Zooplankton,and related phytoplankton cycles,in a eutrophic lake

The seasonal cycles of zooplankton were determined for 18 consecutive months in a sewage-enriched lake in northern Canada and were related to algal availability and utilization, food consumption, temperature and the density of predators. Most of the common species (Daphnia pulex, Daphnia middenorffiana, Keratella cochlearis, Keratella quadrata, Polyarthra vulgaris) increased in abundance in May and June, reached a mid-summer maximum, and declined sharply in the fall. Phytoplankton densities increased sharply in May, peaked early in June and gradually decreased through the summer and fall. Since the quantity of algae in the guts remained constant during this period, algal availability and utilization had no direct impact on the seasonal cycles of any species. Furthermore, the amount of ingested material in D. pulex and D. middendorffiana was similar regardless of time of year, implying that the total quantity of food in the environment did not restrict development. Although temperature was the most important factor influencing variations in the densities of all species, predation by Cyclops spp. probably had little effect on the population dynamics of the fauna.