No. 1 - Tokyo, Japan.

Tokyo is the most populous city in the world. It out ranks Mexico City (the second largest megacity, inland not coastal) by 10 million people. Tokyo Bay covers 1000 square kilometres which, when compared to other bays, is relatively small. However it is a particularly busy port, with many linked industries. Marine Traffic Safety Law has been enforced in Japan since 1973, obliging all large ships and vessels carrying dangerous cargo to be escorted in order to monitor hazards along harbour shipping routes. Additionally, the Marine Pollution Prevention Law requires appropriate environmental conservation measures to be adopted for all ships for the prevention of marine pollution. In 1997 the super tanker Diamond Grace ran aground on reef rupturing its hull and spilling 1,315 tonnes crude oil cargo into Tokyo Bay. Accidents such as this are rare though. Nutrients (in particular a high nitrogen load) and organic material deposited from the land do seem to be the main culprit when considering marine pollution in Tokyo Bay. Despite the development of sewage systems eutrophication is still quite serious. Nutrients accumulate on the ocean bottom creating oxygen depleted zone where marine organisms cannot survive. In addition, increased discharge of chemical pollutants such as heavy metals and PCBs are also a serious threat to marine life.

Land reclamation of 249 square kilometres, to create more room for port facilities and container terminals, has accelerated environmental degradation by damaging tidal flats and seagrass/seaweed beds. Also the deepening of sea lanes in Tokyo Bay has destroyed bottom habitats such as seagrass beds, hardbottom communities and corals. Some estimates of habitat destruction around Tokyo Bay are as high as 90%.

Pollution and destruction of most of the natural coastline have made Tokyo Bay a very inhospitable environment for marine life.