Studies On Phytoplankton In The Central Bohai Sea: Community Structure,Photosynthetic Pigments And Size Characteristics

In the current study,four cruises were carried out in 2015 in the central Bohai Sea(38.21°~39.01°N,118.98°~120.83°E)to investigate the temporal,spacial and size distribution of phytoplankton marker pigments using HPLC method.In addition,microscopic examination and flow cytometry analysis were performed as supplement.Seasonal variation of phytoplankton community and its pattern influenced by environmental factors were also analysed in this study.Results of HPLC show that the phytoplankton chlorophyll a concentrations in October,April,August and June decrease in turn,but the dominating size changes dramatically in different month: microplankton(> 20?m)contributes most of chlorophyll a in April and October,while smaller nanoplankton(2.7~10 ?m)and bigger nanoplankton(10~20 ? m)prevail in June and August respectively.As the second most abundant pigment,fucoxanthin concentrations in October,April,Jun and August decrease in turn,but the dominating size are microplankton in April and October while nanoplankton(2.7~10 ? m)prevail in Jun and August.The results of flow cytometry reveal that Synechococcus dominates picoplankton community in June,August and October,while there is no great disparity between the abundances of eukaryotic picoplankton and Synechococcus in April.Inhomogeneity in spacial distribution of both types was observed.Physical and chemical factors influencing picoplankton vary in different month: in April,there is correlation between temperature and Synechococcus,while there is correlation between eukaryotic picoplankton and silicate;in Jun,both Synechococcus and eukaryotic picoplankton correlate with phosphate and silicate;in August,Synechococcus correlates with salinity and DIN;in October,correlation is found between Synechococcus and salinity,while it is also found between eukaryotic picoplankton and both temperature and salinity.With the analysis of Utermohl method,a total of 130 taxa which belong to 67 genera of 5phyla were identified in the central Bohai Sea.There were 79 species of diatom in 41 generas,42 species of dinoflagellate in 18 generas,6 species of chrysophyte in 5 generas,1specie of phaeophyte in 1 genera and 2 species of cyanobacteria in 2 generas.The resultsof microscopic examination also indicate that the dominant species changed from Dinoflagellates to Diatoms in the studying period,showing obvious seasonal community succession.The indexes of species diversity,ecological richness and community evenness all reach peak values in October.The distribution of community evenness index were similar to that of species diversity index,and the low values of species diversity index were mostly caused by some single overwhelming dominant species.