Variations in Phosphorus Concentrations in Omura Bay.

Omura Bay is located in the central part of Nagasaki Prefecture and is connected with the open sea through Sasebo Bay by two narrow straits. Therefore movement of the bay water is very stagnant and oxygen-deficient water masses are formed at the bottom layer in the central part of the bay every summer.We have been measuring the concentrations of phosphorus in this bay for twelve years.The concentrations of total phosphorus in the surface layer began to increase gradually after 1992.The concentrations in the whole bay become about 10 μg·l-1 higher after September than those in the earlier months every year. It is thought that orthophosphate in the bottom layer at the central part of the bay, which was released from the bottom sediment in summer, causes the increase of total phosphorus in the surface layer after September. This water mass which has been enriched by ortophosphate (as total phosphorus : 70-100 μg·l-1) is diffused by circulation owing to the drop in water temperature of the surface layer and mixing by wind, and the concentration of total phosphorus in the surface layer will rise.