a marine fauna that happened to get cut off from the 

 sea in some past geological period, yet was able to 

 survive as the water became fresh. 



Numbers and species of fish are more concen- 

 trated in the littoral zone of lakes than in the open, 

 deeper waters of the limnetic zone. Limnetic species, 

 however, invade shallow waters for spawning. In 

 deep waters, fish tend to remain close to the bottom, 

 where their food supply is located, unless there is a 

 deficiency in oxygen there. Caged fish, lowered to 

 various depths in a eutrophic lake, did not survive 

 long below the thermocline (Smith 1925). Fish may, 

 however, make short excursions into the hypolim- 

 nion. 



In a study of fishes in six Wisconsin lakes, Pearse 

 (1934) found that Usually most fishes per unit area 

 occur in muddy, vegetation-filled, shallow ponds, but 

 the characteristic fishes (carp, crap pie, sunfish, dog- 

 fish) are not the most desirable jar food. Rich eu- 

 trophic lakes produce considerable quantities of desir- 

 able fishes (perch, largcmouth bass, white bass, rock 

 bass). Oligotrophic lakes produce littoral game fish 

 of good quality and size (smallmouth bass, wall-eyed 

 pike, pickerel) and ciscoes in deep zvater. The aver- 

 age catch with gill nets in two oligotrophic lakes was 

 3.5 per hour ; in two eutrophic lakes, 4.2 per hour ; 

 and in two shallow lakes or ponds, 5.1 per hour. 



In the littoral zone, fish species are segregated 

 according to the composition of the bottom, as are the 

 invertebrates. The species living over rock and gravel 

 bottoms in lakes are mostly different from those in- 

 habiting similar bottoms in streams, but the mud 

 bottom forms are nearly the same as in ponds (Shel- 

 ford and Boesel 1942, Nash 1950). 



Amphibians and reptiles do not commonly occur 

 in lakes except around margins supporting attached 

 aquatic vegetation, and here pond species occur. Such 

 pond mammals as the muskrat, mink, and otter are 

 not typical of lakes as such, although they are fre- 

 quently found in shallow littoral waters. There are 

 a number of bird species, however, that occur most 

 commonly in lakes: American and red-breasted 

 mergansers, loons, pelicans, cormorants, terns, gulls, 

 ospreys, bald eagles, and swallows. These species 

 get their living from the lake, but nest on neighboring 

 shores or islands. In addition, there are many pond 

 and marsh birds that occur along vegetated lake 

 margins. 



FOOD CYCLE 



The lake is a closely knit ecosystem whose 

 inhabitants are largely independent of the rest of the 

 world but very much dependent on each other for 

 existence. It is almost a microcosm in itself (Forbes 

 1887), but it depends on the insolation of the sun 



for energy, rain and snow for water supply, and on 

 minerals dissolved out of the surrounding uplands 

 for the basic nutrient salts essential to the formation 

 and functioning of protoplasm. 



Basic to this food cycle are the bacteria. A few 

 bacteria occur free-floating in the water. For the 

 most part, however, they are either attached to algae, 

 to other plankton organisms, to submerged objects, 

 or occur on the bottom as part of the benthos (Hen- 

 rici 1939). Their number varies from one place and 

 time to another, as do the numbers of other organ- 

 isms ; they are more abundant in eutrophic than 

 oligotrophic lakes. Their action is to transform the 

 dead organic matter into nutrients, especially ni- 

 trates, that the green plants then absorb. 



The phytoplankton are the next link in the food 

 cycle because of their ability to manufacture carbo- 

 hydrates with the aid of sunlight and to anabolize 

 proteins after absorbing nitrogen and other com- 

 pounds dissolved in the water. Rooted vegetation 

 around the lake margin is important in this respect, 

 although in large lakes the proportion of food sub- 

 stances formed by marginal vegetation is small as 

 compared to the amount manufactured by phyto- 

 plankton. In Wisconsin lakes, the daily production 

 of glucose during clear days in August varies from 

 14 to 44 kg per hectare (12 to 39 lb/acre) (Manning 

 and Juday 1941). 



Zooplankton feed upon phytoplankton. Protozoa, 

 bacteria, detritus, and each other. Some species ap- 

 pear to discriminate in their choice of food, but most 

 species filter out and ingest all particulate matter, 

 within size limits, non-living as well as living, with 

 which they come in contact. The ratio of number 

 of entomostraca to number of phytoplankton cells 

 has been found to vary from 1:1,800 to 1:63,000. 

 Ratios of rotifers to phytoplankton vary from 1 :50 

 to 1:37,500 (Pennak 1946). The plant cells, how- 

 ever, are much smaller than individual animals. The 

 mean ratio of zooplankton to phytoplankton by vol- 

 ume is commonly about 1:4 (Davis 1958), but in 

 alpine and northern oligotrophic lakes, the ratio may 

 be reversed (Pennak 1955, Rawson 1956). In the 

 nannoplankton. Protozoa depend largely upon bac- 

 teria, although some forms feed also on algae and 

 detritus ; a few species prey chiefly upon other proto- 

 zoans (Picken 1937). 



When the plankton dies, it settles to the bottom 

 and furnishes food for the benthos. The accumula- 

 tion of dead plankton and other aquatic organisms 

 on the bottom may be extensive enough to form a dis- 

 tinctive brownish layer. The benthic midge fly and 

 other insect larvae, annelids, clams including the 

 sphaeriids, snails, and bottom-dwelling entomostraca 

 feed on this detritus layer, on organic matter held in 

 suspension, and on algal plankton and attached forms. 



The variety of their food habits is reflected in 



74 Habitats, communities, succession 



