ZOOPLANKTON DYNAMICS IN A FRESH-WATER ESTUARY

The zooplankton of Old Woman Creek Estuary, Ohio U.S.A., was sampled twice monthly from May to September 1990. The objectives were to quantify community structure, and to assess population and community responses to natural variations in residence time and water level. Rotifers (Polyarthra, Brachionus spp. and Keratella cochlearis), nauplii and small cladocerans (Bosmina longirostris and Moina micrura) were numerically dominant. Rotifers and tychoplanktonic taxa were especially dominant after storms and during periods of low and variable water level. Zooplankton densities were high, coincident with a high biomass of nano (< 20-mu-m) algae, and the numerical dominance by small herbivores which were readily available to carnivorous species. There was considerable variation in zooplankton community structure among several stations in the estuary. A downstream station near the mouth contained a mixture of lake and estuary species during seiches, and a vegetated station showed the greatest richness of littoral cladocerans. Stations located off the main creek channel were least affected by water fluxes. Frequent flushing events, high silt levels and fish predation were likely the major factors controlling zooplankton dominance in this freshwater estuary.