant planktonts of Lake Mendota. Most of the predominants listed are perennials, and this may be due to the fact that the seasonal conditions of such a lake do not have as wide a range as those of smaller and shallower lakes. Vernal and autumnal forms are chiefly diatoms which appear and increase after the overturn of the thermocline. Estival predominants or prevalents are chiefly algae, par- ticularly species of Anabaena, Lyngbia, and Microcystis. 367] FRESH-WATER PLANKTON COMMUNITIES—EDDY 47 PLANKTON DEVELOPMENT AND RELATED FACTORS The development of the plankton communities studied differs from that of terrestrial communities in that it is a steady progression rather than a succession. The water of streams is motile and to a great extent carries the pelagic community along as it flows from one stage of river conditions to another. Consequently, at any given point in the stream, the plankton is constantly shifting downward, never containing the same set of individuals for any long period of time. This means that at any fixed point, as the water flows downstream, there must be a continual production and replacement of plankton. In the course of the stream, as soon as seasonal and other conditions permit, the first plankton appears as a scanty community; as the water proceeds, this community continues to develop, adding species, all of which, if conditions remain favorable, reproduce and increase in numbers, until eventually the community ap- proaches that of a stable stream. This is a progression of organisms rather than a succession in which one aggregation is succeeded by another entirely different. At any given point in a stream, it is apparent that plankton develops by a series of progressive stages rather than by a succession of forms. True succession in water, in so far as plankton is concerned, depends partly on the point of view as to what constitutes an aquatic habitat. If the moving stream is considered as a continually moving habitat, always created anew at the source and continually moving downstream, then it is conceivable that true plankton succession might occur under certain conditions. Such conditions might exist in a stream, such as the Mis- sissippi, which, flowing a great distance from north to south, passes through several sets of climatic conditions. Shifts in the climatic fac- tors, particularly temperature, may cause some predominants to drop out and to be replaced by others, thus constituting a true succession. Further study may show, however, that such a stream passes from one aquatic climax to another. On the other hand, if the habitat is considered as fixed along with the bottom (and there is some evidence to support this), then the develop- ment of plankton would not be in the course of the stream but at any fixed point, and here it seems to be a progression rather than a succession of predominants. Due to the fact that most streams pass through various stages of size and fall in development, it is necessary to study similar stages, found at present in the upper course, in order to determine the past development at any fixed point. However, such a series of longitu- dinal studies should be within the same climatic area. 48 ILLINOIS BIOLOGICAL MONOGRAPHS [368 In Lake Decatur as an example of newly formed bodies of water, plankton development year after year was found to be a steady progres- sion toward a stable community. There was a progression of forms from the time the lake was first studied in 1923, when it was one year old, until 1929, when the last collections were made (Table 19). Each year a few additional species appeared, and nearly all the species showed a tendency to increase in abundance as the lake became older. No species actually disappeared, although Pleodorina illinoisensis, noticeably abun- dant in the early summer of 1923, was never found to be abundant in following years. Plankton development is comparable to the invasion of barren areas by terrestrial forms. In waters where the plankton appears in only scanty numbers and for only a part of the year, it may be compared to plants in the desert which spring up during the rainy season. No other pelagic organisms previously occupied the waters of the streams studied, and there is no evidence that any plankton forms were actually succeeded by other planktonts. Therefore, in streams, there is no evidence of suc- cession of plankton communities comparable to the usual succession of terrestrial communities. The only such succession was that found in the pond sere, where the water was stationary and the trend was toward a terrestrial climax. The plankton of perennial ponds containing stable- river predominants was succeeded, as the ponds approached littoral con- ditions, by temporary-pond plankton characterized by temporary-pond predominants. As this succession is toward a terrestrial climax, it has little significance in the development of plankton communities. The invasion of new waters by plankton is dependent upon certain conditions which hasten or retard the development of the plankton. The most variable factors in all the waters studied by the writer were age, temperature, turbidity, and level of the water. Dissolved oxygen, hydro- gen-ion concentration, and light, although varying more or less in the different seasons, were always well within the limits of plankton re- quirements. Other chemical factors, though not yet accurately determined to any extent, probably play an important part, particularly in relation to the food supply. AGE OF WATER An opportunity to study the development of plankton in relation to age of water in the course of a stream was found when the Sangamon was dammed at Decatur in 1922. The old river channel from the source to the collecting station at Lost Bridge was 89 miles long. The formation of the lake retarded the waters so that they were many days older when they reached the station than before the dam was built. By computing the age of the water at different points in the lake and stream above, and 369] FRESH-WATER PLANKTON COMMUNITIES—EDDY 49 by tracing the stage of development of the plankton, it was possible to secure data on the age of the water at which plankton production started and the age at which it became heavy. The source of the Sangamon River is in a group of springs and ditches where the stream may be first distinguished 6 miles northwest of Fisher, Illinois. Throughout the course of the Sangamon from its source to the head of the lake at Decatur, there are constant additions from tributaries and springs. There is no information as to the exact amount of water added, but it probably is approximately as much as the water already in the stream at Fisher. Consequently, the actual age of all the water at a given point was impossible to determine accurately. One great difficulty lay in determining the velocity at which the water flowed through the lake. Because of the irregular contour, only approxi- mate results could be secured. The lake extends 10 miles above the col- lecting station and contains approximately 33 times the volume of water formerly in the old river channel. Hence, the lake is theoretically equiva- lent to 330 miles of river channel. It is practically impossible to measure the current as in a normal river channel, because it is perceptible only in the narrowest places, where bridges have been constructed. No doubt, the water in the center of the lake moves faster than that near the shores, but because of the bends and headlands, there is considerable