[M-IS22] Biogeochemistry

[MIS22-09] Analysis of factors contributing to the variation in nutrient concentration and composition among the rivers in the Tokyo metropolitan area

Issei Murabe¹, *Yu Umezawa¹, Ryoutarou Ueba¹, Yuto Tashiro², Satoshi Nakada³, Chikage Yoshimizu⁴, Ichiro Tayasu⁴ (1.Tokyo University of Agriculture and Technology, 2.Akita Prefectural University, 3.National Institute for Environmental Studies, 4.Research Institute for Humanity and Nature)

Keywords:river water quality, treated sewage water, stable nitrogen isotopes, CDOM, Tokyo Ba, red tide

Although water quality in Japan's inner bays has improved since 1979 due to water quality regulations (areawide total pollutant load control), red tides still occur in the inner part of Tokyo Bay, and it is essential to understand the mechanism of red tides. The incubation experiments with nutrients by Hidaka et al. (2022) indicated that the limiting nutrient on phytoplankton abundance and growth in Tokyo Bay fluctuate seasonally, suggesting the importance of understanding the seasonal dynamics of nutrients supplied by the rivers.
The objective of this study was to investigate nutrient concentrations and composition ratios in five representative rivers (Arakawa Riv., Sumida Riv., Tama Riv., Edo Riv., and Nakagawa Riv.) to clarify the factors contributing to nutrient fluctuations using δ¹⁵N (nitrogen stable isotope ratio) and CDOM (colored dissolved organic matter), which indicate contributions of sewage effluent in river water.
The analysis using QGIS (geographic information system) showed that the DSi concentration in each river was defined by the geology of the catchment area and the river or dam that supplied the water to the water purification plant, consequently the treated sewage effluent. The increasing [DIN], [DIP], δ¹⁵N -NO₃, CDOM, and decreasing [DIN/DIP] toward the downstream area indicated that the contribution of treated sewage water largely determines the nutrient concentration and composition ratio in the river. Different from rivers in the Kyushu and Tohoku regions, the seasonal variation of DSi/DIN in the middle reaches of rivers and tidal areas tended to be higher in the rainy season (summer) and lower in the dry season (winter), which was considered to be a trend unique to urban rivers with a significant contribution of treated sewage water. Seasonal variations in nutrient concentrations and their composition in the river well explained the seasonal variations in nutrients that limit phytoplankton growth in the estuarine waters.