Tokyo Bay is a well-known eutrophic inner bay in Japan, and the subsurface hypoxia in Tokyo Bay has frequently been found during the summer months. Although the appreciable pollutant load has significantly influenced the issues with the water quality in Tokyo Bay, there has been very little information on the influences of the non-point sources due to flooding on the water quality in Tokyo Bay. To clarify what the influences of large-scale flooding are on the water quality of the Bay, we performed field measurements on the pollutant loads under flood conditions using automatic water samplers. We also evaluated the long-term trends of the pollutant loads, and investigated their relation with the water quality in the benthic layer of Tokyo Bay.

The results indicated that large-scale flooding caused by the impact of typhoon no. 0709 was a significantly huge environmental event in Tokyo Bay. The comparison between the pollutant loads and DO in Tokyo Bay indicated that the subsurface hypoxia developed in the head of the bay after the large-scale flooding was mainly due to the decomposition of organic materials appreciably transported in the influent rivers.