in- termingling of shore and center waters. This was demonstrated in repeated collections in cross-sections where the only appreciable differ- ences in the plankton content were found in the vegetation close to the shore line. The winds cause considerable wave action on the lake and serve as a factor in keeping the waters well circulated. The river chan- nel extends for 79 miles above the head of the lake to its source. The total distance from the source to the collecting station at Lost Bridge was equivalent to 409 miles of river channel. As the velocity could not be measured in the lake it was necessary to assume that it was the same as that of the river above the lake. This was reasonable, as no appre- ciable difference could be found in the velocity of the river from Ma- homet to the head of the lake or in the river below the lake. The velocity was measured semi-monthly during 1928 at Mahomet and Monticello. In Table 17, there is some discrepancy in the averaged gage readings and velocities because the gage readings were made daily and the veloci- ties only semi-monthly. Occasional measurements were taken at Cerro Gordo and at Coulter’s Mill just above the lake. Broomstock floats were used, and care was taken to select quiet days when there was no wind interference. During flood stages in February, 1928, the water in the river just above the lake reached the greatest velocity recorded, 9,000 feet per 50 ILLINOIS BIOLOGICAL MONOGRAPHS [370 hour, and the age of the water at the Lost Bridge station on the lake was computed at 9 days and 21 hours. Usually, however, it took much longer to reach this station. The longest period, obtained by similar calcula- tions, was 173 days and 20 hours on October 1, 1928, when the velocity in the river was 511 feet per hour. During periods of normal level, a well-developed plankton first appeared in March at Rhea’s Bridge five miles below the head of the lake (Table 17). At the head of the lake only a scanty plankton occurred at the low-water stages. At Monticello from May to October, with a high temperature and low-water stage, a Ficure 8 Graphs showing relation between age of water and volume of plankton at four collecting stations on the Sangamon River, 1928. Government gage readings at Monticello are plotted above the graphs of relative volume of the plankton in each month. On the same diagram are plotted two curves showing for each month the point in the stream where the water had an estimated average age of 6 days and of 40 days, respectively. few plankton organisms were found in water averaging from 6 to 19 days old. The water was nearly always at least 20 days old before plankton organisms appeared in any abundance. Table 18, showing the progression of organisms in the river from Mahomet to Lost Bridge, was based on the June collections of 1928. Fig. 8 shows graphically the relative volume of the plankton in the Sangamon River each month between Lost Bridge and Monticello. The age of the water and the gage reading have been plotted above showing the correlation with the distribution of plankton. Bottom diatoms and 371] FRESH-WATER PLANKTON COMMUNITIES—EDDY 51 protozoans composed most of the catch in the plankton net at Mahomet. These forms decreased as the water proceeded downward, and in about 6 days a few planktonts appeared at Monticello. When the water had reached Rhea’s Bridge, about 20 days from the source, all these forms had increased in abundance, and seven new forms had appeared. At Lost Bridge, about 40 days from the source, thirteen more forms ap- peared, and nearly all had increased in abundance. In other months, there was some difference in the predominant species which appeared at Monticello and Rhea’s Bridge, but they were never many or abundant. According to Kofoid (1903), the Illinois River in low stage replaces its water between LaSalle and the mouth about every 23 days. The water in general at LaSalle may be safely assumed to be already about three weeks old. Therefore, most of the water at Havana, where Kofoid found abundant plankton, was at least 12 to 15 days old or more, even at flood stage, when the velocity was 9,000 feet per hour. Schroder (1897), working on the phytoplankton of the Oder River, first described the relation of plankton to the flow of water by stating that the amount of plankton in running water of the river is in inverse proportion to the slope of the river. Consequently, as a stream ap- proaches maturity, i.e., as it approaches a base level and its current be- comes slower, there should be a proportional increase in plankton pro- duction, and this condition of maturity may be hastened by artificially retarding the waters. Similarly, the farther a stream flows from its source and the more stable its conditions tend to become, the more de- veloped is the plankton element of the pelagic society. Kofoid from his work on the Illinois River concluded that the quantity of the plankton was, within certain limits, directly proportional to the age of the water. TEMPERATURE The fluctuation of river temperature constitutes one of the most marked evidences of climatic changes. The temperature runs through the same general annual cycle year after year with only minor or local variations. Temperature probably affects planktonts by retarding or ac- celerating their growth and reproduction (Kofoid, 1903), and it is partly responsible for the seasonal fluctuations in their abundance. Temperature also affects planktonts by causing variations in viscosity and density ( Wesenberg-Lund, 1908). In the tropics (Van Oye, 1926) where a more uniform temperature prevails throughout the year, temperature does not play an important part as a seasonal factor in plankton abundance. _ Water communities do not exhibit the extreme seasonal fluctuations to which land communities are subject. The seasonal change is gradual, with but little fluctuation. The waters of large streams, lakes, and ponds 52 ILLINOIS BIOLOGICAL MONOGRAPHS [372 studied by the writer did not show as much daily fluctuation in tempera- ture as was found in the temperature of the air. The deeper the water, the more uniform is the temperature from day to day. In Lake Michigan the water cooled more slowly than the air in the autumn and conse- quently was generally warmer; in the spring the reverse was true. Large bodies of water reach freezing temperature on the surface only, and plankton organisms underneath live at a temperature usually higher than that at which organisms live on land. Many species, such as the hiemal and vernal seasonals, show temperature preferences by becoming scarce or rare as the temperature passes certain limits. Others, chiefly the per- ennials, have a large degree of tolerance for temperature extremes, although they are seldom as abundant at the lower temperatures. The rate of fresh-water plankton reproduction—and consequently the abun- dance—at different seasons in the same body of water varies directly with the temperature. Similarly, plankton content varies in bodies of water differing in temperature, as will be discussed later. In the course of any given stream where there is little climatic variation from the source to the mouth, temperature differences are too slight to have much influence on the development of the plankton. VELOCITY AND WATER LEVEL Velocity of current is one of the important factors directly influencing plankton production. Water level, in itself, is not so important in regard to plankton as is the corresponding velocity. Velocity and water level