Influence of suspension-feeding bivalves on the pelagic food webs of shallow, coastal embayments

Bivalve and microzooplankton community grazing on natural plankton assemblages were studied in 2 embayments of the Peconic Bay estuary (West Neck Bay, Long Island, New York) to determine the relative importance of each community on plankton mortality. Each embayment was characterized by a fringing salt marsh. The bivalve populations consisted of a re-stocked population of Mercenaria mercenaria (hard clam) and natural populations of Getikensia deinissa (ribbed mussel) and Mya arenaria (soft-shell clam). Short-term (3 h) grazing experiments were conducted between May and October 2003 and 2004 using 59 1 plastic buckets mounted on a floating platform at each study site. The total grazing pressure on phytoplankton by bivalves (i.e. percent bay volume cleared per day) was substantial, sometimes approaching or exceeding the daily flushing rate of the embayment, and was similar to microzooplankton grazing pressure. Experimental studies also showed that bivalves removed ciliates and copepod eggs of Acartia tonsa from the plankton. At times, bivalve-induced mortality was likely an important regulatory factor on the zooplankton. These findings demonstrate a complex trophic role for benthic suspension-feeders in shallow, coastal embayments. The effects of restoring shellfish populations will likely be beneficial to overall water quality by ameliorating some effects of eutrophication, but the resultant changes in pelagic food web structure are complex and will be difficult to predict.