Sedimentary Record Of The Zhejiang Coastal Eutrophication And Red Tide Of History Study

The study on red tide is a hotspot question to many countries' researchers. Despite of decades of study, the occurrence mechanisms of red tide are still not well understood. Studies have shown that there is a close relation between frequent occurrence of red tide and eutrophication. But this relation is complicated, the species of red tide respond to eutrophication differently. Accordingly we have to study the evolving process of eutrophication.a sedimentary core, about 82 centimeters long, off Zhejiang where red tide occurred frequently before, was collected, and sedimentation rate and age were dated as approximately 1943 on the basis of 137Cs concentration. In this core, the vertical distributions of dinoflagellate cyst, organic carbon, carbon isotope and biogenic silica were studied. The results indicated that the organic matter within the core came from aquatic and terrestrial sources deduced from 13C value (from -26.15% to -19.5%). The dominance of aquatic source is calculated by assuming that the 13C value of aquatic and terrestrial sources are respectively -21% and -27%. The average contents of biogenic silica and total organic carbon within the core are 0.558 and 0.31% respectively, less than those within the core at Pearl River mouth. This is caused by the dilution of a mass of the sand from Changjiang. The tendency of changes in the content of biogenic silica coincides with the organic carbon. The Si/C molar ratio increases as a whole. This seems likely that the eutrophication first stimulated the bloom of diatom that was the dominant species in the region, because diatom seems have stronger ability to reproduce than other species.The increase in the relative frequency of heterotrophic species in dinoflagellate cyst assemblages is caused by decrease in the number of total dinoflagellate, rather than by the increase in the number of heterotrophic species. Besides, the preservation of dinoflagellate cyst is affected by aerobic organic matter decay. So dinoflagellate cyst was an ideal proxy. The amout of dinoflagellate cyst increased in the last ten years, we predict that dinoflagellate will possibly be one of the main red tide species in the future. The inform deduced from the above proxies corresponds to the current facts. So we believe that the study of red tide by using dated sedimentary cores is a good method.