Oligotrophication and its measures in the Seto Inland Sea, Japan

Abstract

The Seto Inland Sea experienced eutrophication during the period of high economic growth and then subsequent oligotrophication due to reduction of inflow load. The nutrient concentration in the seawater peaked in the beginning of 1970's, and thereafter, it has been decreasing. Dissolved inorganic nitrogen (DIN) has remarkably decreased in recent years following the decrease in dissolved inorganic phosphorus (DIP). As a result, Nori (Pyropia) production has decreased in recent years due to bleaching of leaves caused by DIN deficiency. Recent slack in fisheries production was also likely influenced by the nutrient depletion as evidenced by the fish catch using a compact trawl. The process of both eutrophication and oligotrophication that occurred in the Seto Inland Sea can be explained by hysteresis. We introduce several countermeasures to the oligotrophication in the Seto Inland Sea. In particular, to improve Nori production, local nutrient supply was experimentally augmented, such as by application of fertilizer, temporary discharge of dam water, sediment plowing, and ad hoc operations of relaxing sewage treatment water.