are closely related; as the water level rises the velocity increases. At any given point in a stream, fluctuations of velocity result from fluctuations in water level. Velocity is usually higher in young streams, and the con- sequent erosion may increase the turbity and hence reduce the intensity of light. In older streams where velocity is usually lower, the turbidity may decrease and the intensity of light increase. Turbidity, by increasing as the level is raised, actually moves the point of plankton production farther downstream. Kofoid (1903) observed that streams with great velocity usually have more phytoplanktonts than zooplanktonts, and for this he advanced the explanation that the swift current prevents the zooplanktonts from feeding but does not have so much effect on the as- similation processes of the phytoplanktonts. Allen (1920) in his studies on the San Joaquin River concluded that water currents above a very moderate speed are inimical to plankton production. The writer has ob- served that small streams with a swift current, draining lakes or ponds containing an abundant plankton, carry little plankton themselves, and that what little they do carry comes originally from the source body 373] FRESH-WATER PLANKTON COMMUNITIES—EDDY 53 and tends to decrease rather than increase. Velocity is one of the im- portant factors controlling the age of the water and corresponding con- ditions of stability necessary for the production of plankton. Velocity, thus, largely determines the point in a stream at which plankton produc- tion starts, and this point fluctuates up or downstream according to the velocity of the current, being farthest downstream when the velocity is highest and farthest upstream when the velocity is lowest. TURBIDITY Turbidity, an important factor in reducing light and hindering the movements of many planktonts, is partly controlled by velocity of cur- rent, as previously stated. When the current is reduced as the stream approaches stability, there is more tendency for the suspended silt to settle to the bottom. When the turbidity was high in the Sangamon above Lake Decatur, there was a gradual reduction as the water travelled through the lake until at Lost Bridge the turbidity was seldom noticeable, and the plankton was more abundant there than above. In all the streams studied, the turbidity due to suspended silt was highest at the flood stages of spring and summer and lowest in winter when the streams were at normal level. In summer, when the current was usually slow and there was very little suspended silt in the water, a noticeable turbidity was often caused by the increased plankton content. The turbidity in the park ponds was usually high in summer, often giving the water a very muddy appearance although it was due entirely to the heavy plankton content. Suspended silt was practically absent from these ponds because of the lack of disturbing current. In all waters a high turbidity due to suspended silt retards plankton development, and the velocity and other conditions must be such as to reduce this turbidity to proper value before plankton production can be heavy. LiGHT Light, another important factor in plankton existence, especially in regard to the chlorophyl-bearing forms, has long been regarded as one of the factors limiting the vertical distribution of plankton. Aside from the differences produced by turbidity, depth, and seasons, light condi- tions were normal or about the same in most of the waters studied. The length of days in different seasons probably influences plankton abun- dance and may influence the appearance of seasonal predominants, par- ticularly algae. Kofoid (1903) and Allen (1920) both observed indica- tions of “lunar” pulses in the plankton and explained them on the basis of an increased amount of lunar light. The measurable differences, how- 54 ILLINOIS BIOLOGICAL MONOGRAPHS [374 ever, are too slight to be of much significance. In winter the ice reduces the amount of light in the water and no doubt is a factor in lowering plankton production. Intensity of light is reduced in the younger streams because of the silt produced by erosion. In more stable streams the set- tling out of the silt allows increased penetration of light and no. doubt plays a very considerable, though as yet undetermined, part in the de- velopment of plankton. CHEMICAL FACTORS Hydrogen-ion concentration was not an important factor in any water where plankton development was studied. The pH value of natural waters is indicative chiefly of relative acidity, which in turn is largely due to the relative amount of dissolved carbon dioxide, so that pH readings may be regarded as reflecting indirectly the CO, content (Birge and Juday, 1911; Shelford, 1929). The pH value of the water studied by the writer generally ranged from 7.8 in summer to 6.6 in winter, but the fluctuations were not always seasonal. In the park pond at Decatur and in other waters, most predominant planktons were found abundantly at all pH values within the range stated above, indicating that this range was well within the limits of tolerance for most of the planktonts studied. High hydrogen-ion concentration, such as pH 4.0, influences many plank- tonts, but none of the natural waters studied approached such an extreme value, except in swamps and bogs where some of the non-acid rotifers were absent. Harring and Myers (1928) note the absence of certain plankton rotifers from acid waters and state that the pH range for roti- fers is as a rule from 2 to 3 units pH. No marked difference in pH values occurred in the waters of the Sangamon River at the points studied for plankton development, and evidently this factor did not play any part in plankton production in this stream. The dissolved oxygen in the waters studied, as previously mentioned, was never found to be below the requirements of the plankton. Usually it ran higher in the colder months, since the water can hold more gases in solution at lower temperatures. As the plankton, in general, was most abundant in summer when the dissolved oxygen content was lowest, it seems that the dissolved oxygen played little part in seasonal distribution in the shallow waters. Although the writer made no studies on oxygen conditions in deep lakes in the different seasons, these conditions no doubt have more significance there, in regard to seasonal distribution, than in shallow lakes. Birge and Juday (1911) found that the dissolved oxygen content in the surface waters of Lake Mendota was sufficient at all times of the year, but in the deeper waters it was often insufficient, because of the seasonal stratification. In the study of the development of plankton 375] FRESH-WATER PLANKTON COMMUNITIES—EDDY 55 in the Sangamon River no marked differences were found in the amount of dissolved oxygen at different points in the stream, and it is reasonable to believe that this factor, being sufficient for ordinary requirements, had little influence on the development of the plankton. Very few data are available on the dissolved salts and other sub- stances in the waters studied. The samples that were analyzed indicated that there were considerable differences. For example, the waters of the park ponds contained more than 10 times as much chlorine in the form of chlorides as did the Sangamon River. Griffith (1923) has shown that waters free from calcium carbonate produce more desmids than waters bearing this salt. In December, 1928, the Illinois State Water Survey, at the writer’s request, made