THE IMPORTANCE OF ROTIFERAN AND CRUSTACEAN ZOOPLANKTON AS GRAZERS OF ALGAL PRODUCTIVITY IN A FRESH-WATER ESTUARY

In situ grazing experiments were performed on five dates in summer 1990 at Old Woman Creek, a freshwater estuary in Ohio, U.S.A. The objective was to determine the absolute and relative importance of rotifers and crustaceans as grazers of algal productivity. The indigenous phytoplankton (primarily Cryptomonas, Cyclotella, Navicula, Rhodomonas and 3 to 6-mu-m nanoflagellates) was labelled with C-14 and used as the food source in the experiments. Mid-day zooplankton community filtration rates were among the highest reported, with herbivores clearing up to 73 % of the water column per hour. In May and June, nauplii and small cladocerans (Bosmina longirostris, Chydorus sphaericus and Moina micrura) accounted for most of community filtration, while rotifers (especially Brachionus, Keratella and Polyarthra) dominated filtration in July and August. In September, community filtration continued to be rotifer-dominated at two stations in the main creek channel of the estuary, while small cladocerans were more important at off-channel stations with reduced flushing. The results demonstrate the importance of rotifers and small r-selected cladocerans in the freshwater estuarine food web, where frequent flushing and intense fish predation may suppress large herbivores.