Changes in phytoplankton productivity and impacts on environment in the Zhejiang coastal mud area during the last 100 years

A high resolution sediment core was selected in the Zhejiang coastal mud area,which was also located within the hypoxia area.The biomarkers,such as brassicasterol,dinosterol and C37-Alkenone were determined on the 210Pb-dated sediment core.According to the vertical distribution of the biomarkers and ratios in the core sediments,we reconstructed the changes in phytoplankton productivity and community structure over the last 110 years in the Zhejiang coastal region.The results indicated increased phytoplankton productivity during the last 100 years in the mud area.Phytoplankton productivity increased gradually starting in the 1960sand accelerated after the 1980s.The change of phytoplankton community structure showed an increasing relative contribution of dinoflagellates and a decreasing relative contribution of diatoms over the last 100years.The increase in phytoplankton productivity in the Zhejiang coastal mud area corresponded to the increased use of fertilizer and nitrogen fluxes from the Changjiang River during the last 50years.Meanwhile the succession of diatoms to dinoflagellates was probably caused by the increasing trends in N∶P and N∶Si ratios.We suggested that these variations were mainly caused by the increasing anthropogenic activities such as the industrialization,agricultural development and the construction of dams since the 1960sand especially since the 1980s.