chemical analyses of samples from two stations on the Sangamon River and from the park ponds at De- catur and Urbana, with results as shown below in terms of chlorine in chlorides, expressed in parts per million: Satleamom iniver ati Monticello seca cicseeiieie ents thes ciereleleeialnieue 2 Pike susctiir: at Lost: Bridges : oy so suisd clea sean pals ew daes aeers 4 Decatur sParks Pond soe. 6 cae: clay Peer eco uniele aialoato aie ieee 30 lasbyertacap leet Otley ictsvciaier ude tel cis boharak oro uavedaaes eerie tat ocaanlscaie cn alia ve hee 23 The relatively great amounts of dissolved salts indicated by these analyses of the pond waters. may be partly responsible for the heavier plankton in these ponds. In general, it is probable that chemical factors play an essential part in development of plankton. Water flowing down a stream- bed has increasing opportunities to dissolve both organic and inorganic substances, some of which no doubt are made available as food to plank- ton organisms by baeterial action. Griffith (1923) concluded that the presence of plankton depends on the occurrence of suspended organic matter, the decomposition of which provides necessary food materials, and that the amount of plankton depends on the amount of products of fermentation. Pearsall (1922) showed that the plankton increases when there is an increase in certain salts and organic matter. This is in harmony with the observed fact that plankton is more abundant in natural waters of some age, which are obviously more suitable for plankton occupation than juvenile waters near their sources. The determination of the sepa- rate and combined effects of such factors is an indispensable step in understanding the phenomena of plankton production and constitutes a very important field for future investigation. BIoLoGICcAL FAcTors Plankton development requires, first, that the water must be old enough to allow time for the planktonts to grow and reproduce, and 56 ILLINOIS BIOLOGICAL MONOGRAPHS [376 that in streams the current must be slow enough to enable the zooplank- tonts to feed (Kofoid, 1903). Little is known about the rate of repro- duction or length of time necessary for the embryological development of plankton organisms, but these biological factors certainly play some part in determining the point at which any given organisms appear in the course of a stream. Food requirements are at present little understood. Food may not be as important a factor in the distribution of plankton organisms as it is in the distribution of some terrestrial forms. Phyto- planktonts and chlorophyl-bearing zooplanktonts, for example, by utiliz- ing raw materials, are less dependent on the food-factor than are the other zooplanktonts. Even the latter, moreover, are often found where there is little evidence of the phytoplanktonts on which it has been as- sumed that they feed. This indicates that they, too, may utilize other materials. In view of the present status of the question regarding the utilization of the organic content of waters by plankton organisms (Birge and Juday, 1926), the writer prefers to leave this matter for further in- vestigation. INTERRELATIONS OF FACTORS Hydrogen ion concentration and the dissolved oxygen content being usually favorable, as in the waters studied, the important factors ob- served in the production of plankton are temperature and velocity of the water. Temperature controls the rate of vital processes involved in growth and reproduction. Velocity of current is a factor in age of water and must allow time for the organisms to develop and multiply and possibly for suitable conditions of nutrition to be established. Slow cur- rent also permits the zooplanktonts to feed more freely. As the velocity decreases, the turbidity is lowered and more light penetrates the water to greater depths. When all these conditions are favorable, plankton pro- duction conceivably may continue to increase until it automatically checks itself, not only by exhausting the food supply, but also by causing such a high turbidity that the amount of light penetrating the water is in- sufficient for further growth and reproduction. This hypothesis would explain such facts as were observed in the park pond at Decatur, where extreme turbidity was caused by heavy zooplankton production and the ‘algae, being most dependent on abundant light, were scarce. Juday and Wagner (1909) have suggested that plankton may become so abundant that it becomes detrimental to other organisms by the consumption of oxygen through decay. Also it is conceivable that the oxygen content might be depleted by the demand of super-abundant plankton to the point when it might form a check on the development of the plankton. 377] FRESH-WATER PLANKTON COMMUNITIES—EDDY 57 GEOGRAPHICAL DISTRIBUTION AND ECOLOGICAL CLASSIFICATION Very little is known about the plankton organisms in many parts of the world. In vast areas, particularly South America and Australia, our information is limited to a few reports on several groups of plankton organisms. Daday’s reports on some of the plankton organisms from Patagonia (1902) and on the fresh-water organisms of Paraguay (1905) (Lemmermann, 1910) include many forms which are predominant, or prevalent, in the waters studied in this paper. Juday (1915), in a study of some lakes in Central America, found many species which are com- mon here. West (1909), on algae, and Playfair (1912 and 1919), on plankton and dinoflagellates, show that the plankton of Australian waters contains many of the same species in these groups as found elsewhere. Similar results are shown in the papers of Daday (1907), West (1907), and Cunnington (1920) on groups of plankton organisms from African lakes. Van Oye (1926) found a heavy stream plankton in the Ruki in Belgian Congo, which contained many of our common river predomi- nants. Pernod and Schréter (1924) have shown that many of our pre- dominant species of Cladocera are distributed in eastern and southern Asia. The predominants reported by Lemmermann (1907a) from the Yang-Tse-Kiang in China, although differing somewhat in species, are similar to those found in the relatively stable streams studied in this paper. Zacharias (1898, 1898a, and 1909), Lemmermann (1907b), and many workers since have contributed extensive data on the plankton of central European waters. Behning and his associates have done like- wise for the Volga and its tributaries, showing the wide distribution of many of the predominants studied in this paper and the limited distri- bution of other species, particularly those cladocerans and copepods which are not found in America. Wesenberg-Lund (1908), in an extensive geographical classification of plankton, shows that many species are cosmopolitan and others are strictly local. Species scattered commonly throughout the lakes of the arctic, temperate, and tropical regions include many Bacillariaceae, Cyanophyceae, Chlorophyceae, and Flagellata. Rotifers are especially conspicuous and include Keratella cochlearis, Keratella quadrata, Poly- arthra trigla, Asplanchna brightwellu, Triarthra longiseta, species of the genus Brachionus, and Pedalia mira. Other conspicuous forms are Daphnia longispina, Bosmina longirostris, Chydorus sphaericus, Cyclops viridis, and Cyclops leuckarti. The species of the Diaptomidae especially show differentiation in different regions, but the cosmopolitan species were all found to be predominants in the waters studied in this paper. 