Numerical simulation of water quality response to nutrient loading and sediment resuspension in Mikawa Bay, central Japan: quantitative evaluation of the effects of nutrient-reduction measures on algal blooms

摘要

Eutrophication is caused by large influxes of nutrient into closed or semi-closed water bodies due to agricultural runoff, urban waste disposal, and resuspension from the sediment itself. The objective of this study is to examine how effectively various nutrient-reduction measures can improve water quality in Mikawa Bay, central Japan. Both hydrodynamic and water quality variables were simulated using a model which includes a series of hydrodynarnic equations and 13 mass conservation equations related to water quality, such as chlorophyll-a, dissolved oxygen, etc. The calculated spatial distribution and temporal variations of the chlorophyll-a, nutrient, dissolved oxygen, as well as temperature and salinity, showed generally good agreement with field observations. Analysis of various nutrient-reduction measures suggested that nutrient reduction from sediment resuspension can more effectively reduce chlorophyll-a compared with nutrient loading reduction through rivers from land areas. For suppression of alga growth in Mikawa Bay, control of inorganic nitrogen, especially that of NH_4, was a key factor. That is why a decrease in nutrient resuspension from sediment was more effective for reducing chlorophyll-a, since nitrogen resuspension occurred mostly in the form of NK_4.