3. Oligotrophication in the Seto Inland Sea

Over the past 50 years, the Seto Inland Sea has experienced both eutrophication and oligotrophication. In terms of year-to-year variation, the relationship between the phosphorus load and the number of occurrences of red tides shows a clockwise hysteresis, while that between nutrient concentration and fish catch has a counterclockwise hysteresis. Such a difference is the result of the mutual interaction between the water quality and bottom sediment, that is, the organic matter accumulated in the bottom sediment during the period of eutrophication. Therefore, the number of red tides during the period of oligotrophication is greater than during eutrophication at the same phosphorus load due to the release of dissolved inorganic phosphorus from the bottom sediment. The fish catch during oligotrophication is less than during eutrophication at the same nutrient concentration due to the generation of hypoxia in the bottom layer during the stratification period.