58 ILLINOIS BIOLOGICAL MONOGRAPHS [378 All the conclusions reached in this paper in regard to communities are based on plankton evidence only. The question whether various aquatic communities are of formational or associational rank, cannot be answered definitely until the bottom organisms and fishes are studied in relation to communities throughout large areas. If the cosmopolitan predominants in the plankton were accepted as stenoecious species, they would indicate that there is but one fresh-water association in the world. Inasmuch, however, as the fishes—many of which are dominant—and also the bottom organisms—some of which may be dominant—do not show this wide-spread distribution, it may be better to consider the pre- dominant fishes and bottom organisms as stenoecious species determining associational boundaries. Although without much evidence, the writer is inclined to believe that a study of the latter species will show that they define a number of aquatic associations in various parts of the world. Some investigators may consider the cosmopolitan planktonts as compa- rable to the wide-spread soil bacteria, algae, and protozoans (Sandon, 1927). Studies of these minor terrestrial forms, to determine their com- munity distribution and community structure, may show that they occupy the same status on land as is occupied by plankton in water. In view of the results of this investigation it is to be expected, in general, that an abundant and relatively permanent plankton should occur in any sluggish stream which presents the proper conditions of size, flow, and fluctuation, the other factors being favorable as in all the bodies of water studied. Such streams are: 1.—Streams in which natural obstructions render the current slow, as in the Mississippi River where Lake Pepin is formed by the delta of the Chippewa River. 2.—Streams with little fall and slow current, resulting from the occu- pation of ancient and well-worn preglacial channels, such as the Illinois River throughout most of its course. 3.—Streams flowing into the sea and practically at base level through- out most of their course, so that conditions are almost as stable as in large lakes. On the other hand, streams which are at base level near their mouth only, as is the case of the Mississippi, cannot maintain a heavy plankton because the conditions in the upper part are such as to cause a heavy dis- charge of silt which is detrimental to plankton. A summary of the predominant or prevalent species in the various types of waters studied is given in Table 20, showing that there are, in general, four types of plankton communities in the upper Mississippi Valley, which may be distinguished as follows: 379] FRESH-WATER PLANKTON COMMUNITIES—EDDY 59 1.—In rivers and related waters exhibiting some degree of stability, the conspicuous predominants are seven species of rotifers (four of which belong to the genus Brachionus, two to Synchaeta, and one to Filinia) and the cladoceran Moina micrura. These with other predomi- nants characterize the pelagic society of stable rivers and the socies of related ponds and shallow lakes. In younger streams or wherever the conditions are less stable, the socies is characterized by the same species, but to a lesser degree; that is, they are not as abundant and may not all be present at the same time. 2.—In temporary ponds and littoral parts of stable waters with vege- tation, the socies is characterized by the copepod Cyclops serrulatus and the cladocerans Camptocercus rectirostris and several species of Simo- cephalus. These predominants, together with other characteristic species, such as Daphnia pulex, Moina brachiata, and bottom rotifers of the genera Monostyla and Lepadella, may not all be present in a single col- lection, but they will appear often enough in a series of collections to give a characteristic aspect to the plankton. 3.—In deep glacial lakes the socies is characterized by predominants such as the rotifer Notholca longispina, the copepods Diaptomus minutus and Epischura lacustris, the cladocerans Daphmia retrocurva and Bos- mina longispina, and the diatom Striatella fenestrata. At least three of these, and often all four, appeared in every collection examined by the writer. 4.—In moderately deep and shallow glacial lakes the pelagic socies is characterized by the copepod Diaptomus oregonensis and by abundant populations of pelagic diatoms and blue-green algae. The predominants characteristic of deeper glacial lakes are absent or scarce, and the plank- ton more nearly resembles that of stable rivers. Analysis of the plankton of Russian waters as described by Behn- ing (1913, 1921, 1926) and of many other European waters as described by a large number of investigators in the past forty years, shows these same general groups of plankton communities characterized by the same predominants. Certain predominants, such as Cyclops viridis, Cyclops bicuspidatus, Daphnia longispina, Notholca striata, Keratella cochlearts, and Polyarthra trigla, are common in all three types of plankton com- munities. Other predominants, Pediastrum duplex, Diaphanosoma brachy- urum, and Codonella cratera, are most abundant in stable rivers and re- lated waters, but are also found in small numbers in other communities. All the predominants mentioned with the exception of the copepods have been found by the writer in plankton collections from similar bodies of water in many parts of the United States. 60 ILLINOIS BIOLOGICAL MONOGRAPHS [380 Fresh-water plankton should be classified ecologically as the pelagic stratal society or socies of the community of a stream or lake. All the plankton communities studied by the writer showed a common ecological relationship by virtue of their possession of common predominants (Table 21), some of which are seasonals and other perennials, and by virtue of their development from common or related sources. Conse- quently, these plankton communities are considered as belonging to an Mri x Chimax Terrestrial Cornminty Stable Stream Raa Brachionus # 3p. Yinia lorgiseta Abandoned haters IU OR?. 2sp. Brachionus 45p. SURE R Ieee hae Filinia longiset. NMolne micrura Efe K naa Cyclops viridis SUCEEGCES | 25 Diaphanosoma brachyurs Cyclops viridis LDiaphanosorna brachywrutn Polyarthra- keratella Femperary Fond Polyarthra - Keratella Simocephalus 25. Carnptocercus eared seas, clops bicusplidarus Foe tive: tenth Partly Stoble Stream Cyclops viridis paeiay fonus Isp. gq Filinia longiseta Cyclops viridis Polyarthra- kératella ir /tnpounded Waters Artificial or Natural Brochionds 45. Shallaw Lake Filinia longiseta Brachionus 4sp. Synchaeta 2s5p Filia longiseta Cyclops ywiridis Synehaeta 2 sp. Diaphanosoma wh kal Moiha mierura Polyarthra - Keratella lg ne kde very Unstable Stream Polyerthra- Keratella Na Plankton Ficure 9 Diagram showing developmental relations and general trend of plankton com- munities in the region of Illinois, as characterized by some of their common pre- dominants. association, the climax of which is the stable-stream community. Com- munities other than those of stable streams either are developing toward this climax or, if they occupy abandoned areas of streams or lakes, are developing toward a terrestrial climax (Fig. 9). The Plankton of a com- munity which is not mature, therefore, must be designated as a pelagic stratal socies rather than a society, since the latter term should be applied only to the plankton of the stable-stream community. 