Numerical studies on ecological effects of artificial mixing of surface and bottom waters in density stratification in semi-enclosed bay and open sea

Abstract

An apparatus, which mixes waters in upper and lower layers in density stratification and discharges the mixture at the middle depth, has made a remarkable success in enhancing water quality for ecosystems in a small semi-enclosed bay. In this study, we conducted numerical simulations by using a three-dimensional tidal current model and an ecosystem model to predict the effects of the apparatus in two different seas; a larger semi-enclosed bay and the area around a solitary island in the open sea. Isahaya Bay in Ariake Sea was chosen as the former case to see how effectively the apparatus can reduce oxygen-deficient water, and the sea around Okinotori-shima Island represents the latter, where we are interested in the effect of the apparatus on fertilisation; how much primary production can be increased. As a result, it is suggested that the apparatus reduced up to 70% of oxygen-deficient water in Isahaya Bay during the neap tide in high summer, although it is not very effective in fertilising around the solitary island; only 0.3% of chlorophyll-a was increased in the area affected by the apparatus.