Effects of filter-feeding bivalves on the distribution of water quality and nutrient cycling in a eutrophic coastal lagoon

The horizontal variation of water quality was investigated through field surveys in a cross-section of Lake Shinji, the littoral region of which is densely populated by the filter-feeding bivalve, Corbicula japonica. The distributions of chlorophyll a (chl. a) and nutrients (NH4–N and PO4–P) in the lake are greatly affected by the filtration and excretion rates of C. japonica. Minimum values for chl. a and maximum values for nutrients were both found to occur in the littoral region during summer. Chl. a concentration peaked about 1 km from the shoreline, indicating that the presence of the bivalve not only reduces the biomass of phytoplankton in the near shore region through filtration, but also stimulates primary production further offshore by supplying nutrients through excretion. Calculated results of a mathematical model reproduced well the observed horizontal distributions of chl. a and nutrients, indicating that the interaction between the filter-feeding bivalve and phytoplankton populations dominates material cycling and water quality distribution in the lake. Water mass exchange between littoral and pelagic water enables the nutrient exchange between the bivalves in the benthic zone and the phytoplankton in the pelagic zone, thereby forming a benthic–pelagic zone coupling. Other related findings were that seasonal patterns of water quality variations in the north–south cross-section were affected by metabolic rates of C. japonica, and that differences in concentrations between pelagic and littoral regions are largest when water temperature is optimal for bivalve activity.