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The resource of bivalve has been decreasing drastically since 1970s in the Ariake Sea located in Kyushu Island, Japan. Also, water quality environment in the sea has been changed in decades. However, even the data of water quality or the amount of bivalve catchment have been collected for more than 30 years, these data have never been assessed before. In this research, the change of decennial characteristics was assessed in order to identify the environmental change. Using kriging method, all point data of water quality items interpolated into the entire area. Then, each area was classified into six groups by cluster analysis. The result showed that the water quality environment in the Ariake Sea have been significantly changed in these 30 years. It was revealed that the amount of nutrient (DIN, DIP) and organic matter (COD) has been decreased importantly in this area. Then, the relationship between water quality items and bivalve catchment was investigated using Artificial Neural Network (ANN) model with sensitive analysis. ANN model reproduced the annual bivalve catchment well from 10 of annual average water quality items. And sensitive analysis was conducted to evaluate which water quality item is affecting the bivalve catchment the most. The result showed that decreasing amount of nutrients is inducing the decreasing bivalve catchment in the Ariake Sea. This result coincides with the result from cluster analysis. In Japan, the technologies of waste water treatment or sewage coverage have been improved so much that the water qualities in the river become clean which means less nutrients in the water. There are water quality standard to protect the coastal area from eutrophication, but there are no standard to protect the area from oligotrophication in Japan. Therefore, appropriate standard of the water quality items must be established to protect coastal ecosystems in Japan.

To understand aquatic environmental deterioration due to the anoxic state in an organically polluted water body, water quality dynamics under anaerobic conditions were examined through beaker-scale water quality monitoring. This study focused on the dynamic properties of NO_3–N, NH_4–N, PO_4–P, and sulfide from the biochemical reactions point of view, and based on anaerobic respiration activities, such as denitrification, iron reduction, and sulfate reduction. The specific aims of this study were to quantitatively estimate the impacts of the oxidative/reductive state of the sediment surface and the high/low concentrations of dissolved organic matter on the dynamic properties of water quality under anaerobic conditions. The beaker-scale water quality monitoring was carried out through continuous measurements of dissolved oxygen and oxidation–reduction potential (ORP), as well as periodic observations of water quality parameters for six cases that are composed from combinations of three experimental conditions: (1) concentration of dissolved organic carbon (DOC); (2) redox state of the sediment; and (3) concentration of NO_3–N. As a result, the temporal changes in ORP under anaerobic conditions exhibited a five-step process of decline without these conditions. Also, high DOC concentrations and oxidative states of the bottom mud accelerated the release of PO_4–P and sulfide generation through promoting biological iron reduction and sulfate reduction, because these conditions increase respiratory substrates for anaerobic microorganisms, such as iron and sulfate-reducing bacteria. It was concluded that our results would provide important information about the mechanisms of aqueous environmental deterioration due to organic pollution in closed water bodies.