A Century-long Phytoplankton Shift And Environmental Responses In The Adjacent Sea Of The Yellow River Estuary

Under the effects of ocean warming and eutrophication,phytoplankton shifts have happened in many coastal waters over the last several decades,which cause the increase of harmful algal blooms(HABs).As the only Chinese inland sea,the Bohai Sea has experienced abrupt environment change and frequent HABs in the context of eutrophication,decrease of Yellow River input and warming,which have great impacts on fisheries and marine economic development.However,due to the lack,discontinuity and spatial mismatch of long-term observation data,the characteristics and driving factors of phytoplankton shift in the Bohai Sea are still unclear.Therefore,it is necessary to analyze and understand the phytoplankton shift and its responsive mechanism to the environment via the combination of in situ observation and palaeoecology methods,and further analyze long-term variations of diatoms,cysts and geochemical parameters in sediment cores to understand key environmental factors affecting phytoplankton shift.The results are hope to better serve the marine ecosystem health and economic development.In this study,the adjacent sea of the Yellow River Estuary is taken as the research area.We analyze the seasonal and interdecadal variations of phytoplankton and environmental factors based on the observation data during1960s-2010s,1.Seasonal and interdecadal variations of phytoplankton and environment factors in water columnBased on the field survey in the the adjacent sea of the Yellow River Estuary from 2014 to 2015 and the historical data collection during 1960s and 2010s,this study analyzed the seasonal and interdecadal variation of phytoplankton and environmental factors.The results showed that:(1)The biomass peaked in spring and summer.Diatom,Cryptophyta and Chlorophyta were the main groups in spring,Dinoflagellate and Diatom were the main groups in summer,and Diatom was the main group in autumn.(2)The interdecadal variation of phytoplankton showed that the Chl-a contents in the Bohai Sea had increased by 7-fold,and the peaks of Chl-a contents had changed from spring and autumn to spring and summer during 1960s and 2010s.Diatom species and abundance proportion decreased significantly,but dinoflagellate was just the opposite.And the proportion of large-sized diatoms decreased significantly,while small-sized diatoms increased.(3)The SST of the Bohai Sea increased about 0.5°C at the end of 1980s.The SSS increased from 27.5to 31.4 in 1960-2000s,and then decreased slightly.The DIN concentration increased about 7-fold,while DIP,DSi concentration decreased about 2-fold,3-fold since the1960s.Asymmetric changes between N,P,and Si caused DIN/DIP increasing about15-fold and DSi/DIN decreasing about 20-fold since the 1980s.Correlation analysis showed that the seasonal variation of phytoplankton was mainly affected by water temperature and nutrients.The interdecadal variation of phytoplankton might be related to the increase of DIN,DIN/DIP and the decrease of DSi/DIN.2.Long-term shifts of diatoms,cysts and controlling factors in sediment coresIn this study,diatom frustules,dinoflagellate cysts,grain size,total organic carbon(TOC),total nitrogen(TN)and carbon and nitrogen isotopes(?¹³C,?¹⁵N)in sediment cores from the adjacent sea of the Yellow River Estuary were analyzed to reconstruct a century-long phytoplankton shift and environmental responses.The results showed that two significant shifting points were identified from the varying trend of diatoms and cysts.The first one occurred in the 1850s,when the Yellow River outlet relocated from the southern Yellow Sea to the Bohai Sea,as evidenced by finer grain size,lower sea salinity and nutrient input,causing a significant increase in total biomass and brackish species(Actinocyclus ehrenbergii,Gonyaulax)proportion,and decrease in marine species(Protoperidinium,Pyrophacus steinii)proportion.The other shift happened after the 1970s,when significantly increased fertilizer usage and wastewater discharge led to nitrogen enrichment in the water column,causing a marked increase in total biomass,small-sized species(Paralia sulcata,Actinoptychus undulatus,Thalassiosira eccentrica),eutrophic species(Lingulodinium polyedrum,Polykrikos schwartzii)and HABs species(Scrippsiella,Alexandrium)proportion.Redundancy analysis between diatoms,cysts and environmental factors indicated that the nutrient enrichment and Yellow River input had a more important role in regulating phytoplankton shifts than rising temperature after the 1970s.However,when water temperature gone up at the end of the little ice age(1550s-1850s),the cold temperate species(Protoceratium reticulatum)corresponded well to the warming.3.The correlations of phytoplankton and environmental factors between sediment and upper waterThis study analyzed the correlations between diatom frustules,cysts and phytoplankton,geochemical parameters and environmental factors in sediment and water column,respectively,and further evaluated the accuracy and existing problems of sedimentary palaeoecology in indicating environmental change.The results showed that:(1)P.sulcata,Coscinodiscus,Thalassiosira,Protoperidinium were both dominant species in sediment and water;After the 1970s,the variations of diatom frustules,cysts abundance in cores and Chl-a concentration in upper water corresponded well.The proportions of eutrophic species,HABs species and small-sized diatoms in sediment and water increased,while the proportion of large-sized diatoms decreased;Increased values of TOC,TN,BSi,and?¹⁵N corresponded well to the eutrophication process after the 1970s.(2)However,there were some dominant species(such as Chaetoceros,Ceratium)with thin cell wall or no cysts were not detected in the sediment.Therefore,the multiple-proxy paleoecological method is reliable,which can better reveal long-term shifts of phytoplankton assemblages with the combination of in situ observation data.As one of the most frequently diverted rivers in the world,this study choose the adjacent waters of the Yellow River Estuary as a breakthrough to reveal the impact of the Yellow River avulsion on the phytoplankton shifts in the Bohai Sea.The results are of great significance for understanding the phytoplankton shifts in the coastal waters under the control of large rivers.