Biomarker Records And Forcing Mechanisms Of Phytoplankton Productivity And Community Structure Changes In Min-Zhe Coastal Mud Area Over The Last 100 Years

With the increasing anthropogenic activities and natural changes, the ecological environment in marginal seas have been altered significantly during the past decades, such as the increasing occurrence frequency and area of harmful algal blooms, the increasing area of hypoxia and so on. But it is still a challenge to distinguish the forcing mechanisms between anthropogenic activities and climate change. However, due to the lack of actual observation, it is unclear how the productivity in marginal seas varied in the past hundred years. Marine sediments preserved long-term continuous organic carbon burial records. We can reconstruct the history and pattern of phytoplankton productivity and explore the forcing mechanisms by the analysis of organic matter in sediments. In this paper, we used several indicators to reconstruct the phytoplankton productivity and community structure of study area in the past hundred years, aiming to distinguish the forcing mechanisms between climate change and anthropogenic activities to the ecological environment preliminarily through the variation of sediment record.High time-resolution biomarker records of three 2loPb-dated sediment cores (CJ12-1269,28.9°N,122.5°E;#34,28.5°N,122.3°E; T08,28.5°N,122.5°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 increased in the past 100 years. Phytoplankton productivity in core CJ 12-1269 and core T08 began to rise significantly from the 1960s, and phytoplankton productivity in core# 34 stations began to rise significantly from the 1970s. And productivity of core CJ 12-1269 was higher than that of core #34, which was higher than that of core T08. Phytoplankton community structure variations displayed opposite patterns in the three cores. The increasing Dinosterol/Brassicasterol (D/B) ratio of core CJ12-1269 since the 1960s indicated increased dinoflagellate contribution to total productivity. In contrast, the decreasing D/B ratio of core #34 since the 1960s revealed increased diatom contribution to total productivity. While the D/B ratio of core T08 increased first and then decreased since the 1960s, indicated increased dinoflagellate contribution to total productivity first and then increased diatom contribution to total productivity.The improve productivity in the study region may be caused by the increasing input nutrients from the Changjiang River due to the increased fertilizer consumption since the 1950s. And the levels of phytoplankton productivity reflect that core CJ 12-1269 was strongly affected by anthropogenic activities, core T08 was partly affected, and core# 34 was weakly affected. On the other hand, the increasing coastal upwelling brought more nutrients, which may cause the increase of phytoplankton productivity. And the different trends of phytoplankton community structure were caused by the different impacts on different cores. Both the productivity increase and the increased dinoflagellate contribution in core CJ 12-1269 since the 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 mainly caused by increased coastal upwelling, with higher nutrient concentration and higher Si/N ratios. While core T08 was impact by comparable influence of the Changjiang River and the coastal upwelling, it had an uncertain change of phytoplankton community structure.Our preliminary conclusion can be used to distinguish the effects of anthropogenic activities and natural changes on ecosystem. And by comparing more records with better spatial and temporal resolution, distinguishing natural change and human activities quantitatively in future research, it can be better constrained.