Effect of the northern quahog Mercenaria mercenaria on the development of blooms of the brown tide alga Aureococcus anophagefferens

Three experiments were carried out in 300 l mesocosms using natural seawater from the Peconic Bays ecosystem, Long Island, New York, to examine the ability of the northern quahog Mercenaria mercenaria to prevent blooms of the brown tide alga Aureococcus anophagefferens. Nutrient enrichment and mixing of the mesocosms was conducted according to previous methods that we have employed to induce brown tides. Treatments with and without clams were examined. Abundances of A. anophagefferens increased dramatically during 8 to 9 d experiments in mesocosms without bivalves (average peak abundances >600 000 cells ml–1). The brown tide alga constituted >50% of the total phytoplankton biomass in these mesocosms by the end of the experiment. In contrast, algae in mesocosms with high abundances of clams did not develop brown tides and A. anophagefferens abundances in these mesocosms were 2 orders of magnitude lower. Bivalves not only prevented a buildup of total phytoplankton biomass but also prevented the shift in phytoplankton species composition to dominance by A. anophagefferens observed in treatments without clams. Experiments to test the efficacy of different abundances of clams for preventing blooms of A. anophagefferens demonstrated that population clearance rates by clams of approximately 40% of the mesocosm volume d–1 were sufficient to prevent the buildup of phytoplankton biomass and net population growth of the brown tide alga under the environmental conditions and nutrient enrichment that we employed. This turnover rate by suspension-feeding bivalves is similar to the same magnitude of bivalve filtration pressure estimated for Great South Bay, Long Island more than 2 decades ago, prior to the outbreak of brown tides. We conclude that the feeding activities of northern quahogs in shallow bays can exert considerable control on total phytoplankton biomass in the overlying water column, and specifically on the ability of A. anophagefferens to dominate the phytoplankton assemblage and form brown tides.