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Which nutrients limit primary production in coastal marine environments? Using large (13 000 1) mesocosms with sediments we explored nutrient limitation in Narragansett Bay, Rhode Island, USA, over a 9 wk period. Separate and combined additions of phosphorus and nitrogen were made to the enclosures. Phytoplankton biomass, daytime whole system oxygen production and nighttime whole system respiration showed an approximately 5-fold increase in nitrogen and nitrogen + phosphorus treatments, whereas phosphorus treatments had somewhat lower phytoplankton biomass and metabolism than controls. In these whole system experiments nitrogen was the nutrient most limiting to primary production.

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Gross sedimentation of l4C labelled carbon was 58 % greater in mesocosms (13 m3) containing the bivalve Mercenaria mercenana (16 ind m-2) relative to controls without this filter feeder. This difference was attributable 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 benthc community respectively. Permanent biodeposition by the clams contributed the least (21 O h ) . 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 dyes 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

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