Water Quality of Rainwater and Its Impact on Primary Production in the Osaka Bay

Abstract

A nutrient load by rainfall has an important role in the primary production in an enclosed bay. We made analyses of rainwater qualities around the Osaka Bay, and calculations for the impact of the precipitation on the water quality and primary production in the Osaka Bay, using a 3D hydrodynamic water-quality model. The rainwater had high concentration of T-N about 0.7mg/L on average for observed rains. The total precipitation loads of T-N, T-P were 1, 300, 11 ton/year, which are only about 3% and 0.4% of total inflow load in the Osaka Bay, respectively. However, calculation results show that the precipitation make phytoplankton biomass increase in the surface layer about 3μg/L in the southwest area of the bay, where the tidal exchange is small and the DIN concentration is very low.