Seasonal change in a filter-feeding bivalve Musculista senhousia population of a eutrophic estuarine lagoon

Filter-feeding bivalves often predominate the benthic biomass of estuaries, although their population size may drastically fluctuate due to physical and biological disturbances. To examine the recovery of a mussel population after periods of severe predation and anoxia, and to estimate the amount of nutrients removed from the system through mussel production, we surveyed, over 2 years, the Musculista senhousia population in the estuarine lagoon, Lake Nakaumi, Japan. Predation by diving ducks (Aythya fuligula, Aythya ferina and Aythya marila) during winter dramatically reduced the mussel biomass in both years, but recruitment of juvenile mussels sustained the population. Anoxia during the second summer severely reduced the mussel population, resulting in less biomass than in the autumn of the previous year. Potential annual removal of nitrogen and phosphorus from the lagoon water through burial of M. senhousia shells under oxic conditions was estimated to be 7.1 and 5.1 tons, respectively. These are equivalent to 0.7% and 4.9% of the nitrogen and phosphorous annual load entering the lagoon via the main river. Under anoxic conditions, removal would decrease to only 5.6% of the potential amount.