Shift of the Circumstances in Effluent Pollutant Loads and Water Pollution in the Seto Inland Sea.

As water pollution and red tide due to aquatic eutrophication in the Seto Inland Sea became a social issue at the beginning in the 1970's, various countermeasures were carried out by peculiar laws for environmental improvement in the Seto Inland Sea. We investigated how water pollutant loads in the basin and the water qualities in the sea changed by the countermeasures. The changes of pollutant loads from effluent, census data associated with the loads and water qualities in the Seto Inland Sea were examined from public statistical tables and official data of national Environment Agency. Though the population in the basin increased remarkably after 1970, it has no influence for the change of pollutant loads, because the sewerage treatment systems also increased simultaneously. There are also little changes in industrial products and farm animal numbers. Nevertheless, the COD loads decreased by 63.2% after 1968, and the reduction of nutrient loads during 1979 and 1994 were 5.3 and 30.3%, in T-N and T-P, respectively, because of pollutant load restriction by the laws.The water quality in the Seto Inland Sea has not shown a clear improvement at all, in spite of these load reductions. It got well a little in COD concentration, but worse in transparency on the whole. On the other hand, T-N and T-P concentrations increased during 1985 and 1989, then decreased a little.In the relations among pollution elements in the sea, there was a significant relationship between COD and Chl.a concentrations, which means organic pollution is strongly affected by algal growth by aquatic eutrophication in the Seto Inland Sea. The comparison of the primary productivity with effluent loads in COD demonstrated that the former was about 20 times larger than the latter.