NITROGEN-METABOLISM OF THE FILTER-FEEDING BIVALVE CORBICULA-JAPONICA AND ITS SIGNIFICANCE IN PRIMARY PRODUCTION OF A BRACKISH LAKE IN JAPAN

Uptake of particulate nitrogen and nutrient excretion by the filter-feeding bivalve Corbicula japonica in Lake Shinji was measured simultaneously by two methods: a continuous flow laboratory incubation with an intact sediment and an in situ measurement with a benthic chamber. The excretion rate of ammonium by C japonica estimated from the in situ measurement exceeded the uptake rate of particulate organic nitrogen (PON), suggesting the involvement of bioturbation or other nonmetabolic process in the upward flux of ammonium. On the other hand, the results from the continuous incubation method agreed well with the growth rate of C. japonica under natural conditions. In the natural habitat of Lake Shinji, C. japonica excreted 4.5 mg-atoms N m-2 d-1 as ammonium and 4.6 mg-atoms N m-2 d-1 as feces and pseudofeces while taking up PON at a rate of 10.4 mg-atoms N m-2 d-1. These results suggest that C japonica plays an important role in stimulating primary production by maintaining rapid recycling of nitrogenous materials.