2022 Volume 45 Issue 5 Pages 239-244
Dissolved oxygen in the bottom layer (Bottom DO) was newly added to the environmental water quality standards for protecting the living environment in 2016. Tokyo Bay is a highly closed water area, so red and blue tides occur every year, and Bottom DO is often lower than 2 mg L-1 in summer. To consider the cause of the reduced Bottom DO, the vertical distribution of water quality characteristic, that is, temperature, salinity, chlorophyll-a, and DO, has been measured several times a month from 2003 to 2017 at fixed points in Tokyo Bay. During this time, the stratification of water temperature has been observed every summer. From autumn to winter, the stratification disappears. The decrease in salinity has been observed with the inflow of fresh water caused by rain fall. At St.A (the point of a dredged trench on the bottom) , as a result of the decrease in depth, the mass of hypoxic water has been decreasing. These data should be useful for studies of Tokyo Bay.
