Ecosystem changes of the coastal areas of the East China Sea (ECS) during the past 30 years have been characterized by increased productivity and increased occurrences of dinoflagellate red tides; both were commonly attributed to be caused by anthropogenic input of nutrients. However, climate-driven regime shifts were found in the Pacific, but they have not been observed in the ECS owing to the lack of continuous high resolution data. Hence, it is still a challenge to distinguish the influences of anthropogenic activities and climate changes on ECS shelf ecosystem. We compare phytoplankton biomarker (brassicasterol, dinosterol, and alkenones) data of five box cores from the coastal area of the ECS, which afford phytoplankton productivity and community structure records at decadal-scale resolution for the last 100 years. Phytoplankton productivity in all cores increased during the past 50 years, but community structure changes showed different trends. Relative diatom contribution revealed a decreasing trend during the past ca. 50 years in cores DH5-1 and CJ43 collected near the Changjiang Estuary, which were likely caused by increased N/Si and N/P ratios, providing conditions more favorable for dinoflagellates growth. In contrast, the relative diatom contribution increased in cores 32, 34, and DH6-3 further away and south of the Changjiang Estuary, suggesting limited influence of the Changjiang water. Instead, diatom increasing trend is likely caused by increased supply of phosphorus and silicate, as a result of larger Kuroshio intrusion flux and the corresponding strengthened coastal upwelling. Therefore, both anthropogenic activities and climate-driven coastal upwelling have contributed to increased productivities, but these two forcing mechanisms have resulted in contrasting community structure changes at different sites in the coastal area of the ECS.
The East China Sea (ECS) and the Southern Yellow Sea (SYS) ecosystem is undergoing dramatic changes, but the spatiotemporal patterns and forcing mechanisms of phytoplankton variations remain understudied. Phytoplankton lipid biomarkers are useful proxies for productivity and community structure changes, and they were measured in suspended particles of more than 81 sites from spring and summer of 2011 in the ECS and SYS. In spring, the concentrations of brassicasterol (4.7-127 ng L À1 ) and dinosterol (0.7-37 ng L À1 ) were markedly higher in the northern and central SYS, while C 37 alkenones (0-15 ng L À1 ) were detected at only seven sites in the ECS. In summer, brassicasterol (25.3-1178 ng L À1 ) and dinosterol (0-125 ng L À1 ) showed high values off the Changjiang River Estuary (CRE), while C 37 alkenones (0-410 ng L À1 ) had high values in the northwest and central SYS. The mean concentrations of the three lipid biomarkers in summer were 3 to 61 times higher than those in spring. Spatiotemporal patterns of biomarkers reveal higher ratios of diatom/dinoflagellate and diatom/haptophyte in higher productivity areas, off the CRE in summer and the northern and central SYS in spring. This study validates the applicability of brassicasterol, dinosterol, and alkenones as proxies of productivity and community structure of the three phytoplankton taxa: diatoms, dinoflagellates, and haptophytes. The results indicate that nutrients (in summer) and turbidity-induced photosynthetic available radiation (in spring) play important roles in regulating spatiotemporal variations of phytoplankton in the ECS and SYS.
Phytoplankton productivity and community structure in marginal seas have been altered significantly during the past three decades, but it is still a challenge to distinguish the forcing mechanisms between climate change and anthropogenic activities. High time-resolution biomarker records of two 210 Pb-dated sediment cores (#34: 28.5°N, 122.272°E; CJ12-1269: 28.861 9°N, 122.515 3°E) from the Min-Zhe coastal mud area were compared to reveal changes of phytoplankton productivity and community structure over the past 100 years. Phytoplankton productivity started to increase gradually from the 1970s and increased rapidly after the late 1990s at Site #34; and it started to increase gradually from the middle 1960s and increased rapidly after the late 1980s at Site CJ12-1269. Productivity of Core CJ12-1269 was higher than that of Core #34. Phytoplankton community structure variations displayed opposite patterns in the two cores. The decreasing D/B (dinosterol/brassicasterol) ratio of Core #34 since the 1960s revealed increased diatom contribution to total productivity. In contrast, the increasing D/B ratio of Core CJ12-1269 since the 1950s indicated increased dinoflagellate contribution to total productivity. Both the productivity increase and the increased dinoflagellate contribution in Core CJ12-1269 since the 1950-1960s were mainly caused by anthropogenic activities, as the location was closer to the Changjiang River Estuary with higher nutrient concentration and decreasing Si/N ratios. However, increased diatom contribution in Core #34 is proposed to be caused by increased coastal upwelling, with higher nutrient concentration and higher Si/N ratios. . 2014. Upwelling and anthropogenic forcing on phytoplankton productivity and community structure changes in the Zhejiang coastal area over the last 100 years. Acta Oceanologica Sinica, 33(10): 1-9,
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