FATE OF BACTERIAL PRODUCTION IN A HETEROTROPHIC ECOSYSTEM - GRAZING BY PROTISTS AND METAZOANS IN THE HUDSON ESTUARY

The Hudson River Estuary is a heterotrophic ecosystem with high rates of planktonic bacterial production. We measured grazing on bacteria in monthly experiments during 1990 to determine: (1) the primary consumers of bacteria; (2) whether absolute rates of grazing on bacteria were high relative to other systems; and (3) whether bacteria were a sufficient carbon resource for consumers. Fluorescent minicells were used to measure the ingestion of bacteria by heterotrophic flagellates and ciliates. Using the abundances of bacteria, rotifers and cladocerans in the river, we also estimated zooplankton clearance rates. Total grazing of the plankton community was assessed by measuring the disappearance of fluorescent minicells over 24 and 48 h. Heterotrophic flagellates were the primary consumers of bacteria with ciliates and the cladoceran Bosmina longirostris also important seasonally. Ingestion of bacteria by heterotrophic flagellates, ciliates, and B. longirostris was sufficient at most times to satisfy estimated carbon requirements of these consumers. Large bacterial size and relatively low consumer populations in the Hudson account for the apparent sufficiency of bacteria as a carbon source. We conclude that grazing on bacteria in the Hudson River is similar to the levels of grazing observed in other systems. Nevertheless, bacteria are a potentially significant resource, because the dominant planktonic consumers are capable of ingesting bacteria at rates sufficient to support carbon demands.