NUTRIENT DECREASE IN THE SETO INLAND SEA, JAPAN, AND THE RESPONSE OF THE PHYTOPLANKTON ASSEMBLAGE TO THE DECREASE

Abstract

The nutrient decrease in the eastern part of the Seto Inland Sea and its influence on the lower trophic levels of the coastal ecosystem are discussed based on the information obtained during our previous study. During the high economic growth from the 1960s onwards, the Seto Inland Sea became heavily eutrophicated. Since the enactment of the Law for Conservation of the Environment of the Seto Inland Sea in 1973, nutrient concentrations gradually decreased after 1970 and dissolved inorganic nitrogen (DIN) concentration in particular decreased after 1990. One fundamental explanation for the gradual decrease of DIN concentration from 1970 to the present is the enactment of the above law by the Environmental Agency. However, the reason for the recent decrease of DIN concentrations is still unknown. It was thought that the decrease of the upward nutrient flux across the overlying water-sediment interface greatly affected the water column nutrient concentrations. The response of phytoplankton assemblages to the decrease of nutrient concentrations seemed to be no apparent decrease of biomass but a change of species group and species composition.