Picophytoplankton Community Structure In The Western Pacific Ocean And Western South China Sea

In marine phytoplankton community,picophytoplankton can grow rapidly with the advantage of its small size,thus it plays the key ecological role in the coastal and oceanic waters.Especially in oligotrophic regions,picophytoplankton is predominated,and it has a potential high impact on the nutrients cycles in an ecosystem.It is basic for microbial food web in oligotrophic regions and is essential for supporting the upper water food web,as well as the symbiotic phytoplankton.Besides,it is of great significance in the biogeochemical cycle of marine ecosystems.In order to study the picophytoplankton community and explore the environmental factors which affect the dynamic changes of picophytoplankton community.In this paper,we were using the flow cytometry to determine the cell abundances and groups of picophytoplankton,had described the picophytoplankton community structure in the western Pacific Ocean from 2017 to 2020,and in the western South Sea from 2019 to 2020.There are a total of six cruises,cover three seasons of summer,autumn and winter,with a time scale of four years.Our study is expected to be a deeper and systematic understanding of the role and status of picophytoplankton community in these marine ecosystems.To understand the picophytoplankton community in the western Pacific Ocean we have carried out four cruiese along the transect 130°E,Beside this,142°E,165°E and equatorial transect were added in 2019,to make a more comprehensive understanding the biogeographical features of picophytoplankton structure in the western Pacific Ocean.We found the three major types of picoplankton in the western Pacific Ocean,they are Synechococcus,Prochlorococcus and Pico-eukaryotes.The average cell abundance of Synechococcus,Prochlorococcus and Pico-eukaryotes were 1.84x103 cells/mL,9.19×103 cells/mL and 4.99×102 cells/mL,and the precentages were 16%,79.75%;4.25%,respectively.The precentages of Synechococcus in the picophytoplankton community was decreasing year by year,while Prochlorococcus was increasing year by year.The average carbon biomass were 0.46 ?g C/L?0.49 ?g C/L?0.48 ?g C/L,and the proportions were 32.1%?34.3%?33.6%,respectively.On the horizontal distribution of picophytoplankton,the cell abundances near the equator were higher,and the cell abundances gradually decreased with the increase of latitude.In vertical distribution,Prochlorococcus was more sensitive to light,so it was distributed in deeper water,it was thriven in 100m water layer.While Synechococcus was abundant in upper water layers of 50m and rarely found in 100m.Pico-eukaryotes was concentrated in subsurface,and cell abundance was low.The correlation between hydrological characteristics and picophytoplankton community in the marine was analyzed.We concluded that the concentration of nutrients had a negative correlation with the picophytoplankton community.In addition,Synechococcus had a negative correlation with depth and salinity,and there was a positive correlation between temperature and Synechococcus,while Prochlorococcus and Pico-eukaryotes had a positive correlation with salinity.The research areas in the western South Sea were divided into two parts:near the Pearl River estuary area and Qiongdong coastal area.Several stations had been set up in the coastal area of Qiongdong,including coastal and oceanic stations,which had basic significance for the preliminary study of the difference of picophytoplankton community in nearshore and offshore.There were Synechococcus,Prochlorococcus,and Pico-eukaryotes in the western South Sea.The average abundances of Synechococcus,Prochlorococcus,and Pico-eukaryotes were 5.17×103 cells/mL,1.725×104 cells/mL,1.77×103 cells/mL,respectively.The abundances proportions of the three algae were 21.4%?71.3%?7.3%,respectively.Compared with 2019,the proportion of Synechococcus in 2020 had decreased.On the contrary,the proportion of Prochlorococcus had increased.The pico-eukaryotic changed little.Compared the abundances in the western South Sea and the western Pacific Ocean,it was found that the abundances of the picophytoplankton community in the western South Sea was slightly higher,and the proportions of the three picophytoplankton in different areas were also different.The average carbon biomass of Synechococcus,Prochlorococcus,and Pico-eukaryotes were 1.29 ?g C/L?0.91 ?g C/L?1.71 ?g C/L,and the proportions were 33%?23.3%?43.7%,respectively.In horizontal distribution,the abundances of picophytoplankton in the Pearl River estuary were significantly lower than that in the coastal areas of Qiongdong.It was related to the summer monsoon brought flood season to Pearl River estuary,large amounts of nutrients and other substances were imported into the ocean from terrestrial,resulting in higher concentrations of nutrients near the Pearl River estuary.The distribution of picophytoplankton in the coastal area of Qiongdong showed that the cells abundance inshore was high,and the abundance gradually decreased from inshore to offshore.In terms of vertical distribution,picophytoplankton had different distribution characteristics.Synechococcus was concentrated on the surface layer,and Pico-eukaryotes were concentrated in the shallow water layer above 50m.When the water depth exceeded 100m,Synechococcus and Pico-eukaryotes were rarely seen.Prochlorococcus was concentrated in the 75m layers.The results of correlation analysis with environmental factors showed that the horizontal distribution of picophytoplankton would be affected by summer monsoon and circulation and water masses changed caused by summer monsoon.The vertical distribution was mainly affected by light,salinity and nutrients.The Pacific Ocean and the South Sea are interlinked,the influence of the equatorial currents throughout all year.The equatorial region is also a place where tropical storms and typhoons are occurrence.The summer monsoon in the South Sea is also of great significance to atmospheric circulation and climate change in the northern hemisphere.Based on these complex marine environments,this paper studied the temporal and spatial varied of picophytoplankton in the western Pacific Ocean and the western South Sea,and discussed the relationship between dynamic changed of the biological community and marine environmental factors,laid a foundation for the study of the role and status of picophytoplankton in ocean.