381] FRESH-WATER PLANKTON COMMUNITIES—EDDY 61 SUMMARY The present study, based on more than 2,000 collections of plankton from streams, lakes, and ponds (mostly in the United States), has shown that the plankton element of fresh-water communities consists of organ- isms whose behavior is comparable in certain respects to that of the organisms of terrestrial communities. Some species which are conspicu- ous or abundant may be called predominants or prevalents, in the same sense as these terms are used for the abundant organisms of terrestrial communities. Some predominants are perennials and others seasonals. Some predominants are common in all kinds of relatively stable fresh- water communities and indicate an associational and formation-like structure comparable to that of terrestrial associations and formations, in which the predominants are of two classes, those which characterize the particular association and those which characterize the formation and serve as binding species. In all the streams studied the plankton element gave evidence that stream communities—inasmuch as they belong to an aquatic association, the climax of which is a stable-river community—should be considered as separate from terrestrial communities. Some predominants which are seasonal in the early stages of plankton development in a stream become perennials when the stream presents more stable conditions throughout the year. There is a progressive development of the plankton element in streams comparable to the invasion of the barren area by terrestrial communities. This begins with the first evidence of stable conditions in the course of the stream in summer. Downstream, as the conditions become more stable, there is an increase in the number of species and the abundance of each. The most mature communities studied were found to contain about forty predominant organisms. Apparently, the number of peren- nials increases as the climax is approached. The predominants in perennial ponds and shallow lakes are the same as the predominants in stable rivers. Many such ponds or lakes are abandoned parts of streams, and others which are not in any way con- nected with streams but contain similar predominants, may be con- sidered as natural or artificial reproductions of abandoned parts of streams. The communities of these bodies of water are retrograding from the aquatic climax and are at some stage of succession towards a terrestrial climax and, hence, properly belong to a terrestrial association. The last aquatic stage in the succession of pond communities toward a terrestrial climax is found in temporary ponds which contain a scanty plankton characterized by several predominants partly littoral in origin. 62 ILLINOIS BIOLOGICAL MONOGRAPHS [382 Three seasonal groups, or socies—hiemal, vernal, and serotinal—and possibly a fourth, estival, can be distinguished in the plankton of shallow lakes and streams of Illinois and are characterized respectively by seasonal predominants. The important factors influencing the development of plankton are age of water (i.e., distance from source ~ velocity), temperature, and turbidity. In the streams studied, other factors such as light, dissolved oxygen, and hydrogen ion concentration seemed to be always sufficient to meet the requirements of the plankton. Observations on the plankton of water of different ages showed that, all other factors being favorable, a few plankton organisms usually appeared in water 6-10 days from its source, while an abundant plankton appeared in water 20 days or more from its source. In the bodies of water studied, there are, in general, four types of plankton society or socies which may be characterized by their predomi- nants as follows: 1.—Rivers and related waters exhibiting some degree of stability: four species of Brachionus, two of Synchaeta, Filinia longiseta, and Moa micrura. 2.—Deep lakes: Notholca longispina, Striatella fenestrata, Daphnia retrocurva, Bosmina longispina, and Diaptomus minutus. 3.—Temporary ponds:. Cyclops serrulatus, Camptocercus rectirosiris, and two species of Simocephalus. 4.—Moderately deep and shallow glacial lakes: Diaptomus oregon- ensis, pelagic diatoms, and blue-green algae. If the vagile elements of the pelagic portion of the community and the bottom society or socies show similar differences in various areas, these aquatic communities are of associational rank. FRESH-WATER PLANKTON COMMUNITIES—EDDY 63 CHECK LIST OF NAMES OF SPECIES 383] Key: Acroperus harpae Baird............. (@! Waona aftinis, (Leydig)... 5 s.<<.0i Cl Anabaena circinalis (Kiitz.) Hansg...Al Anabaena spiroides Lemm........... Al Anuraea aculeata Ehr. = Keratella tps tly ae ICON SI) eee Ro Anuraea cochlearis Gosse = Kera- tella cochlearis (Gosse)........ Ro Aphanizomenon flos-aquae (Linn.) ‘Eg is. | \ e eeeee e Al IOCADSAl SD; <.2\e)s\<'s,010 0) © s.0'n:sis,e 4.0.5 Al OO Ue 2 re Pr Asplanchna brightwellii Gosse....... Ro Asplanchna priodonta Gosse......... Ro Asterionella gracillima Heiberg...... Al Bosmina coregoni Norman & Brady..Cl Bosmina longirostris (O.F.M.)....... Cl Bosmina longispina Leydig........... Cl Brachionus angularis Gosse......... Ro Brachionus budapestinensis Daday...Ro Brachionus calyciflorus Pallas....... Ro Brachionus capsuliflorus Pallas...... Ro Brachionus havanaensis Rousselet...Ro Brachionus patulus O.F.M........... Ro Camptocercus rectirostris Schoedler. .Cl GarmhocatnptusrSpe.c. cece <2 onelare else's we Co Centropyxis aculeata Stein.......... Pr Ceratium hirundinella O.F.M......... Pr Ceriodaphnia lacustris Birge......... Cl Ceriodaphnia pulchella Sars......... Cl Chilodon cucullus O.F.M............. Pe CHEOGCOCEISWSD7.. dasie flea sieals ce wciae Al Chydorus sphaericus (O.F.M.)....... Gl Closterium acerosum (Schrk.) Ehr...Al Closterium moniliferum (Bory) Ehr..Al Codonella cratera (Leidy) Vorce....Pr Coelosphaerium naegelianum Unger..Al Goleps:. hirtis “Baye. esas. Os velo d Pr Conochiloides natans (Sel.)......... Ro Canochilus volvox: Ebr.) 2.3%.6./00 4 Ro Scisaaa rit Stipa sissies erie tice dee Al EEMETO EIA SPDs aise.sints te elk orale tio save Al Cyclops bicuspidatus Claus....... Co Ceclaps leuckartt Clans cosa o 225.04 sic Co Cyclops oithonoides Sars............ Co Cyclops serrulatus Fischer........... Co Cyclops strenuus Fischer............ Co Cyclops viridis Jurme.........5-0.68 Co Daphnia longispina (O.F.M.)........ Cl Daphnia longispina var. cucullata BAGS. oi caste sete he hae hed Se meals Cl Al=Algae; Cl=Cladocera; Co=Copepoda; Pr=Protozoa; Ro=Rotatoria. Daphnia longispina var. hyalina RG VOe fo cy hu enchinntand cone saan Cl Daphnia pulex (de Geer)........«... Cl Daphnia retrocurva Forbes.......... Cl Diaphanosoma brachyurum (Liéven) .Cl Se HEIe cia oovectreitcid see mee Mee Cl Diaptomus ashlandi Marsh.......... Co Diaptomus. gracilis, Sars... ... 00.5... - Co Diaptomus graciloides Sars.......... Co Diaptomus leptopus Forbes.......... Co Diaptomus minutus Lillje........... Co Diaptomus oregonensis Lillje........ Co Diaptomus pallidus Herrick......... Co Diaptomus sanguineus Forbes....... Co Diaptomus shoshone Forbes......... Co Diaptomus sicilis Forbes............ Co Diaptomus siciloides Lillje.......... Co Diaptomus vulgaris Schmeil......... Co Diffogiavacuminata Ebr... o2s)d.5.2 Pr Difflugia globulosa Duj.............. Pr Difflugia lobostoma Leidy........... Pr Difflugia pyriformis Perty........... Pr Dinobryon sertularia Ehr............ 1205 Epischura lacustris Forbes.......... Co Epischura nevadensis Lillje.......... Co fudorma, elevans Ebr... e002 - 085 Pr Buglenavacus” Bht cncussoe ste nee ete Pr Euglena acutissima Lemm............ Pr Euglena oxyuris Schmarda.......... Pr Euglena spiroides Lemm............. 1B BPnglena viridis Kar, 30. 6. e octene Pr Pilinia loneisetaCEht.): .225.0 soe Ro Fragilaria crotonensis (Edw.) Kitton. Al Glenindinivea Spies t s/s chia soa cele ae Pr Gyrosigma acuminatum (Kitz.)Cl...Al Keratella cochlearis (Gosse)........ Ro Keratella quadrata (O.F.M.)........ Ro Lecane ungulata (Gosse)............ Ro Lecquereusia epistomium Penard..... Pr Lepadella acuminata (Ehr.)......... Ro Leptodora kindtii (Focke)........... Cl Limnocalanus macrurus Sars........ Co Lysigonium (Melosira) granulatum (Bir, ). ghz ete ott wae ae age 8 Al Lysigonium (Melosira) varians Coe ais Yaigice a ate atare le cas age ae Al Microcystis aeruginosa Kutz......... Al Moina brachiata (Jurine)........... Cl Mois: smicritte, Wuirz sae soba t a oe ce Cl 64 ILLINOIS BIOLOGICAL MONOGRAPHS Navicula spp. ..---eeecsereeeceeetees Al Nitzschia sigmoidea (Nitz.) W. Sm..Al Notholca longispina Kellicott........ Ro Notholca striata (O.F.M.)........-- Ro Oscillatoria "Spay lis foie se ales ss eine Al Paramoecium bursaria Ehr.......... Pr Pedalia mira (Hudson).........---> Ro Pediastrum duplex Meyen..........- Al Peridinium Spp.....---e eee ee cree eeee Pr Peridinium tabulatum (Fhr.)........ Pr Phacus acuminata Stokes.........--- Pr Phacus longicauda (Ehr.) Duj....... Pr Phacus pleuronectes (O.F.M.) Duj...Pr Platyias quadricornis (Ehr.)........ Ro Pleodorina illinoisensis Kofoid...... Pie Pleuroxus denticulatus Birge......... Cl Polyarthra platyptera Ehr.= Poly- arthvatrigia HE. 2. eee ciel ie 5 oe Ro Polyarthra trigla Ehr............---- Ro Pompholyx complanta Gosse........ Ro Scapholeberis mucronata (O.F.M.)...Cl Scenedesmus quadricauda (Turp.) Simocephalus exspinosus (Koch)... HC [384 Simocephalus vetulus (O.F.M.)...... Cl Sphinctocystis eliptica (Kiuitz.) Kuntze. Al Sphinctocystis librilis (Ehr.) Hass...Al Staurasirum: ‘Spp.... ssc asia ieee ene Al Stentor Coeruleus Ehr............... Pr Striatella fenestrata (Kiitz.)......... Al Strombidumi sp!) 0-4 se eee eee PE Surirella robusta ‘Ehr.//)22 eee eee Al Synchaeta -pectinata Ehri2e ese eee Ro Synchaeta stylata Wierz.i2. (cae eneae Ro Synedra acus (Kutz) (Guaieee eee Al Synedra ulna ‘(Nitz.) Ebr ieepeeees Al Synura uvella, Hhr:. .. /c.caxieemeeeen Pr Testudinella patina (Hermann)...Ro Tintinnidium fluviatilis Stein........ Pr Trachelomonas hispida (Perty) Steins 6. 30S foe Se Pr Trachelomonas volvocina Ehr....... Pr Triarthra longiseta Ehr. = Filina longiseta’ (Ehr.)'.)..') 2) ohare Ro Trichotria (Dinocharis) tetractis (Ehr.) ‘Harring, 20.5. see Ro Trochosphaera aequatorialis Semper. Ro Volvox globator Leeuwennoek....... Pr FRESH-WATER PLANKTON COMMUNITIES—EDDY 65 385] *IayaUI DIqNd Jad QOT uULY} sso, = + . 4 : ee ee Peat sans ee: e: z zu Be sz, bee Eee ie] - weit t on MMi shes Sic eA a ann ees vat es ere oe «: ee estin tt TAMOMATd Re “OONWDON “NH SSS ea aca od 109 ~ Sn ian [-e) . _ . Shes TRAN a = Braet elas one eh ales ted a aicasieatt ea ana ocie i octpedictentantachictnte stentahanhe isis ea . N “NO CntMomaMNhnnm Ott nua w = . 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BT[IIIOS teres eeeeecesees sepsonqos BIsnwyiqd veteeererees sisniopMAles snuomseig ceeeeeererecsccee eS} BIYVWEA[Og vere eeeeees*PITIQOAIOA SEUOMIO[AYIEIL tee ereseseres *PSOUISNIO® SIFSADOIOIA soteds FRESH-WATER PLANKTON COMMUNITIES—EDDY (aque 19nd sag spuDsnoy [) SHILINAWWOD INAAAMAIG AHL AO NOILVIAY AHL ONIMOHS ‘(Lsnodny aNv ‘AN ‘ANO[) SHOVAHAY AAWWAS NO Gusvg ‘AaLV\\ JO SAIGOG AAILVINASAAdAY NI SINVNINOGIA NOLUNVIg JO ANNVGNNAY— |Z AAV] 405] [406 ILLINOIS BIOLOGICAL MONOGRAPHS 86 = FMA ANDAAOS . ‘Adanind N “N ewes ¢°7et'st L°8SL eeee eee oe o°esz L’OLV oe *Ja}IUI DIqnd Jad OOT UeYy ssay = + “syueuTMmOpeid puod Areioduia ls ‘syuvuTMOpald axe] [PNelD, a —————————— eee se eeee eee eae ween eee eee ee ee wees eee eee wee eee ee - . OMS Aaa Oo S wy = -cooooormroeoveeo i=) = TaMaMtS + -= Tee ep nD ines ERS Me Sapte Netley wo) Tips se eee eeeees puog puog uleval1S wes14S weal}S Are1odua [etuueieg suno,z poepunoduly 2421S (papnjauo0) )—17 ATAVL, *s}URUIMIOPeId IDATI 9[G23S_ sees Taialale i= rei=i*/9)5 TAETIOEY einyosidq See eeeeeee sees cnanurT snmojdeiq stresses es ssistauoseIO SnUI0JdeIG, tereeeeeeeesee ss raypyonal sdopAd *** *;euldsisuo] euruisog Fetes eee eee ss pamoaoIjeI eluydeq sect e twee ene * ,euIdsIsu0] eoTOYUION, SECC) (10 (= (os (0b fa) BUelISel yy Ce ace *sninzed snuomorig teeeeeeeses ssnqemoquep snxoino[qg seeeeeess epyeuoOIONU suaqeoydeas ee ee ‘ sTulOoLIpenb seléye[g trtereeeeeeeesesessyomnd eruydeq Seteeeee esses epapnTnoe elepedoT trreeeeesessppidsry SeuUOmIOPOYIeLL RRR) 11 |=. sn]eydss0ulis terse es “STIysOIl}eI snoJeo0jydule| teteeeeeeeeee ss ongemuitas sdopAsd Petes eeeeeeeese ss sSTIBZMA BI[IIVW ee ee | *SI[eUIDIIO euseqeuy tresses eeeeeeeesods esdeoousydy Pereeeeeesseees -Bapléqs BJBYOUAS cence rn sererce *eyeuTyood eyoeyouAS teteeeee esses *gaprojpis snurojdeiq trereeeeesees+ ss puidsisuoy emydeq ee ee *snpryjed snwojdeiq Pees e eee ees s-BIQUIpUNITY WINT}eIID Testes esse SISMOBURARY SNUOTYORIG STOIC MOISES OO yo) 18 e10pozyde] ee ee ee ee * ;PaNIOTUI euloyy satvedS 407] FRESH-WATER PLANKTON COMMUNITIES—EDDY 87 BIBLIOGRAPHY ALLEN, W. 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Animal Ecology of an Illinois Elm-Maple Forest. Ill. Biol. Monog., 9:7- 93. WESENBERG-LunpD, C. 1908. Plankton Investigations of the Danish Lakes. Copenhagen. 389 pp. West, G. S. 1907. Report on the Freshwater Algae including Phytoplankton of the third Tanganyika Expedition conducted by Dr. W. A. Cunnington. Jour. Linn. Soc., Bot., 38:81-194. WINELER, L. W. 1888. Die Bestimmung des im Wasser gelosten Sauerstoffes. Berichte d. deutsch. Gesellschaft, Jahrgang XXI, 2:2843-2845. WHIPPLE, G. C. : 1898. Classification of Lakes according to Temperature. Amer. Nat., 32:25-33. 1927. The Microscopy of Drinking Water. John Wiley and Sons, New York. 586 pp. ZACHARIAS, O. 1898. Das Potamoplankton. Zool. Anz., 21:41-48. 1898a. Das Heleoplankton, Zool. Anz., 21:24-32. 1909. Das Plankton. Leipzig, 213 pp. Arcella vul garis Bottom fauna INDEX (See also pages 63-86.) MPS rh seed 0% banig sa reitante. tones ee 34,57 Cunnington, W.A......... 19) 24 28R4A5048 9 Current). verges. ados oa os . | OARS Be Oe Senet 14,31,56,59 Cyclops bicuspidatus...15, 16, 18, 27, 31, 33 91 ik ks 12, 49, 56 edtdo ue. fees o, 17,5153 34, 35, 39, 40, 44, 59 210 BEE On ADEE EE oie eo 46 TELE 8s. ohenicoens Gly 32, 33, 34, rH EMILE ACINEQHE fo 65.5 <1 ties wes ols dle as 33 WEEBDEOLAES 25 5550/20) o\0\s nwlatile he Ve Anuraea cochlearis...........00000: 33 SCTLULGINS Sho Hate oohetatee le 24; 35% 30 Aphanizomenon flos-aquae............ 14 SFHEMMUSH r,t Aarsssicbal aster. a eee 33 ee Casa Rec as MOA a 35 viridis... .14, 16, 18, 31, 33, 34, 35, 57, 59 Bre Rotts x6 ot5 sna staves es ue Cypressswampsd f2o5 3 2 nett ees: OEE Ve Shales 16, 33,4457. Daday, Hoo Vote. gecvcsic toatl ss ed 57 PME Tu cs haters whee te ee 31,33 Daphnialongispina......15, 16, 31, 33, 34 Bs A tisfehey so Meee 7, 26, 58, 60 40, 44, 46, 57, 59 Astertonella gracillima. ...16, 19, 31, 34, 44 PUES Shwe ah alae 32, 33, 35, 59 PMA a Ee Nah atse tenes crcl shactass ote eharale 57 TEL OCCUPY UP ei A vag wnsiee 31, 34, 59 Dawson; (Lake 3/2502 1 ee as ae 25 2 ON ER ORE ETO 8 Decatur, Lake.......17,27, 45, 48, 53,55 CE Is es ee 57, 59 Deer Crabhe i230. Reh Ris ee PRE tt Apis Sisle Ae mea 5 Dtaphanosoma brachyurum.. .18, 33, 34, 35 euthoeplanktoney s 4). /s shee See eS! 8 43, 44, 46, 59 EPA TAGTOES 2 5 hog 25... aes Db 2 55 leuchtenbergianum.............. 