DIEL CHANGES OF NITROGEN SPECIES IN SURFACE AND OVERLYING WATER OF AN ESTUARINE LAKE IN SUMMER - EVIDENCE FOR BENTHIC PELAGIC COUPLING

Diel changes in the concentration of nitrogen species [ammonium, nitrite, nitrate, dissolved organic N, and particulate organic N (PON)] and some biological parameters were monitored in the water column of Lake Shinji, a shallow estuarine lake in Japan. The sediment of the lake was densely inhabited by the filter-feeding bivalve Corbicula japonica. An overlying water sampler showed that PON at the sediment-water interface (less than a few millimeters above the sediment) was not depleted; nevertheless, active uptake of PON by C. japonica (10.4 mg-atoms N m(-2) d(-1)) is reported. Dissolved and particulate N species showed vertically uniform profiles at night, which we attributed to nocturnal water mixing associated with cooling surface water. These observations suggest benthic-pelagic coupling between filter-feeding bivalves and phytoplankton which enables the removal of nitrogen from the eutrophic lake through fisheries yield of bivalves.