CAN SUSPENSION-FEEDING BY BIVALVES REGULATE PHYTOPLANKTON BIOMASS IN LAKE WACCAMAW, NORTH-CAROLINA

Suspension feeding by bivalves has been hypothesized to control phytop lankton biomass in shallow aquatic ecosystems. Lake Waccamaw, North Ca rolina, USA is a shallow lake with a diverse bivalve assemblage and lo w to moderate phytoplankton biomass levels. Filtration and ingestion r ates of two relatively abundant species in the lake, the endemic union id, Elliptio waccamawensis, and an introduced species, Corbicula flumi nea, were measured in experiments using natural phytoplankton for dura tions of 1 to 6 days. Measured filtration and ingestion rates averaged 1.78 and 1.12 l ind.(-1) d(-1), much too low to control phytoplankton at the observed phytoplankton biomass levels and growth sates. Measur ed ingestion rates averaged 4.80 and 1.50 mu g chlorophyll a ind.(-1) d(-1), too low to support individuals of either species. The abundance of benthic microalgae in Lake Waccamaw reaches 200 mg chlorophyll a m (-2) in the Littoral zone and averages almost an order of magnitude hi gher than depth-integrated phytoplankton chlorophyll a. Total microalg al biomass in the lake is therefore not controlled by suspension feedi ng by bivalves.