Seawater Pollution Monitoring in a Net Cage Fish Farm in Daya Bay, South China

Seawater pollution monitoring was conducted in a cage fish farm area and its vicinity (as control area) in Daya Bay, South China from 2001 to 2002 on a seasonal basis. The results showed that water environment factors of dissolved oxygen (DO) in summer and acid volatile sulfide(AVS) in spring both exceeded the criteria of the second class of sea water quality standards for fisheries in China with exceeding rate of 62.5%, the other water environmental factors were general within the criteria. DO and chlorophyll a (Chl-a) in the fish farm were both lower than in control area significantly (t-test, p<0.05), while the other environmental factors did not display evident differences between fish farm and control area. The seasonal variation of seawater temperature, salinity, transparency, suspended solid, DO and pH were mainly determined by the subtropic monsoons climate in Daya Bay. The seasonal variation of chemical oxygen demand (COD_Mn), five days biochemical oxygen demand (BOD₅) and AVS appeared to be related to the seasonal characteristics of fish farming. The nutrients such as dissolved inorganic nitrogen (DIN), PO₄-P and SiO₄-Si were mainly regulated by the rainfall and phytoplankton status; and the Chl-a was mainly related with temperature and atomic ratio of N:P.