2023 Volume 46 Issue 6 Pages 163-171
The spatial–temporal distributions of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations and changes in the limiting nutrients of phytoplankton in the Seto Inland Sea since the 1980s were analyzed for clustered areas. The DIN concentration decreased in all areas, whereas the DIP concentration decreased in areas strongly affected by the land and in transition areas where the effect of the open sea is strong. These areas account for approximately 20% of the Seto Inland Sea. On the other hand, no significant decrease was observed in the area strongly affected by the open sea, which accounts for approximately 80% of the Seto Inland Sea. The decrease in DIN concentration was greater than the load reduction rate of total nitrogen from land over the past 40 years in the areas strongly affected by the land. The decrease in DIP concentration was equivalent to the load reduction rate from the land. The decrease in DIN concentration was approximately 30% to 40%, even in the area strongly affected by the open sea. The limiting nutrient was phosphorus in the areas strongly affected by the land, whereas it was nitrogen in the area strongly affected by the open sea. The area with nitrogen as the limiting nutrient increased in autumn and winter.
