Biomarker Records Of Phytoplankton Productivity And Community Structure Changes Of The North Yellow Sea And Its Influencing Factors Over The Last 100 Years

As new geochemical indices, biomarkers which are produced only by certain species of organism and have relatively stable chemical properties have been widely used in reconstructing paleo-ecological environments in several open oceans and marginal seas. Many researchers also have used aliphatic hydrocarbons, fatty acids, polycyclic aromatic hydrocarbons, sterols, straight-chain alcohols and alkenones to reconstruct phytoplankton productivity, community structure, terrestrial input and sea surface temperature (SST) in chinese marginal seas.However, there were few biomarker records in the North Yellow Sea (NYS) and the Bohai Sea (BS). With global warming and the intensification of human activities, the ecological environment of the Yellow Sea has undergone great changes. But ecological observation works in our country have carried out for only a few decades, there was not enough data to support hundred years or even longer scale study of the Yellow Sea ecological evolution. So it is necessary to reconstruct long-time scales phytoplankton productivity and community structure changes of the North Yellow Sea, which can provide data for the research of regional environmental change.In this paper, contents and ratios of various biomarkers in three 2l0Pb dated cores (B24/B25/BF01) from the North Yellow Sea were applied for the reconstruction of changes of phytoplankton productivity and community structure over the last 130 years. Through the analysis of influencing factors, the main results are as follows:(1) 210Pb dating results showed:210Pb vertical distribution of core B24 and B25 corresponded with a "decay-balance" model, indicating a stable sedimentary environment and uniform deposition rate over the past 100 years.210Pb vertical distribution of core BF01 consistented with a "mixed-decay-balance" model, reflecting the mix and disturbance of core-top, which was caused by the strong hydrodynamic conditions.(2) Contents of brassicasterol, dinosterol, C37-alkenones and other biomarkers indicated that phytoplankton productivity increased gradually over the last 130 years and increased rapidly after 1950. It is suggested that strengthened East Asian Winter Monsoon (EAWM) which increased terrestrial inputs and strengthened seawater mixing caused higher phytoplankton productivity. Atmospheric deposition also affected the North Yellow Sea phytoplankton productivity levels which cannot be ignored.(3) Biomarker ratios indicated that the North Yellow Sea phytoplankton community structure changed significantly since 1960 and showed different trends in core B24 and B25. As for B25, from 1960 to 1990, the relative contribution of dinoflagellates increased and the relative contribution of diatoms decreased for the reason of decreasing Si and increasing N/P ratio which were caused by enhanced anthropogenic activities. After 1990, the trend was opposite caused by decreasing SST and changing nutrient regime. As for B24, the relative contribution of coccolithophores increased and the relative contribution of diatoms decreased for the reason of strengthening Yellow Sea Warm Current. After 1990, the relative contribution of coccolithophores decreased and the relative contribution of dinoflagellates increased which were mainly related to the increasing nutrients by terrestrial inputs.(4) Compared with core B25 and BF01, the relative contribution of coccolithophores of core B24 was significantly higher. This core might be affected by the Yellow Sea Warm Current which can bring warm water with high sanility and more coccolithophores, while core B25 was more influenced by coastal currents with low temperature and low salinity. The relative contribution of coccolithophores in core BF01 was the lowest because of low water depth and strong coastal currents. The main controlling factors of phytoplankton community structures in core B24 and B25 was different. As for B24, the increasing of relative contribution of coccolithophores reflected the Yellow Sea Warm Current was the main controlling factor. As for B25, the changing of relative contribution of diatoms and dinoflagellates reflected nutrient regime which affected by anthropogenic activities was the main controlling factor.