The Dynamics of Aquatic Systems: A Comparative Study of the Structure of Four Zooplankton Communities

This study compares the structure of the limnetic zooplankton communities of four very different lakes ranging from an oligotrophic Great Lake to a small dystrophic reservoir. Only filter feeding cladocerans and calanoid copepods are considered. Using Levins' Theory of the Niche analysis, I measured habitat overlap values for each species pair to determine to what extent habitat selection can explain the number of limnetic zooplankton species coexisting in these lakes and to what extent other factors, such as resource allocation or differential predation, must be invoked to explain the observed diversity. A substantial portion of the ecological overlap of zooplankton populations can be alleviated by habitat selection. Zooplankton communities are dominated by two to three species, in both numerical abun- dance and grazing intensity. In Lake Michigan (large, oligotrophic), habitat selection was the major mechanism of competitive coexistence and it was achieved by extensive vertical migrations (mean a = .47). In Gull Lake (small, mesotrophic), habitat selection via vertical migration also predominated (mean a = .37) as the major mechanism of competitive coexis- tence. Three species, Diaptomus sp., Daphnia retrocurva, and Daphnia galeata exhibited high habitat overlap coefficients. In Cranberry Lake, a dystrophic reservoir, most species possessed high overlap coefficients (mean a = .62) and minimal migration amplitudes. Diaptomus spp. and Daphnia catawba predominated. In Lake George, a medium-sized oligo- trophic lake, mean habitat overlap was also high (mean a = .65). The three dominant species were Diaptomus sp., Diaphanosoma leuchtenbergianum and Daphina galeata. For the latter two communities, which possessed similar high and mean overlap values, the basic equation was weighted by the standing crop and turnover rate of phytoplankton. The spatial-temporal positioning of the Lake George populations were correlated with algal standing crop and turnover patterns while the Cranberry Lake populations were not.