Phytoplankton Community Structure In The Eastern Equatorial Indian Ocean During Spring Intermonsoon 2016

As primary producers,phytoplankton,although small in size,is numerous and widely distributed in the oceans;they have worked much on promoting material circulation and energy flow;and they are able to use photosynthetically fixed organic carbon to support marine ecosystems and promote biogeochemical cycles in the oceanosphere.Therefore,the study of phytoplankton community structure can provide a better understanding of phytoplankton systems and ecosystem dynamics processes and provide a scientific basis for marine environmental protection and exploitation of biological resources.Though the study related to phytoplankton community in the Indian Ocean has been more and more,the study about phytoplankton community structure of the Eastern Indian Ocean is still a lack of adequate understanding.Therefore,this survey was based on the voyage in the Eastern Equatorial Indian Ocean(80.00?96.00°E.S.02°S ? 10.02°N)during Spring intermonsoon,2016.For the water samples taken from 29 stations including sections of 11,12,and 13,among them,phytoplankton were identified and counted by Utermohl method,then the datum about the ecological characteristics of phytoplankton communities in the investigated waters such as species composition,cell abundance,dominant species and community diversity were analyzed later.In order to estimate the contribution of phytoplankton carbon biomass,we have calculated the cell volume and converted it with certain formula.All the water samples collected from the voyage were statistically analyzed,and a total of 71 genera and 260 species of phytoplankton were identified,including dinoflagellates,diatoms,cyanobacteria and chrysophyceae.Among them,dinoflagellates had the largest number of species,tolerating 32 genera and 190 species.Diatom came to second contribution of the total species numbers;it has 32 genera and 63 species.Meanwhile,2 genera and 4 species of cyanobacteria were identified.Tropical extraneous species and warm extraneous species prevailed the main ecological types of species in all stations of the survey area,which also reflect the climatic characteristics of tropical and subtropical areas from the biologicalperspective.During this period,the average cell abundance in the investigated sea area was 1.207×103 cells/L,the average cell abundance of diatom was 0.029×103 cells/L,the average cell abundance of dinoflagellate was 0.044×103 cells/L,the average cell abundance of cyanobacteria was 1.1327×103 cells/L,and the minimum cell abundance belonged to chrysophyceae.Overall,because of the impact of the Wyrtki jets and diluted water,horizontally,the high value of phytoplankton cell abundance appeared in waters near Sumatra island,and the cell abundance was mainly contributed by Trichodesmium.Vertically,the high value of phytoplankton cell abundance appeared in the 50 m water layer.Among them,the dominant species included Trichodesmium thiebautii,Trichodesm i um erythraeum,Planktoniella Formosa,Oxytoxum pachyderme,Coscinodiscus subtilis,Protocentrum compressum,Prorocentrum lenticulatum,Scrippsiella trochoidea,Thalassionema nitzschioides,Thalassiosira excentrica and so on.The shannon-wiener diversity index of phytoplankton community in the survey area ranged from 0.04 to 4.79,with an average value of 2.65.The range of Pielou evenness index ranged from 0.002 to 0.981,and the average value was 0.74.Through the correlation analysis of phytoplankton and various environmental factors,it can be deducted that although phytoplankton cell abundance is greatly affected by temperature,depth and salinity,some dominant species have great correlation with the concentration of nitrate and phosphateWe have used cell volume to estimate the phytoplankton carbon biomass in the survey area.The maximum phytoplankton carbon biomass occurs in the 25 m water layer.The contribution of phytoplankton carbon biomass differed with the various species,among which,dinoflagellates contributed the most,accounting for an average of 74.64%of the phytoplankton carbon biomass.The average contribution of diatom accounted for 10.42%of phytoplankton carbon biomass.The largest carbon biomass of cyanobacteria was contributed by Trichodesmium thiebautii,which accounted for 54.78%of the cyanobacteria carbon biomass.The high value of surface phytoplankton carbon biomass distributed at stations on the west side of Sumatra island and the south side of Sri Lanka island.According to the relative percentage of cell abundance and carbon biomass of dominant species,smaller cells could also contribute much to the phytoplankton carbon biomass when they accounted for a large number in cell abundance and were in an absolute dominant position.The correlation between environmental factors and phytoplankton cell abundance and its carbon biomass showed the difference between cell abundance and carbon biomass.By comparing the phytoplankton carbon biomass in different sea areas,it can be seen that except the survey area,sampling time and other objective factors,phytoplankton carbon biomass is also influenced by the comprehensive effects of cell abundance,species composition and calculation method.