tenia and otlier fungi ; ami among tla- zooplankton, 

 all classes of Protozoa except Sixirozoa, Rotatoria, 

 Kntoniostraca (especially Cladocera. Copepoda, and 

 Ostracoda), some immature Diptcra, the statoblasts 

 and gemmules of bryozoans and sponges, the rare 

 fresh-water jellyfish, Crast^edacusta. and occasional 

 aquatic mites, gastrotrichs, and others. Fresh-water 

 plankton lack many forms common in the plankton 

 of the ocean. On the other hand, the rotifers, aquatic 

 insects, and water-mites are mostly absent from the 

 sea, and the Cladocera are only poorly represented. 

 It is probable that plankton evolved from benthonic 

 forms occurring near the shore (Ruttner 1933), and 

 many species of groups listed above, notably Ostra- 

 coda and Rotatoria, are still largely benthonic in be- 

 havior. 



The algae in fresh water may vary in numbers 

 from hundreds of thousands to tens of millions of 

 cells per liter ; Protozoa, from thousands to hundreds 

 of thousands of individuals per liter ; and the rotifers 

 and entomostracans, from less than ten to hundreds 

 per liter. 



Distribution 



Many species of plankton are nearly world- 

 wide in distribution, particularly those that occur in 

 the larger lakes. Cosmopolitan distribution and the 

 many primitive types of the plankton community in- 

 dicate that its origin is ancient. Some plankton, how- 

 ever, such as species of the genus Pseudodiaptomns, 

 have a very limited distribution. 



The plankton found in the open water of small 

 to medium-sized lakes is seldom more than one to 

 three species of copepods, two to four species of 

 cladocerans, and three to seven species of rotifers, 

 although the species change from one time of the 

 year to another. It is also unusual to find more than 

 one species of the same genus at the same time. 

 When two do occur, one of them is usually much 

 more abundant than the other. It is commonplace to 

 find that 80 per cent or more of all limnetic copepods 

 present belong to a single species : 78 per cent of all 

 cladocerans to a single species, and 64 per cent of 

 all rotifers to a single species (Pennak 1957). 



In any one lake the horizontal distribution of the 

 plankton may be irregular because of water currents, 

 inflowing streams, irregularity of shore line, or 

 swarming of a particular species in local areas. The 

 vertical variations in the composition and abundance 

 of species is even more striking. The chlorophyll- 

 bearing algae require light and are most numerous 

 in the upper stratum, although diatoms commonly 

 occur at greater depths (Fritsch 1931). The verti- 

 cal distribution of zooplankton varies widely with the 

 species, but it is strikingly affected by light, food, 



FIG. 6-5 Common invertebrates found in laites. (a) Copepod, 

 (b) cladoceran, (e) ostracod, (d) the snail Amnlcola //moso, 

 (e) the snail Volyafa iricarinafa, [i] the ghost larva Chaoborus 

 albipei, (g) the fingernail clam Pisidium. (Modified from various 

 sources, Pennak 1953.) 



gravity, dissolved gases, particularly oxygen, and 

 thermal stratification. Few zooplankton occur in the 

 hypolimnion of eutrophic lakes during the summer 

 stagnation period, but occur at all depths during the 

 spring and autumn overturns. 



Diel movements 



Several species of net zooplankton exhibit pro- 

 nounced vertical migrations, moving upward into 

 surface strata during the night and returning to 

 greater depths during the day. In some instances 

 this daily shifting of position may extend to 60 or 

 more meters, in other instances it may be only a frac- 

 tion of a meter, and some species do not exhibit the 

 phenomenon at all (Langford 1938). A common ex- 

 planation of these movements is that the animals are 

 negatively geotatic by nature, but that during the day 

 this drive is suppressed by a negative phototaxism 



Lakes 67 



