Modeling of the climate and anthropogenic influences to the eutro-dynamics in Dapen Bay, Taiwan

A three-dimensional numerical model has been constructed for simulating the hydrodynamics and the bio-geochemical responses in coastal lagoons. The model has been applied to study the eutro-dynamics in Dapeng Bay, southwest of Taiwan. Dapeng Bay is the largest single entry lagoon in Taiwan with a area more than 530 ha. The hydrodynamics in the lagoon is dominated mainly by tidal variations from the coastal seas through a tidal inlet during the dry season. Fresh water discharged into the lagoon is concentrated at the rainy season that comes with the southwest monsoon during summer. Constant freshwater sources are untreated domestic sewerages and aquaculture drainages surrounding the lagoon throughout the year. Vertical structure of the hydrodynamics are considered in order to take density effect into account, thus salinity and temperature has to be calculated for computing currents. The lagoon was an important fishery ground where various aquaculture activities had been settled, mainly oyster farming and cage net farming. An eutro-dynamic model has been constructed and coupled with the hydrodynamics for studying the water quality and the eutrofication conditions in the lagoon. Eight state variables have been included in the bio-geochemistry model, i.e. micro-plankton, detritus, dissolved nutrients and dissolved oxygen. The plankton growth is controlled by water temperature, light intensity and the nutrient supplies. The basic assumption for nutrient supply was that Silicate and phosphate are fully available in the lagoon. Therefore, the model only simulates the nitrogen cycle of the lagoon. Tides predicted by a regional model are used for specifying the level at the mouth of the lagoon. Various boundary conditions are obtained from measurements. The responses of the eutrodynamics to weather and anthropogenic influences are discussed in this paper.