Origins of Phosphorus and Nitrogen in Ariake Bay, Japan

Origins of total phosphorus (TP) and total nitrogen (TN) were investigated in Ariake Bay, Japan, using the unit response function method. About 50% of TP and 67% of TN in Ariake Bay were found to have originated from the open ocean, 14% of TP and 33% of TN originate from rivers, 36% of TP originated from sea sediments and 3.3% of TN was emitted to the air via de-nitrification. The limiting nutrient for primary production in the head of Ariake Bay, that is the main occurrence area of red tides and oxygen deficient water, is phosphorus (Yanagi and Abe, 2005). We consequently suggest that it would be very difficult to prevent red tides and oxygen deficient water in Ariake Bay by managing land-derived TP, because its proportion (14%) is not large. It may be possible to prevent red tides and oxygen deficient water by reducing the bottom release of TP (proportion of 36%), by applying civil engineering method such as the placement of a sand cover over the present sea sediments.