[A-CG40] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG40-P05] Analysis of differences in nutrient concentrations and composition ratios among rivers flowing into Tokyo Bay

*Issei Murabe¹, Yu Umezawa¹, Ryoutarou Ueba¹, Hiroya Kondo¹, Satoshi Nakada², Ryo Sugimoto³ (1.Tokyo University of Agriculture and Technology, 2.National Institute for Environmental Studies, 3.Fukui Prefectural University)

Although water quality in Japan's inner bays has improved since 1979 due to water quality regulations, red tides still occur in the inner part of Tokyo Bay, and it is essential to understand the mechanism of red tides. It is known that nutrient concentrations and composition ratios affect phytoplankton growth rates, biomass, and dominant species. However, water quality surveys conducted by public agencies in representative rivers and coastal areas lack data on some nutrients, including silicate (DSi), and the accuracy of analysis of low-concentration samples is poor. In this study, nutrient surveys including DSi, groundwater inflow, cations and anions, and fluorescent dissolved organic matter (FDOM) were conducted in five rivers (Arakawa River, Sumida River, Tama River, Edo River, and Nakagawa River). The nutrient concentrations and composition ratios of each river, and the differences in characteristics of each river were clarified, and the contribution of these differences to the ecosystem change in the inner part of Tokyo Bay was discussed by analyzing the factors that contribute to these differences.

The analysis revealed that the Arakawa, Sumida, and Tama Rivers showed a marked increase in nutrient concentrations and FDOM concentrations toward the estuary. These results indicate that these three rivers are heavily influenced by sewage treatment effluent, which increases nutrient concentrations and contributes to algal blooms at the estuaries. The Edo and Nakagawa Rivers, which are part of the Tone River water system, showed high DSi concentrations in the upper reaches, and the ratio of DSi to dissolved inorganic nitrogen was also found to be proportionally high in the river water. This reflects the geology of the Tone River basin, and these rivers contribute to the growth of diatoms in Tokyo Bay.