Nutrient discharged in Tokyo Bay flows to the Pacific Ocean through the mouth of the Bay. The temperature of Tokyo Bay is also affected through the mouth of the Bay by the Pacific Ocean, especially the Kuroshio Current. Thus, the characteristics of water and mass exchange through the mouth of the Bay should be clarified in order to grasp present and future water environmental condition of Tokyo Bay. Water exchange is caused by tide, fresh water discharge, wind and etc. However, it is still unknown how these factors affect the water exchange rate of Tokyo Bay because of the lack of current observation data.
　Tokyo bay is the heavily eutrophied semi-enclosed bay. The large scale anoxic water occurs at the bottom of Tokyo Bay and endangers the sea life. The development of anoxic water is caused by oxygen consumption of sediments and algal blooms. However, the effect of freshwater discharge and wind is still not clear. For example, the fresh water discharge drives the gravitational circulation and water exchange. On the contrary, the fresh water seems to strengthen the stratification and anoxic water. Although the south wind transport highly oxygen concentrated water to the head of Tokyo Bay, it decreases the gravitational circulation and water exchange.
The object of present study is to clarify the freshwater discharge and wind effect to the water exchange and anoxic water of Tokyo Bay.

Main results of this study are as follows.
1) Since 2003, long term current and water quality observation at the mouth of Tokyo Bay has been conducted using a ferry (“Tokyowan Ferry Kanayamaru”). Before this observation, it was difficult to conduct the observation because of heavy marine traffic. In this study, current and water quality observation data obtained by HF radar and Chiba monitoring post are also analyzed with the ferry data.
2) Yearly averaged residual current at the mouth of Tokyo Bay is 3 layer structures, that is, surface layer outflow, middle layer inflow and bottom layer outflow. This structure is almost same every year. The monthly averaged residual current has a seasonal trend. Water exchange rate is derived by this current data. The maximum water exchange rate (6 year average) is 12,000(m3/s) in October. On the contrary, it is smaller in winter (Jan – Mar) and summer (Jun – Aug). The minimum rate is 6,200(m3/s) in January. Residence date of fresh water is derived from these exchange rate and is 18 in Oct and 38 in Jan. Water exchange in summer is small, which is not coincident with Unoki(1998)’s result, that is, the water exchange is large in summer because of rainfall.
3) 100m3/s fresh water discharge drives gravitational circulation and 480m3/s water exchange. Thus, yearly average fresh water discharge into Tokyo Bay drives 2,035m3/s water exchange rate. 1m/s wind drives wind driven circulation and 900m3/s water exchange rate. Winter averaged wind is 2.5m/s NE which increases 2250m3/s water exchange, on the other hand summer averaged wind is 1.9m/s SW which decreases 1710m3.s water exchange. In Tokyo Bay, south wind develops wind driven circulation which is opposite to gravitational circulation. In summer, south wind is dominant which weaken the gravitational circulation and decreases the water exchange rate.
4) Comparison between water surface velocity observed by HF radar and water exchange rate observed by Ferry has strong correlation. Water surface velocity is strongly affected by wind. In summer, when SW wind is larger than 2.5m/s, wind driven circulation is larger than gravitational circulation. Monthly averaged water surface velocity shows that clockwise circulation develops in the head of Tokyo Bay from spring to autumn. The center of this circulation moves on the line latitude 35.56 degree. Larger SW wind makes the circulation larger and moves it to East.
5) Using 3-D numerical flow simulation, responses of water exchange rate by freshwater discharge, wind, temperature are examined. The responses of water exchange rate by wind and freshwater discharge are almost same.
6) Using ecosystem numerical simulation, responses of summer anoxic water inside Tokyo Bay by freshwater discharge, wind, temperature are examined. The clockwise circulation induced by southerly wind spreads the freshwater from Arakawa River and strengthens the stratification at the head of Tokyo Bay. The strong stratification enlarges the anoxic water.
7) The water quality data observed at the Chiba monitoring post were analyzed. From Jun to Aug, the increase of freshwater discharge causes sea surface salinity, (stratification and low layer DO), lower, (stronger and worse). This is remarkable when monthly wind direction is southerly. These data support the numerical simulation results. From 1970s, both freshwater discharge and southerly wind have been increasing and make the environment of Tokyo Bay worse.