Phytoplankton productivity and community structure variations over the last 160 years in the East China Sea coast in response to natural and human

Eutrophication has caused drastic changes to the marine ecosystem of the East China Sea during the past decades. However, there is relatively sparse evidence of historical changes, as well as the explicit effects of climatic changes and anthropogenic activities on the primary productivity of marine coastal ecosystems. In this study, surface and core sediments from the Zhejiang-Fujian coastal mud area, East China Sea coast, were analyzed using the bulk and molecular biomarkers. The results showed that ecosystem changes were characterized by increased phytoplankton productivity and a fluctuant transition from blooms mostly dominated by diatoms to red tide events dominated by dinoflagellates. Variations from the early 1850s to the early 2010s can be divided into a nature-dominated period (the early 1850s–1960s) and a human-impacted period (1960s–the early 2010s). Particularly, natural forcing such as heavy floods (e.g. 1998, 1954, and 1931) in the whole of the Yangtze River catchment, variations in the intensity of East Asia Monsoon, and strengthened or weakened Kuroshio intrusion/positive or negative Pacific Decadal Oscillation phase in the coastal mud area have substantially affected the phytoplankton productivity and community structure during the nature-dominated period. In contrast, changes in nutrient supply and compositions were more apparent during the human-impacted period, which could have been because of increased fertilizer usage, discharges of industrial wastewater and domestic sewage, and large-scale human projects (e.g. Danjiangkou Reservoir and Three Gorges Dam) in the Yangtze River drainage area, leading to significant phytoplankton productivity and community structure variations in the coastal mud area system of East China Sea