Seasonal And Interannual Changes Of Net Phytoplankton Biomass And Community Structure In The South Yellow Sea

Variations in phytoplankton biomass and community structure will impact the structure and function stability of marine ecosystem,and further impact the global climate change.Based on recent data collected in different months in 2011—2012,seasonal variations in phytoplankton biomass,proportion of dinoflagellates and dominant genus were analyzed.We further analyzed the long-term changes of net phytoplankton by comparing with historical data from 2000—2003 and 1958—1959,aimed to reveal the evolution trend of phytoplankton community structure in China's coastal waters.Based on the analysis results of net phytoplankton samples collected from five months,i.e.April,June,August,October and November,using Flow Cytometry and Microscopy(FlowCAM),seasonal variations in phytoplankton were analyzed.The results indicated obvious seasonal differences in total phytoplankton carbon biomass and spatial distribution.In spring and autumn,carbon biomass decreased from inshore to offshore regions while in summer abundant phytoplankton existed in the southern region.The maximum carbon biomass of total phytoplankton was(46192.9±117905.1)?gC/m3 in the summer,while the minimum was below 1000?gC/m3 in autumn.The proportion of dinoflagellate was lowest in summer(14.05%)based on carbon biomass,whereas in April or November the proportion was about 48%.Dinoflagellate proportion was relatively lower in the southern region.The environmental factors that influenced phytoplankton communities in the South Yellow Sea were discussed.Correlation analysis revealed that phytoplankton was impacted by temperature,salinity and the dissolved inorganic nutrients in each season.In spring,dinoflagellates negatively correlated with temperature,salinity and phosphate.In summer and autumn,phytoplankton was positively correlated with temperature and negatively correlated with salinity.Dissolved inorganic nitrogen and reactive silicate were the key environmental factors that influenced phytoplankton in summer.There was a difference in the selection of dissolved inorganic nitrogen between diatoms and dinoflagellates.Silicate concentration was the main factor that affected the proportion of diatoms in phytoplankton communities.Phosphorus limitation was found in autumn but not summer.In autumn,both the carbon biomass of diatoms and dinoflagellates were significantly correlated positively with phosphorus.Redundancy analysis(RDA)revealed that the adaptability towards environmental factors was different among various phytoplankton groups.Significant regression relationships for net phytoplankton biomass and abundance were established in the form of 10 10 log C(28)k ?log abundance(10)b,where C represents carbon biomass(?gC/m3),abundance represents cell abundance based on cell counts(cells/m3),and k and b are constants.Based on these relationships,carbon biomass could be estimated from cell abundance.The values of k and b were 0.48 and 0.49 respectively for total net phytoplankton.Regression equations for diatoms,dinoflagellates and common dominate genus,i.e.Chaetoceros,Coscinodiscus,Pseudonitzschia,Skeletonema,Ceratium,Protoperidinium and Pyrophacus,were also established respectively.The results also confirmed the good agreement between converted carbon biomass by counting and by volume measurement.Using C:abundance relationships may simplify the methodology and analysis of experimental and model simulations of fluxes in marine ecosystem biomass.Based on recent data collected in 2011—2013 and historical data in 1958—1959 and 2000—2003,long-term changes of net phytoplankton community in the South Yellow Sea were analyzed.The results indicated that the biomass of net phytoplankton increased dramatically in spring.Statistical analysis indicated that both phytoplankton abundance and carbon biomass in spring of 2011 were significantly higher than those in 1959.The high value of carbon biomass in spring were distributed in the inshore areas in each year.High value of phytoplankton biomass was also found offshores in 2011 and the area of low carbon biomass significantly narrowed.In spring,the proportion of dinoflagellates increased obviously and the area where dinoflagellate dominated expanded based on carbon biomass.Rhizosolenia was dominated in spring in the study area.Compared with 1959,the number of dominate genus increased in 2011,and Ceratium became dominated in the community.In summer,obvious interannual fluctuation in phytoplankton abundance and biomass were found.Both abundance and carbon biomass of net phytoplankton in 1959,(2.02±3.31)×104cells/m3 and(62.99±71.33)?gC/m3,were significantly lower than those in 2012—2013.The maximum of abundance,(18.73±46.68)×104cells/m3,and carbon biomass,(1110.71±2107.82?gC/m3),were found in 2013.Compared with 1959,the abundance proportion of diatom in 2012—2013 decreased from 93% to 50%—60% and the carbon biomass proportion decreased from 70%—85% to 18%—28%,respectively.The area where dinoflagellate dominated expanded.The proportion of dinoflagellates carbon biomass increased because of the low carbon density in diatom and the increase of dinoflagellate abundance.Chaetoceros,Pseudonitzschia,Ceratium and Noctiluca became the new dominant genus in 2012—2013.The increased trend in phytoplankton in spring and summer was most obvious in the coastal regions which indicated that the change of marine physical and chemical factors from terrestrial input induced by human activities had great impact on phytoplankton communities.Different from spring and summer,the change in phytoplankton biomass was not obvious in autumn and the distribution patterns were in accordance in different research stages.The proportion of dinoflagellates also increased in autumn.