Analysis of water quality in Lake Hamana using a coupled physical and biochemical model

The mechanism of water pollution in Lake Hamana, one of the typical highly eutrophicated semi-enclosed estuaries in Japan, has been studied using a numerical 3D physical–biochemical coupling model. In this study, the chemical oxygen demand (COD) was adopted as an index of the lake water quality, and the spatial pattern of the average COD concentration during the summer from 1988 to 1991 was simulated by the model. It was suggested from the numerical result that the water quality in Lake Hamana was essentially influenced by the phytoplankton production, although the dissolved organic matter accounted for the major fraction of COD constituent in the lake. The evaluation of the biological processes over the whole lake region showed that the autochthonous COD flux associated with the primary production may be almost 10 times larger than the external loading flux. Because the phosphate turned out the most limiting nutrient to the phytoplankton production and most of the phosphorus pool in the water column appeared to be supplied from the sediment, it was considered that the benthic phosphorus flux would be the most critical factor to the autochthonous COD in the lake.