Spatiotemporal changes in chlorophyll a concentration in the inner area of Tokyo Bay from 2016 to 2020

Tokyo Bay is one of the most productive coastal systems in the world. Surrounded by a growing urban metropolis, the bay has experienced large fluctuations in water quality over the past decades due to eutrophication and regulatory requirements for wastewater treatment. However, the spatial and temporal variability of chlorophyll a (Chl a) and associated environmental factors remains unclear. In this study, water temperature, salinity, light condition, Chl a and nutrient concentrations was investigated monthly at three stations in the inner area from November 2016 to October 2020. In the surface water, the Chl a and nutrient concentrations largely fluctuated seasonally. In the summer season (May–September), with sufficient nutrient input from rivers and wastewater treatment plants, Chl a concentration in the northwestern part of the bay (St. AO) was positively correlated with water temperature with extremely high concentration (mean, 90.5 µg L⁻¹). On the other hand, those in the northern (St. CB) and central (St. F3) areas were negatively correlated with nutrient concentrations, especially DIP and DSi which sometimes decreased to below the detection limit (BDL). In the winter–spring season (January–April), the Chl a concentration was relatively high at St. CB (mean, 25.9 µg L⁻¹), which could be attributed to light availability, sufficient light penetration to the bottom, and vertical mixing of the entire water column. These results indicated that the factors controlling Chl a differed among areas and seasons in the bay.