Phytoplankton Community Structure And Its Main Influencing Factors In The Eastern Indian Ocean Affected By Indonesian Throughflow During Winter

Relying on the"global change and Air Sea Interaction(Phase II)"Special East Indian Ocean Water Body Comprehensive Survey Project,this paper focuses on the species composition,dominant types,cell abundance and species diversity characteristics of micro-phytoplankton and nano-phytoplankton,based on the phytoplankton data and synchronous environmental data observed in the low latitude tropical East Indian Ocean(107°E-115°E;20°S-11°S)from January to February2020,The horizontal and vertical distribution pattern of phytoplankton community structure under the influence of Indonesian throughflow and the main influencing factors were comprehensively analyzed and compared with historical data.The main achievements are as follows:(1)Community characteristics of micro-phytoplankton in the area affected by the Indonesian throughflow in winter in the Eastern Indian Ocean:(1)A total of 268species of small phytoplankton belonging to 57 genera of 4 phyla were identified in the area affected by the Indonesian throughflow in winter 2020 in the East Indian Ocean,including 129 species of diatoms belonging to 38 genera,136 species of dinoflagellates belonging to 17 genera,2 species of chrysophyta belonging to 1 genus,and 1 species of cyanobacteria belonging to 1 genus.The ecological types are mainly oceanic warm water species,tropical nearshore species,and tropical exocean species.Diatomaceous algae and dinoflagellates are dominant groups,with dominant species mainly being Chaetoceros spp.,Chaetoceros messanensis,Thalassiothrix frauenfeldii,Leptocylindrus mediterraneus,Bacteriastrum comosum,Pyrocystis noctiluca,Thalassiothrix longissima and Trichodesmium spp;(2)The average cell abundance of phytoplankton is 1.55×10~4cells/m~3,with high values of phytoplankton cell abundance in the northern and central western sea areas;(3)At a 45%similarity level,small phytoplankton can be roughly divided into 5 communities,among which the main species causing differences between communities are Chaetoceros spp,Leptocylindrus mediterraneus,Chaetoceros messanensis and Thalassiothrix frauenfeldii;(4)Compared with historical data,it was found that the dominant species in the tropical East Indian Ocean have always been Chaetoceros,Planktoniella and Pyrocystis noctiluca,while the dominant species in Chaetoceros clusters have increased.(2)Nanophytoplankton community and main influencing factors in affected area of Indonesian throughflow in winter in the East Indian Ocean:(1)In the winter of2020,a total of 8 phyla,117 genera,and 363 species of microphytoplankton were identified in the Indonesian throughflow affected area of the East Indian Ocean.Among them,49 genera and 144 species of diatoms,55 genera and 202 species of dinoflagellates,3 genera and 7 species of cyanobacteria,4 genera and 4 species of cyanobacteria,1 genus and 1 species of naked algae,1 genus and 1 species of cryptoalgae,and 1 genus and 1 species of green algae were identified.Diatoms and dinoflagellates are the main groups of microphytoplankton in the study area.The dominant species(taxa)are mainly Cryptomonas,Gymnodinium,Nanophytoplankton,Nitzschia,Gyrodinium,Lepidodinium.There are obvious temporal and spatial differences in the distribution of cell abundance.0 m,30 m and 50 m high value areas are basically concentrated in the central north and central south of the study sea area,and the distribution trends of 75 m,100 m and 150 m are roughly the same.The highest abundance value occurs in the 75 m water layer of DM2-9 station(110.6°E,15.3°S),at 268.8×10~2cells/L,with the main groups being Nitzschia,Chaetoceros,Thalassiothrix frauenfeldii,Synedra,Nanoflagellate;(3)From the vertical perspective,the micro phytoplankton community structure in different water layers is significantly different.The dinoflagellate species in the upper mixed layer(0 m,30 m)are significantly dominant.In the 75 m water layer,the number of dinoflagellate species reaches the peak.With the deepening of water depth,the proportion of diatom species increases.In terms of cell abundance,the contribution rate of dinoflagellates in the upper mixed layer(0 m,30 m)is over 70%,while diatoms in the lower thermocline layer(100 m,150 m,and 200 m)are the main contributors to abundance;(4)A comparative analysis of microphytoplankton species in the 90 degree ridge area of the East Indian Ocean revealed significant differences in the composition of microphytoplankton species between the two sea areas at 75 m.The main species causing the differences were Nitzschia,Gymnodinium,Lepidodinium,Cryptomonas.(3)The main influencing factors of phytoplankton community structure in the impact area of the Indonesian throughflow in winter in the East Indian Ocean:(1)Temperature is the main influencing factor of the nano-phytoplankton community structure in the study area.The water temperature in the study area is lower than that in the tropical oligotrophic sea area of the Indian Ocean at a depth of 30 m in the upper mixed layer,and higher than that in the tropical oligotrophic sea area at a depth of 75 m-150 m.The overall temperature of the vertical thermocline increases.The Indonesian throughflow passes through the study sea area,cooling the surface temperature while heating the thermocline;(2)The correlation between a single nutrient in the study area and the abundance of phytoplankton cells is not significant,and it tends to be influenced by the combined effects of multiple nutrients.In the mixed layer water bodies(0 m,30 m,and 50 m)in the winter study area,the vast majority of stations have P limitations,some stations have N potential limitations,and the 75 m water layer has lower P limitations,with no overall Si limitations.Compared with the distribution characteristics of nutrient concentration in the 90 degree ridge area,the nutrient concentration in the thermocline and above water in this study area is significantly higher than the latter,which is more conducive to the growth of phytoplankton in the study area.