Analysis Of Phytoplankton Community Structure And Evaluation Of Water Environment Quality In The Coastal Waters Of Sanya Bay

In this study,phytoplankton in the coastal waters of Sanya Bay were investigated for two periods,in the summer（June）,winter（December）of 2021 and in the dry season（April）,wet season（July）,normal season（November）of 2022,the temporal and spatial characteristics of phytoplankton community and water environment factors were analyzed,water quality was comprehensively evaluated,and the relationship between phytoplankton and water environmental factors was explored.（1）The spatial and temporal changes in species composition and abundance of phytoplankton community were obvious,and the overall community structure was diatom-dinoflagellate type.In 2021,a total of 130 phytoplankton species in 59 genera and 6 phyla（99 species in the phylum Bacillariophyta,25 species in the phylum Pyrrophyta,2 species in the phylum Xanthophyta,2 species in the phylum Cyanophyta,1 species in the phylum Chlorophyta,and 1 species in the phylum Euglenophyta）were identified in summer,and 111 phytoplankton species in 54 genera and 6 phyla（80species in the phylum Bacillariophyta,26 species in the phylum Pyrrophyta,2 species in the phylum Xanthophyta,1 species in the phylum Cyanophyta,1 species in the phylum Chlorophyta,and 1 species in the phylum Euglenophyta）were identified in winter,with the number of species showed summer>winter;In 2022,160 species of phytoplankton in 70 genera and 7 phyla（117 species in the phylum Bacillariophyta,34species in the phylum Pyrrophyta,2 species in the phylum Xanthophyta,4 species in the phylum Cyanophyta,1 species in the phylum Chlorophyta,1 species in the phylum Euglenophyta,and 1 species in the phylum Chrysophyta）were identified in dry season,132 species of 65 genera and 6 phyla（97 species in the phylum Bacillariophyta,28species in the phylum Pyrrophyta,1 species in the phylum Xanthophyta,3 species in the phylum Cyanophyta,1 species in the phylum Chlorophyta,and 1 species in the phylum Euglenophyta）were identified in wet season,and 127 species of 61 genera and6 phyla（91 species in the phylum Bacillariophyta,30 species in the phylum Pyrrophyta,2 species in the phylum Xanthophyta,2 species in the phylum Cyanophyta,1 species in the phylum Chlorophyta,and 1 species in the phylum Euglenophyta）were identified in normal season,with the number of species showed dry season>normal season>wet season.From 2021 to 2022,the number of phytoplankton species was lower at the two end stations than at the central stations.In 2021,the average abundance of phytoplankton in the coastal waters of Sanya Bay was 11.07?10⁵ cells/L,summer>winter;in 2022,the average abundance was 11.55?10⁵ cells/L,normal season>dry season>wet season.From 2021 to 2022,the abundance of phytoplankton at the two end stations was significantly lower than that at the central stations.（2）In 2021,there were 18 dominant phytoplankton species in the coastal waters of Sanya Bay,and diatoms were absolutely dominant.The most dominant species in summer was Achnanthes catenate,with a dominance index of 0.123,and in winter was Rhizosolenia alata,with a dominance index of 0.109.In 2022,there were 17 dominant species,all of which were diatoms.The most dominant species was Leptocylindrus danicus in both dry season and normal season,with a dominance index of 0.278 and0.517,respectively,and Pseudo-nitzschia delicatissima in wet season,with a dominance index of 0.097.（3）From 2021 to 2022,the Shannon-Wiener diversity index（H’）,Pielou uniformity index（J）,Marglef richness index（d）and diversity threshold（Dv）of phytoplankton at each station in the coastal waters of Sanya Bay indicated that,the water quality of the sea was non-polluted to medium polluted;the phytoplankton species were abundant,with complex community structure,good stability and strong resistance of the ecosystem to disturbance.（4）In 2021,the BOD₅ at each station in the coastal waters of Sanya Bay met the second class seawater quality standard,and the rest of the water environment factors met the first class seawater quality standard;In 2022,the BOD₅ of each station was still in the second class seawater quality status,the inorganic nitrogen of station S6 in dry season and station S1 in normal season met the second class seawater quality standard,and the rest met the first class seawater quality standard.From 2021 to 2022,the eutrophication index at each station was at a low level,indicated that the coastal waters of Sanya Bay were in a poor nutrient state.（5）The relationship between phytoplankton community structure and environmental factors in the coastal waters of Sanya Bay was close.The results of RDA analysis showed that COD,DO,S,p H,DIN and DIP were the key drivers of phytoplankton community succession in the coastal waters of Sanya Bay in 2021;the key drivers in 2022 were T,DO,COD,BOD₅,NO₂-N,NO₃-N and DIP.