Numerical study on amelioration of water quality in Lakes Shinji and Nakaumi: a coastal brackish lagoon system

摘要

Aiming at estimating the effect of an amelioration scheme for water quality in a highly eutrophicated and strongly enclosed estuary, especially placing emphasis on the alleviation of a oxygen-depleted water body, a three-dimensional (3-D) time-dependent coupled physical and ecological model was applied to Lakes Shinji and Nakaumi, a typical coastal brackish lagoon system in Japan. First, the model was run to reproduce the seasonal variation in water temperature, salinity, dissolved oxygen, and other ecological constituents during the period from April 1998 to March 1999. The daily change regimes of the observed sea surface winds, tidal elevation at the open boundary, river discharges, and meteorological parameters were incorporated into the forcing variables of the ecosystem model. Next, as a numerical experiment, a scenario simulation was carried out under four premises to alleviate the oxygen-depleted water body. The model results agreed fairly well with the field measurements obtained through the regular environmental monitoring in the lagoon system. It was found that an oxygen-depleted water body appears in the eastern part of Lake Nakaumi in early May and develops to cover the whole lagoon in midsummer to leave it almost anoxic. The scenario simulation revealed that each amelioration scheme has a considerable effect in alleviating this hypoxic water body. It was found from the model results and the additional flux analyses that the oxygen concentration will improve as a result of (1) change in the flow field by cutting the dikes built up during lake reclamation and (2) decrease in benthic oxygen consumption by filling up deep holes once created by dredging activities.