Large-scale restoration of tidal flats and shallows to suppress the development of oxygen deficient water masses in Mikawa Bay [Japan]

Abstract Development of hypoxia has been confirmed in the inner-part of almost every major bay of Japan on the Pacific coast from Tokyo southward. Mikawa Bay, where Japan's most serious hypoxia occurs, is used to this report to present the effects and causes of hypoxia, such as impact upon fisheries, historical development and nutrient budget between sediment and water. Although hypoxia basically results from an increase in nutrient load input, intense reclamation in Mikawa Bay of about 1,200 ha of shallows in the 1970s, including tidal flats, has drastically accelerated a deficiency in dissolved oxygen. This is mostly due to losses in the rich filter-feeding macrobenthic community that largely control the high water-purification capacity of those areas. Currently, oxygen deficient water masses in Mikawa Bay are large enough to strip the precious water purification capacity of the remaining shallows by killing the remaining filter feeders. Consequently, the considerable shallows have turned from being a purifier of water quality to a source of excess nutrients, thus sending the Bay into a spiral of deterioration. In order to break this vicious cycle, the dissolved oxygen deficiency of the Bay must be contained to the extent that the purification capacity of the shallows can be brought into full play. To this end, the first thing to do is to restore the tidal flats and the shallows having the effective depth so designed as not to be affected by oxygen deficient water masses, over an extensive area. This may be a more urgent imperative than reducing the nutrient load input. Since 1998 to 2001, about 350 ha of artificial shallows, including tidal flats, have been restored in Mikawa Bay using sand dredged from the Nakayama sea channel. Recovery of abundant benthic organisms, such as bivalves, has been confirmed already by monitoring. Additional tidal flat restoration' is now in progress.