Research On The Sedimentary Records Of Environmental Changes In The Daya Bay, South China Sea

Sediment cores from length of 50 to 90cm were collected from six sites in two sea areas of Daya Bay)from 2005 to 2006.Vertical distribution of biogenic silica (BSi),total organic carbon(TOC),tatal nitrogen(TN) and dinoflagellate resting cysts were investigated in order to understand the efficiency of these sedimentary parameters as indexes for anthropological activities such as marine aquaculture,the activities of nuclear power station and port on environment.The values of BSi,TOC,TN,the terrestrial organic carbon(TOC₁),dinoflagellate resting cysts in the sediments of cultural sea area in Daya Bay increased obviously, which indicated that the increasing terrestrial nutrition inputs had promoted the eutrophication due to human activities.At the shellfish area and cage fish area in Dapeng Ao area,the values of the aquatic organic carbon(TOCs) decreased in recent sediments,whereas the molar ratios of BSi to TOC_s(BSi/TOCs) increased from 0.616, 0.858 before mariculture(about 1980,30 cm sediment) to 0.942,1.192 after mariculture,respectively.On the contrary,the values of TOC_s increased recently in the cage fish area of Aotou area,and BSi/TOCs decreased evidently from 4.182 before mariculture to 0.687 after mariculture.It is suggested that annual primary production decreased and the proportions of diatoms increased in the cultural sea area of Dapeng Ao area,oppositely in the cultural sea area of Aotou area.The annual primary production increased evidently after the operation of the Daya Bay Nuclear Power Plant(DBNPP).The average values of BSi,TN,TOC_s, dinoflagellate resting cysts in the core near DBNPP were 0.62%,0.16%,0.63%,283 cysts/g D W_t and 1.37%,0.21%,1.08%,1324 cysts/g D W_t before and after the operation of DBNP,respectively.However,BSi/TOCs decreased from 0.84 to 0.53, which indicated that the contribution of diatoms to the total annual production decreased as the operation of DBNP.The annual primary production decreased obviously after the operation of the Huizhou Harbor.The average values of BSi,TN,TOC_s in the core near the Huizhou Harbor werel.63%,0.41%,1.08%and 1.13%,0.17%,0.26%before and after the operation of the Huizhou Harbor,respectively.But,BSi/TOCs increased from 1.64 to 3.54,which demonstrated that the contribution of diatoms to the total annual production increased as the operation of the Huizhou Harbor.An obvious increase of dinoflagellate resting cysts and overwhelming dominance of Scrippsiella cysts(the maximum relative frequency of 68%) was observed in surface sediments,which indicated the progressive eutrophication in Daya Bay, especially at Aotou area.The cysts of Alexandrium spp.,a toxic PSP causative species, distributed widely in Daya Bay,especially in shellfish cultural area of Dapeng Ao area with the maximum concentration of 269 cysts/g D W_t,which might provide sufficient seed bed for its blooms.Hence,monitoring should be enhanced in order to control the further deterioration of water quality and occurrence of toxic blooms.