Role of the bivalveCorbicula japonica in the nitrogen cycle in a mesohaline lagoon

The effect of the bivalveCorbicula japonica on the nitrogen cycle in Lake Shinji, a mesohaline brackish lagoon in Japan, was examined quantitatively based on field surveys and laboratory experiments carried out in the summer of 1982 and 1983. The biomass of the flesh ofC. japonica comprised 97% of the total biomass of the macrozoobenthos in summer. Total biomass ofC. japonica in the lake was estimated 30 986 t fresh wt. The concentration of suspended solids immediately above the lake bottom, whereC. japonica filters the water, was 1.5 to 4 times higher than that in the surface water. In laboratory experiments, the filtration rate was 5.0 litres g dry flesh wt^-1 h^-1, the excretion rates of ammonia and of feces and pseudofeces ofC. japonica were 200×10^-6 g N g dry flesh wt^-1 h^-1 and 33.4 mg dry wt g dry flesh wt^-1 h^-1, respectively at 27°C, the average summer water temperature in the lake. From our study, we estimated thatC. japonica filters almost the same amounts of particulate organic nitrogen produced in the lake by phytoplankton and that supplied from rivers, and that it excretes 30% of filtered nitrogen as feces or pseudofeces and 18% as ammonia.