Abstract
There are signs of oligotrophication and decrease of fishery production in some estuaries in Japan. Major causes could be a hasty removal of nutrients from sewage and impoundments of rivers. In Japan, phosphorus removal from sewage was successful in alleviating eutrophic conditions of the coastal waters and resulted in decrease of red tide occurrence. However, it seems not only to have decreased the primary production but also to have changed the phytoplankton species composition by the imbalance of N:P ratio in the discharging water. Dams trap nutrients on the bottom of the reservoir as a result of phytoplankton blooms, and thereby decrease nutrient level in the discharging water. Although clean transparent water is generally desirable, moderate nutrient loading is necessary to support sustainable fisheries production. It could be achieved by artificially controlling the amount and the timing of the nutrient discharge from dams and sewage treatment plants. It could be efficient to take the reservoir water out from the lower outlet of the dam, and might be needed to add nutrients in some occasions to support sustainable fishery production.
