An assessment of the zooplankton annual succession in a hypereutrophic southern Minnesota lake utilizing two sampling methods

Zooplankton samples were obtained utilizing two sampling methods to determine: 1) the annual succession of zooplankton taxa; and; 2) correlation between samples obtained by a simple composite sampler and a Birge/Juday trap sampler. Duck lake-small, hyper-eutrophic, with simple basin-had a early summer littoral zone dominated by Potamogetan crispus macrophytes. It was reasoned that the abrupt transition of large-bodied cladacera (primarily Daphinia) populations to small-bodied cladacera and copepod was due to centarchid (panfish) predation following the rapid senescence of Potamegeton crispus. After the loss of ideal macrophyte habitat (covering 50% of lake by surface area), the centarchids likely migrated to deep water where feeding upon large bodied zooplankton commenced at a great rate. This conclusion, while not a part of the original scope of sampling, was based upon the timing and thoroughness of the two events. It was assumed the rapidity of large-bodied zooplankton decline could not be explained by documented factors of temperature, food availability, or loss of hypolimnetic refuge alone. In addition, simultaneous sampling by composite sampler (two meter PVC tube at surface) and a Birge/Juday sampler (at discrete depths) allowed for sampler comparison. A high degree of correlation was established both at the surface strata and at depths below two meters (due to homogenous distribution of zooplankton).