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Gross sedimentation of 14C labelled carbon was 58% greater in mesocosms (13 m3) containing the bivalve Mercenaria mercenaria (16 in m-2) relative to controls without this filter feeder. This difference was attributed to the activities of M. mercenaria and presumably due to filtration of particles from the water column. Of this increase, 32% and 47% were attributable to assimilation into clam tissue and respiration by the benthic community respectively. Permanent biodeposition by the clams contributed the least (21%). The ability of 8 filtration rate models to predict the increase in gross sedimentation was examined. Those models (4) which were based on data for bivalves filtering natural suspensions of particulate matter gave estimates which agreed well with observed differences. Those models (4) which yielded poor predictions used dues or algal monocultures to generate data and overestimated gross sedimentation due to bivalves by up to an order of magnitude. Such overestimation may exaggerate the role of bivalves in enhancing sedimentation and controlling phytoplankton biomass in shallow waters.