Long-Term Change in COD in the Open Waters of the Pacific Ocean near Japan, and its Effect on COD in the Seto Inland Sea

In the Seto Inland Sea, the largest enclosed sea area in Japan, a gradual increase in the chemical oxygen demand concentrations in the seawater (hereinafter referred to as COD) is apparent, and the achievement ratio of the environmental standards for the COD has not reached 80%. In order to solve this problem, it is necessary to analyze the influence of the open ocean water on enclosed sea areas, in addition to the inflow load and amount of marine-derived organic matter. The present mean values of COD at 0 to 50 and ≥ 100 m depth from the sea surface in open ocean water at the offing of Tokushima (site E9) were 1.2 and under 0.9 mg L-1. The trend that COD at this site is high value near the surface and decreases with increasing water depth was observed. The COD at site E9 was higher than those from 20 to 30 years ago in the open ocean near site E9; thus, the trend that the COD in the open ocean increased over the past 40 years to the present was confirmed in this study. The trend that the annual mean concentration of COD increased in the midwestern Seto Inland Sea area after the 1980s was obtained from the existent data. In these areas, high positive correlations between annual mean values of COD in some areas (bay and sea) of the Seto Inland Sea and those of background (seawater at the offing of Tokushima and Kochi) were confirmed. These results suggested that the COD in the Seto Inland Sea was affected by the background water, namely, the water of the Pacific Ocean.