. Bulletin. Natural history; Natural history. 350 Illinois Natural History Survey Bulletin Vol. 32. Art. 4 Shelbyville all year, Cyclops vemalis was dicyclic in Lake Sangchris in late spring and autumn, -while Mesocyclops edax ap- peared in the heated reservoir 2 months earlier than in Lake Shelbyville. The ap- pearance of Macrocyclops albidus seemed random in both lakes, and thus no specific trends were associated with this species. These preliminary observations led me to conclude that the reproductive period for some Lake Sangchris zooplankters (mostly rotifers) appeared to increase or decr
. Bulletin. Natural history; Natural history. 350 Illinois Natural History Survey Bulletin Vol. 32. Art. 4 Shelbyville all year, Cyclops vemalis was dicyclic in Lake Sangchris in late spring and autumn, -while Mesocyclops edax ap- peared in the heated reservoir 2 months earlier than in Lake Shelbyville. The ap- pearance of Macrocyclops albidus seemed random in both lakes, and thus no specific trends were associated with this species. These preliminary observations led me to conclude that the reproductive period for some Lake Sangchris zooplankters (mostly rotifers) appeared to increase or decrease depending upon the temperature preference of the individual species. Increased growth periods could result in increased fecundity and produc- tion of zooplankters during normally nonreproductive periods, thus precipitating the development of a more substantial food base for newly hatched fry and young-of-the-year planktivorous fishes. Such data, combined with biomass estimates, would be of great practical im- portance to fishery scientists interested in creating conditions conducive to increased fish production in cooling reservoirs. Number of Species Many classical zooplankton studies have shown that the abundance of these organisms undergoes extensive seasonal variation. The typical curve of seasonal zooplankton abundance usually has a large spring pulse; a decreased population during the summer; and a second, less pronounced pulse in the fall, followed by a very small population in winter. Furthermore, the number of species may also follow a somewhat similar pattern unless the natural periodicity is modified or eliminated by various biological, chemical, or physical parameters. The mean numbers of zooplankton species in Lake Shelbyville are compared to the mean numbers in the cooling loop and control arm of Lake Sangchris from September 1975 to May 1977 (Fig. 2). As is typical of nonheated reservoirs in this region, the mean number of Lake Shelbyville species decreased
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