Abstract
The long-term variations in water quality in the Seto Inland Sea of Japan were examined based on the routine observation data from the local fisheries’ experimental stations from1972 to 2013. The water temperature increased due to global warming, and the increase rate in autumn was especially high. The transparency increased after the 2000s in Osaka Bay, whereas it increased after the 1990s in the Seto Inland Sea, excluding Osaka Bay. The nutrients decreased due to oligotrophication. In the Seto Inland Sea, excluding Osaka Bay, the DIN concentration decreased rapidly in the1970s and decreased again after the2000s. In Osaka bay, however, the DIN concentration did not decrease in the 1970s, but it markedly declined after the 1990s. The high concentration of DIP in the 1970s was decreased around 1980. Thereafter, the concentration remained stable in the Seto Inland
Sea, excluding Osaka Bay, but it continued to decline in the surface layer in Osaka Bay. The reduction of DIN and DIP concentrations was largely affected by land load reduction. It is also thought that they were affected by a reduction in supply from the bottom mud and open sea, and increase in the total solar radiation in recent years.