33, 34 Birge, E. A., and Juday, C....... LADD OL. (Did peOmIiGdaeya sen. s,s csajlactoleaneactie 57 oi, 4G"54 . Diaptontusas deat ded ibn ess ood sue 45 BOSMtNG COVEgONt. .. 1.2.2 ee ea 33 GSHIENEE > 2) REO ee 31, 32, 34 / } 16, 18, 33, 35, 44, 57 GOUENEES hohe AH ad odo enka che aa 33 A A A hat Ar Ee 3. HEM 31, 59 GRICUOES.. 3) 2.5 Side ata LEY ea BEN Neate a hale vo te 27,43 PEPLODUE) h.050 3 had he Vel Haid eee .25, 32, 26, 28, 30, 36, 57, 59 PUB BES HE bos od bees oa dkde 31, 32, 59 eS eee ae 14, 16, 33, 44 OT ELBUCTISUS ow 8 ol hehe 31, 32, 33, 34, 59 budapestimensts : 2.020... .0.00..4: £5, 16 pallidus........... 16, 24, 32, 43, 44,45 } “14, 16, 18, 44, 45 IRIE LEEUIEES FEI Shogo said cal ween 46 Readat coshcnth el Uist ait ee ees 14, 16, 44 SHOSHONE Sos AS ot ee ee ae SHE AG d SRS asin Cee ee Ye 16 sictloides..........16, 18, 32, 43, 44, 45 Mey No ARP ARIE Se ALS SEEDER EN Poo bi wo sO Le ee, 40 Rite eet ee kee area 33 CEN a OL Rae Bae OPUS RRR 8 I Fs Dintaniss 95. Pls ie, 30, 50, 59 Paya ce eee 2 oh 21 Diufflugia globulosa..............16, 31, 44 peracid etnhadon.s sais. ao SIE 9 lobostoma...........+..14, 15, 16, 18, 44 Camptocercus rectirostris........... 27,59 Dinobryon sertularia............... 1; 46 Canmthacam pes 6 556 ee) wie ates Zigae. Dounnante, oo. oN es elon 9, 58 Ceratium hirundinella... . 16, 31, 33, 34, aa: Dptiglas Lalse cis...) nye 0 eee 31 SEE Cede Ere ie Siok ete oo Bddy, Sisk oe) coe T826, 2a Si 46 Brel eee lee dead acta Whe Sh): Bisbee drole 0s) appr eee) ee APE SB costs SOAS PO See ao) Batomostiaed: 27 ou) ood dosti IO pes 16, 27,31, 35,57 LEpischuralacustris........ 31, 32, 34, 59 oh es A Ny a WRN HY LM 14, 31, 59 NEVEMENSIS N08 Hi aS eh ee OS OL Reet 1 ca Sama eee 2,8 Brie bakes ncintas foods dows see eee RAE aire he RP Seem 66:47) 60:.. Eubranchipus:.- 3...) i20e.).(o0 2 4 Closterium acerosum............... 16,44 Eudorinaelegans............... 14, 16, 44 Codonella cratera............ PEELS Mupleda oo) Joe vedi ethos Saee 21 31, 39, 44, 49 AGUS IONS 5 RS ee 16, 44 Conochiloides natans......... 16, 40, 44, 45 ONYUPIS na cia eles MO ee Oe ote 16, 44 Conochilus volwox.\. 3s 248 ba Se 32, 33 WALES dk eS eh Ne 16, 44 Ned Ps Rae wee 14, 31,32,59 Euryoecious species................. 7 92 ILLINOIS BIOLOGICAL MONOGRAPHS Facultative planktonts.............. 8 Filinia longiseta. . .14, 16, 32, 40, 44, 45, 59 Finger Wakes iinet belay alekiowie maibiernts 31 BUS pee sete ee a a a oe alee Y Bada rules hie ea La ed 6 Forbes: Sil eye tiie cco pe enous (efeumeee 34 Ore A ele Gadi aie nate i i Rae a ve 22 Pox Riven cmn Wie cin eeu enacts uals 16 Fragilaria crotonensis.............. 31, 35 GaltsomiPaSe leh wc raiaepinys 14, 15,17, 19 Gary, ponds near..............-+-+- 34 Greatiwakes e708. ele tie) sce es 30, 31, 32 Green Wake ie iy Gece aia 22, 31, 32 Green|River (oss sls Me ee 21 Grif BID asa slats ae ee anette Una 55 Harring, H. K., and Myers, F.J..... 25, 54 Pleleoplamictony o aii). aie ie ois tips iaisea as 8 HRensem (Vie eo eae le cca aoe ates 7 Horseshoe Lake..............-... 25, 39 Hydrogen-ion concentration,.12, 18, 20, 23 24, 25, 26, a 39, 54, 56 Illinois-Mississippi Canal....... 17, 28, 44 Illinois State Natural History Survey. 39 Illinois State Water Survey.......... 55 Illinois River...... 14, 15; A, 37, 45, 51, e Ineidentalseia ic Meike ae ee ae Oe Min enerveS sire sie ce eile aaah abr etaee lat i Juaday, (Gils Me ANS ge. as ie ae 29, 32, 57 Juday, C., and Wagner, Geo.......... 56 Keokuk Dame ici icisiieiatele tao orks 19 Keokuk Wake ies aie ial sleikelns aap apaye 30 Weratelllay yeas au ied Qu ie ais alata 30 CCULERIA HRP ieee he aes aval 33 cochlearts.......... 14, 15, 16, 18, 31, 34 35, 36, 39, 44, 57, 59 GUCETONE ON e\e ciao 16, 31, 32, 40, 44, 57 Kofoid, C. A...... 14, 15, 21, 25, 27, 34, 44 45, 46, 51, 52, 53, 56 Kollewitze Roe ie A eh Kolkwitz, R.,and Marsson, M......... 8 FRGIOBSTS WV 2) oa ileal el a kia cha taeda en tal cs cial 8 Lakes, classification................. 29 physiographical study........... 10, 29 Lemmermann, E..............-+.+- 57 Lepadella ee re NTO dS iia 59 Leptodora kindtii........... 16, 33, 43, 44 Levels of water............. 20, 25, 50, 52 | ea 01 Bes a MER eae na MS ISO E 53 Lamnocalanus macrurus............ 31, 32 Limnoplankton..............------ Hohmann Pais see aahel nea eh uate 37 Buono Lake ym asl Beale 31 Teuhualke ood se beeen 36 Leyne ial sie Bie els x cusgevecuie ade rercsionelan ees 46 Lysigonium granulatum...... 15, 16, 18, 31 Melntyre Bakeries is Me where eee 25 Mia comiualke sy o)aie rears sevekeyeustercac dots gt haeste 2S [412 MarshiGa Dino eadieeneniee 22, 30, 31, 32 Mendota, Lake................ 31, 46, 54 Methods})0))'0 s.),¢.2 See eee 11 Michigan, Lake...... 31, 32, 34, 35, 46, 52 Microcystis aeruginosa... .14, 16, 18, 44, 46 Minnesota, Lakes................... 26 Mississippi River........ 14, 17, 30, 57, 58 Moma brachiata. . 05.2 eee oe 59 MVUCV UT. oo 0s) chau toie date ae 16, 44, 59 Monostyla ic. 24.00...) : te 27,59 LunarIs. 3). 22 a ee 35 Murray, James... 05...) 2.) see 26 Mysis. ci jcls4 eins + nag Ne ts Naumann Be oe) oe 8 Nordquist, Harald.:,.).,:../:..4. 090s 33 Notholca longispina......... 31, 32, 33, 59 SPIE Sie Mol at, 16, 21, 27, 31, 32, 39 40, 44, 46, 59 Obligoplankton.. 2... 2.77 .2/oaeeeeee 8 Oconomowoc Lake................ 31, 35 Oder. Riveri:. .is.05 . Wee ae ot Ohio River’. ). oo. eae Oxbow pond, Urbana................ Oxygen....... 12; 18, 23, 24, 27, 39, 54, 56 Paraguay. :.;sc-.sis« 3 sealer 57 Patagonia... .js..\s)5+ 4/1) 57 Pearsall, W.. He 2... 2 ake Pecatonica Rivers... jae Pedalia mira..........4. 15, 16, 33, 44, 37 Pediastrum duplex....... 14, 31, 40, 44, 59 Pepin, Wale? ./.t. ait. hs Sar ee 19, 30, 57 Pernod, M....))).5. 0.5 hs. ee 57 Perennials.) 2 aah ese 10, 21, 39, 44 Pewaukee Lakes. 0). 04.052). ae 36 Phacus longicauda..............-- 16, 44 Plankton, classification............. 8,57 COUNEING:: { .)s). 650.04). ee 11 definition. .......05) oo 0) J ae 7,8 development. :).....,..\i. «(cee 47 ecological classification............ 57 geographical distribution.......... 57 IE SSA PAM avails Sic 6 22,29 stable Streams... 5). 0.4 see 13 temporary ponds. . .......... 12,53 Me Ove se eis Kath he eS, 51557 WelOGiE ys River cic top Aer Sevseele os ate 52, 56 Wolgta Biwens 6 Oo ica Sa aici s ee em 57 Wabasht Riversic.).siin cece cid oe ales 16 AW EIN brenvenaatns ance iiel = ater eich ae. 31 Ward, H. B., and Whipple, G.C...... 34 Weese, A. O. Prato DAU A MN ae mam Paez yk 18, 43 Wesenberg-Lund, C............. 8,51, 57 Wests MGS en Ne TU ST ge 34, 57 Wihippler Ge Caan fone ios o35 28 22, 29 Warlclen tilde Wises cteiaictana Siete iaid sie. arkente oe 12 Winnebago, Lake.............. 2253132 SY Nob gl Ben cia RS eV ee aT A 24 Yellowstone Park..................- 34 VWioung Streams syc Nols ctl raks carotene = 45 Zacharias Osi d bars we ks svete oes 57 sha eds is beat ty, MEN hE RHA 5 AEs CU Riana eg eu UNIVERSITY OF ILLINOIS BULLETIN Vol, XXXI IssuED WEEKLY—JuLy 1, 1934 No. 45 A Study of Fresh-water Plankton Communities BY Ae SAMUEL Eppy | “9 0 PRICE $1.00 ILLINOIS BIOLOGICAL MONOGRAPHS Vol. XII No. 4 PUBLISHED BY THE UNIVERSITY OF ILLINOIS URBANA Entered as second-class matter, Dec. 11, 1912, at the post office at Urbana, Illinois, under the Act of August 24, 1912. Acceptance for mailing at the special rate of post- age provided for in section 1103, Act of October 3, 1917, authorized July 31, 1018. 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