Annual Variation Of Phytoplankton Community Structure In Equatorial Region And Its Adjacent Areas Of Eastern Indian Ocean During Spring Intermonsoon

Annual variation and in-year features of phytoplankton community structure were investigated in the tropical eastern part of Indian Ocean during the spring inter-monsoon period of 2012, 2014, 2015 and 2016.A total of 525 taxa of phytopllankton that belong to 88 genera of 4 phyla were identified,in which 49 genera and 143 species belonged to Bacillariophyta, 33 genera and 372 species were Pyrrophyta, 4 genera and 6 species were Chrysophyta, and 2 genera and 4 species were Cyanophyta. These species were tropic oceanic and warm oceanic species mostly,which indicated that the climate of the was tropical and subtropical,Phytoplankton cell abundance showed the decrease trend from 2012 to 2016, the abundance was 93.788 ×103 cells/ m3, 64.362 ×103 cells/m3, 32.762 ×103 cells/m3 and 26.950 ×103 cells/m3 (except St. 1107) respectively, which could be caused by El Nino during survey period. Phytoplankton cell abundance was contributed by diatom and dinoflagellate abundance mainly, but phytoplankton distribution was mainly determined by diatom distribution. The highest abundance occurred in equator section both in 2012 and 2016. However, the highest abundance was observed in southern Bay of Bengal transection that paralleled with equator in 2014, and near Sri Lanka in 2015.The dominant species in 2012 were Pseudo-nitzschia delicatissima,Climacodium frauenfeldianum,Thalassionema nitzschioides, Scrippsiella trochoidea, Thalassiosira excentrica, Fragilariopsis doliolus and so on. The dominant species in 2014 were Climacodium frauenfeldianum, Leptocylindrus mediterraneus,Chaetoceros coarctatus,Chaetoceros lorenzianus, Chaetoceros atlanticus var. neapolitanus, Chaetoceros messanense and so on. The dominant species in 2015 were Pseudo-nitzschia delicatissima,Fragilariopsis doliolus, Thalassionema nitzschioides, Climacodium frauenfeldianum,Planktoniella foromsa, Thalassiosira excentrica and so on. The dominant species in 2016 were Chaetoceros atlanticus var, neapolitanus, Pseudo-nitzschia delicatissima,Fragilariopsis doliolus,Planktoniella foromsa,Chaetoceros messanense,Bacteriastrum comosumand so on.From 2012 to 2016, the S-W diversity was 5.26, 5.04, 5.20 and 5.14 respectively, and Pielou evenness index was 0.80, 0.82, 0.81 and 0.78 respectively. Richelia intracellularis cell abundance decreased year by year, and existent forms changed little. Three species were observed in the survey area, in which T. thiebaultii was predominant exclusively in four years. During the four years, the average trichome abundance of Trichodesmium was the highest in 2012, followed by 2016, and then 2014, and the lowest trichome abundance was found in 2015. Three species had similar patterns in each year. But Trichodesmium in different years had different patterns. The abundance in the survey area was much lower than that in other seawaters, Trichodesmium abundance was in significant negative relationship with salinity in 2014. The highest similarity index detected between 2012 and 2016. The relationship between phytoplankton cell abundance and temperature and salinity was not obvious. The relationship between cyanobacteria, temperature and salinity varied a lot, the environmental factors had complex relationship with diatom, but they had quite good relationship with Chrysophyta. Most dominant species was related to cyanobacteria positively.