BACTERIVORY BY SESTONIC PROTISTS IN A SOUTHEASTERN BLACKWATER RIVER

As part of a study examining the trophic base of secondary production in the Ogeechee River, Georgia, bacterial grazing by sestonic protists was measured directly with fluorescently labeled bacteria. For the period April 1988 through September 1989, flagellates and ciliates ingested an average of 18.6 (range, 1.7-43) and 314 (range, 25-1,140) bacteria individual-1 h-1. These grazing rates are comparable to those of protists in marine and lentic habitats. When combined with bacterial and protist densities in the river, an average of 15.6% (range, 0.2-59%) of the water column was cleared by protists per day. On the basis of literature values of microbial C content and assimilation efficiencies, this percentage translates to a production rate of 11.7-mu-g C liter-1 d-1. Protist bacterivory can facilitate carbon flow to higher trophic levels by turning bacteria into larger parcels of carbon and nutrients that are then available to a greater portion of the filter-feeding community. The microbial loop in this lotic system differs from pelagic systems; bacteria are derived from extemal sources, and bacteria and protists are directly consumed by macroinvertebrates.