The abundance of microzooplankton and their grazing impact on phytoplankton were studied using the dilution technique from May 1990 to November 1991 in northern Hiroshima Bay, a typical eutrophic area in the Seto Inland Sea. Microzooplankton, dominated in number by tintinnid ciliates, were abundant from June to September when chlorophyll-a concentrations were high. Maximum density of microzooplankton ranged from 3.8โข to 3 1 25.4x10 ind 1-. During the period of investigation, mean microzooplankton density and mean chlorophyll-a concentration of the <20-gin fraction increased toward the inner region of the bay. The microzooplankton grazing on phytoplankton increased from summer to early autumn, and decreased from late autumn to winter. At an offshore station, the annual means of the daily grazing loss for total chlorophyll-a and the chlorophyll-a of the <20%tm fraction were 12 and 15% of the initial standing stock, respectively. At an estuarine station, the microzooplankton grazed 19 and 29% of the total and <20-~tm initial standing stock, respectively. The quantity of grazed chlorophylla correlated positively and linearly with the potential production of chlorophyll-a at both stations. The quantity of chlorophyll-a grazed by microzooplankton and the potential production of chlorophyll-a were nearly equivalent in the <20-gm fraction at the estuarine station. This suggests that the microzooplankton assemblage was able to consume almost all the nanoplankton newly produced in the eutrophic estuary.
Various field investigations and laboratory experiments have demonstrated that these ciliates can grow rapidly, occur in abundance, and feed efficiently on nanoplankton (Beers et al.