Long-term variability of nutrient and dissolved organic matter concentrations in Tokyo Bay between 1989 and 2015

Tokyo Bay is surrounded by a growing urban metropolis and has experienced varying levels of water quality over time. Vertical profiles of nutrients and chlorophyll a (Chl a) were obtained monthly from 1989 to 2015 at two sampling stations located in the center of Tokyo Bay. From 2006 to 2015, profiles of dissolved organic nitrogen (DON) and phosphorus were also obtained. Mann-Kendall trend analysis was used to establish the significance of the long-term trends for all parameters. Water-column integrated nutrient concentrations showed significant decreases at both stations, except for integrated phosphate concentration at one station. Water-column integrated Chl a concentration also decreased, indicating that active nutrient consumption due to an increase in primary production was not responsible for the declining nutrient concentrations. Rather, the decreases reflect the reduced loading of nutrients into the bay due to the implementation of advanced wastewater treatment techniques. Phosphate concentrations in the surface waters of Tokyo Bay were frequently below detection limits during spring and summer after 2000. During the 10-yr period following 2006, dissolved organic phosphorus concentrations decreased significantly at the surface at both stations, whereas DON concentrations did not change significantly. The ratios of total dissolved nitrogen to total dissolved phosphorus in the surface waters were much higher than the Redfield ratio for both stations, and these ratios increased significantly during the most recent decade. Continued declines in total dissolved phosphorus concentration could strengthen phosphorus limitation of primary production in Tokyo Bay.