Prediction of effect of seawater-quality enhancer installed in Isahaya Bay by ecosystem model

Abstract

It is said that the tidal current in the Ariake Sea has changed since the construction of the Isahaya Dyke. The change may bring on the environmental deterioration in the Ariake Sea. Especially in the Isahaya Bay, the emergence of oxygen deficient water has been prominent. In this study, firstly, we simulated the emergence of oxygen-deficient water by incorporating an ecosystem model into an ocean current model and elucidated the mechanism of such a phenomenon both from physical and biological viewpoints. Secondly, we proposed a local engineering solution, i.e. a sea water-quality enhancer, which mixes bottom and surface waters and discharges the mixture at the depth of euphotic layer, and numerically simulated its effects on water quality and ecosystems. The numerical model considered 8 principal rivers because the nutrient-rich density current of fresh water largely affects the ecosystems in the Ariake Sea. It was shown that 16% of the oxygen deficient water in the Isahaya Bay was reduced by the apparatus, which emits 1-million-ton water per day, 8 weeks after the start of operation