Oxygen-deficient waters along the Japanese coast and their effects upon the estuarine ecosystem

Development of hypoxia in Japan has been confirmed in the inner part of almost every major bay of Japan on the Pacific Coast from Tokyo southward. This paper presents multiple aspects (present condition, hydraulic mechanism, effect upon fisheries, historical progress and nutrient budget between sediment and water) using Mikawa Bay, where Japan's most serious hypoxia occurs, as an example. Although hypoxia basically results from the increase of nutrient load input from domestic and livestock sources, the intense reclamation of shallows (including tidal flats) and the large reduction in river flow due to farmland irrigation drastically accelerated dissolved oxygen deficiency. Oxygen-deficient waters in Mikawa Bay are large enough to strip the water purification capacity of the remaining shallows. Unfortunately, the shallows have turned from a purifier to a source of nutrient load. These conditions are more or less common in all bays where the dissolved oxygen-deficient waters have been reported. To break this cycle, dissolved oxygen deficiency must be contained to the extent that the purification capacity of the shallows can be restored to an efficient level. For this purpose, the first thing to do is to restore tidal flats over an extensive area and to recover sufficient water flow, which may be a more urgent imperative than reducing the nutrient load input.