Minimum Nutrient Concentrations for Sustaining Marine Ecosystem Productivity: Measurement during in situ Growth Experiments of Bivalve and Sessile Organisms

The relationship between seawater nutrient concentrations and ecosystem production was studied by in situ growth experiments, which have been conducted along the nutrient concentration gradient from the eutrophic Osaka Bay head to the oligotrophic adjoining Harima Sea. Bivalves (asari clam Ruditapes philippinarum) were cultured in suspended cargo, and sessile organisms were collected on a suspended plate at each station. The distribution of the primary production was also calculated within the study area. The wet weight of the molluscous part and the condition index of asari clam are strongly correlated with the annual mean total nitrogen (TN). The primary production (PP) is significantly correlated with TN in the area of TN > 0.2 mg L. In contrast, PP is small and of a pelagic nature in the area of TN ≦ 0.2 mg L. The annual mean TN is strongly correlated with TP, and TN = 0.2 mg L is equivalent to TP = 0.029 mg L.