Inter-Annual Fluctuation of Water Quality in Seto Inland Sea: Relationship with Sea Levels at the Entrances of the Inland Sea

Abstract

The comprehensive water quality survey in the Seto Inland Sea has been operated by the Ministry of Land, Infrastructure and Transport (MLIT), Japan. The water quality survey Program was established for the purpose of making data sets available to know actual conditions of water pollution, and to generate products that would be valuable for scientific analysis of climate-related processes. The program started in May 1981. Water qualities (salinity, water temperature, DO, pH, turbidity and clearness) were observed in February, May, August and October. The observations were carried out at 200 points (142 points in 1989–1999) located on 8 km × 8 km grid. We examined the physical factors affecting inter-annual and seasonal changes in water quality of the Seto Inland Sea by using the data (salinity, temperature, sea level etc.) measured/collected by several institutions, mainly MLIT and JMA (Japan Meteorological Agency). From the present study, it is revealed that the water mass inside the Inland Sea is strongly related to the sea level along the Japanese coast facing the Pacific Ocean. High salinity water that is a part of the Kuroshio intrudes into the Inland Sea. In the past twenty years, the Kuroshio path moved periodically. The Kuroshio was often observed to take a large meandering path in the 1980s and to take a northerly path in the 1990s. Because of this change in the Kuroshio path, the salinity distribution in the Inland Sea has dramatically changed. Geographical and inter-annual fluctuation of primary production was studied in relation to currents of the Inland Sea.