東京湾の環境修復

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  • Environmental Restoration of Tokyo Bay.
  • トウキョウワン ノ カンキョウ シュウフク

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Development during the past few decades has irrevocably damaged coastal ecosystems, and ongoing projects threaten to further disrupt the environment. A Tokyo Bay restoration project must harness the area's natural ability to recover from environmental damage. The recovery is accomplished through three mechanisms: sea-water exchange, sedimentation, and biological production.<BR>The reclamation projects had also created more intractable problems. The Bay floor surrounding reclaimed land is scarred with deep trenches where the earth was dredged up to build the artificial islands. Hydrogen sulfide released from water stagnating in these trenches created toxic blue tides that threatened to exterminate life in the Bay. The Bay restoration project could include filling such trenches, measures which would prevent the blue tide and help keep the Bay waters moving.<BR>Sedimentation is the Bay's second method of self-purification. The average molecule of water remains in the bay for about a month and a half. Heavy metals found in sewage are settling into the bay bottom before they can be drawn out to sea; as sediment, they are rendered effectively harmless. Sedimentation is useless, however, against pollutants which remain suspended or dissolved in the bay water. Phosphorous, a chemical indicator of water pollution, dissolves and decomposes easily. So do nitrogen-incorporating nutrient salts. They stay in the bay nearly as long as the water in which they are dissolved, and are easily reintroduced into the ecosystem.<BR>Tokyo Bay's tidal flats and shallow waters yielded over 100, 000tons of seaweed and clams annually. The latter fed on plankton brought by tidal currents from spring to early fall; seaweed thrived in the nutrient salt-rich environment from fall to winter. The ecosystem of the tidal flats thus functioned to purify the Bay's water by processing organic substances year-round. When reclamation projects were built on the tidal flats and seaweed beds which had hosted this seasonal ecosystem, they disrupted the food cycle and wiped out 50% of the fish catch. The disappearance of the tidal flats also meant an end to their ability to purify the Bay.<BR>The coast is the interface between land and sea-its ecosystem has developed over thousands of years. Coastal restoration projects must be designed to work with the powerful cleaning system of the Bay itself if they are to assist truly sustainable development.

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  • 応用地質

    応用地質 43 (6), 382-389, 2003

    一般社団法人 日本応用地質学会

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