Long-term trends in eutrophication in the inner part of Tokyo Bay, Japan, from 1998 to 2023

Nutrient loading is a critical issue in coastal waters that leads to eutrophication and anoxic water masses and impacts production in fisheries. In this study, therefore, long-term and seasonal variations in the eutrophication index (EI) using dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and chemical oxygen demand (COD) were investigated from 1998 to 2023 in the inner part of Tokyo Bay. Furthermore, the contribution of DIN, DIP, and COD to eutrophication was analyzed. The DIN concentrations showed a significant long-term decreasing trend, whereas DIP and COD showed no trend. EI also showed no long-term trend. The DIN and DIP concentrations showed a significant negative correlation with salinity, suggesting that the DIN and DIP concentrations in the inner part of Tokyo Bay were influenced by river water. COD showed a significant positive correlation with chlorophyll a concentration, suggesting that the COD in the inner part of Tokyo Bay was largely caused by endogenous factors. The EI was higher during the seasons with higher DIN and DIP concentrations and a lower salinity. The EI in the inner part of Tokyo Bay appeared to be higher during the season and at locations which the influence of river water was greater. The highest contribution to the EI was found for COD (> 40 %), followed by DIN and DIP, suggesting that COD is an important factor in the eutrophication process.