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

Abstract

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.

Coastal urbanization alters carbon cycling in Tokyo Bay

“…If a GPP/NCP ratio of 2.0 was assumed, the extrapolated GPP in this study was 414 gC m −2 year −1 , which was lower than the GPP values in the 1970s and 1980s but similar to those from the 1990s (Table 1 ). This supports previous findings that surface-water Chl a concentrations decreased between the 1970s and 1980s 39 but remained constant between the 1990s and 2015 7 : Annual mean Chl a concentrations were 40–50 μg L −1 in the 1970s and 1980s 39 , 40 and decreased to 26.3, 23.7, and 27.4 μg L −1 in the 1990s, 2011, and 2012, respectively 11 .…”

“…Although dissolved inorganic nitrogen concentrations increased to 54 μmol L −1 in the 1990s, it decreased to 30.5 μmol L −1 in 2011. In contrast, phosphate concentrations decreased to 1.1 and 0.7 μmol L −1 in the 1990s and 2011, respectively 11 . Decreasing nutrient loads from the rivers reduced primary production in the bay 42 , 43 .…”

“…Nutrients and organic carbon inputs from terrestrial sources have also decreased significantly from the 1970s to 2015 11 , 23 , 26 . Annual mean dissolved inorganic nitrogen and phosphate concentrations were about 40 and 1.5 μmol L −1 , respectively, during the 1970s and 1980s 42 .…”

“…Tokyo Bay is surrounded by Tokyo metropolitan area, the world’s largest megacity. The nutrient concentration in the bay increased markedly with urbanization of the watershed between the 1950s and 1980s but gradually decreased thereafter because of advances in effluent treatment and sewage treatment efficiency 11 . Carbon cycling in Tokyo Bay may have changed with this decrease in nutrients.…”

Barriers and Bridges in Abating Coastal Eutrophication

“…In the Suo Nada, the westernmost portion of the Seto Inland Sea between Honshu and Kyushu, chlorophyll a levels declined substantially, particularly in shallow waters, between 1984 and 2004, closely following reductions in loads of phosphorus and nitrogen (Nishijima et al, 2016). The introduction of advanced treatment for some wastewater discharges has also yielded reductions in nutrient concentrations and chlorophyll a in Tokyo Bay (Kubo et al, 2019). For at least for some Japanese coastal waters, reduced primary production has led to concerns by some fishers, politicians and even scientists that abatement of anthropogenic nutrient loads has been too hasty and has led to severe phosphorus limitation and declines in the secondary production of exploited resources (Yamamoto, 2003).…”