Eutrophication and Oligotrophication in Japanese Estuaries

“Eutrophication and Oligotrophication in Japanese Estuaries”, one in the series of books entitled “Estuaries of the World”, addresses the question: What is the present states of Japanese estuaries from the viewpoint of eutrophication and oligotrophication? We describe three typical situations, namely, (1) eutrophication problems in Tokyo Bay, (2) oligotrophication problems in the Seto Inland Sea and (3) the disappearance of hypoxia in Dokai Bay. Total TP (total phosphorus) and TN (total nitrogen) loads control law has played an important role in these three bays. However, the results of the application of the law differ among the three bays as the characteristics of material cycling are different.

Tokyo Bay, located in the centre of Japan, has a temperate, humid climate. The bay is an enclosed environment, which is heavily populated and densely used. It is approximately 50-km long and 20-km wide and has an average depth of 15 m. Since 1950, increase in the concentration of population and industry in this river basin has caused radical changes in the coastal areas of Tokyo Bay. Some of these changes were eutrophication and a decline in the area of shallow sandy flats in the bay. As a result of eutrophication, Tokyo Bay has been affected by red tides (algal blooms) approximately 50 times a year and by blue tides (upwelling of hypoxic bottom water) approximately 3 times a year. The area of tidal flats has decreased by approximately 90 % over the last 100 years. These changes can affect the distribution of living organisms in the bay. The iconic richness of biological production in the shallow and tidal flats is under the threat of disappearance. On 26 March 2003, the Tokyo Bay Renaissance Promotion Council endorsed the ‘Action Plan for Tokyo Bay Renaissance’. The goal is to restore the beautiful coastal environment for people to enjoy and sustain its natural biodiversity. The challenge of restoration is just beginning. Important and practical future aims are as follows: (1) to sustain the monitoring campaign with the cooperation of various stakeholders; (2) to create and maintain habitat for benthos, sessile organisms and fish larvae using an ecosystem approach; and (3) to establish public participation mechanisms.

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.

Dokai Bay, in northern Kyushu, Japan, though small in size is an important port for the huge heavy and chemical industrial facilities that surround the bay. In the past, it was also a discharge area for heavily polluted industrial waters. Long-lasting hypoxia events were observed over large parts of the bay in the 1990s, but loads of total phosphorus (TP) and total nitrogen (TN) in wastewater have been controlled since 1980 and 1996, respectively, by government regulations. In consequence, TP and TN concentrations derived from nutrients such as phosphate-phosphorus and ammonium-nitrogen have substantially decreased. In addition, the concentration of acid-volatile sulfides in sediments has decreased drastically.

Because of decreased eutrophication, the severity and duration of the hypoxia events have decreased, and, although the red tide formation frequency has not changed, the diversity of red tide organisms has increased. Macroalgae have expanded their distribution to inner and deeper parts of the bay as the light penetration has increased. Sessile animals as well as fish, shrimp, and crabs are significantly more abundant on the bottom of the inner bay because of the increased dissolved oxygen content, especially in summer.