Hainan Province is surrounded by the sea on all sides, and the ecological environment quality of its surrounding waters has become an important factor restricting sustainable social and economic development. This dataset is based on the results of a survey on the composition and species diversity of phytoplankton communities in the surrounding waters of Hainan conducted by the Ministry of Agriculture and Rural Affairs's financial special project "Nanfeng Special Project" Phase III in August 2023. This dataset aims to investigate the composition and species diversity of phytoplankton communities in the surrounding waters of Hainan, in order to evaluate the water quality of the waters. It can provide data support for water quality monitoring and water environment prevention in the surrounding waters of Hainan Province, and has reference value for environmental and biodiversity research in related waters.The survey site for this dataset is selected from four sea areas in Hainan Province, including Lingao Huanglong Port, Dongfang Basuo Port, Wanning Shimei Bay, and Wenchang Xinbu Sea. Water samples were collected using a shallow water type III plankton network, and portable water quality detection instruments were used to measure the temperature, water temperature, pH value, conductivity, salinity, and dissolved oxygen at each survey site. The samples were observed using an optical microscope, CX41, and the water quality was evaluated using data processing and analysis methods such as Shannon Wiener diversity index (H ′), Pielou evenness index (J), dominance index (Y), and richness index (d).This dataset collected a total of 36 microalgae and seawater samples for water quality testing and microscopic observation analysis. The survey results are as follows: A total of 253 species of phytoplankton were identified through microscopic examination of samples collected from four sea areas, including 214 species of diatoms, 24 species of dinoflagellates, and 15 species of blue-green algae. The diatom phylum accounted for 84.48% of the total species, with Shimei Bay being the most abundant in phytoplankton species and Xinbu being the least abundant in marine floating plant species; The diversity indices of the eight ports and Shimei Bay plots are both greater than 3, with Shimei Bay having the highest diversity index and Huanglong Port having a diversity index of less than 1; The evenness index of the four sample bays is less than 1, with the highest being 0.81 in Shimei Bay; After water quality testing, it was found that the COD, ammonia nitrogen, total nitrogen, total phosphorus, and nitrite nitrogen in Shimei Bay were the lowest in all four sea areas. The water quality indicators in Huanglong Port were higher than those in the other three areas, and the density of Microcystis in Huanglong Port accounted for 92.32%, The data set can be used to monitor the phytoplankton population, density and dominant species in the coastal waters around Hainan, and provide data support for relevant departments to prevent natural disasters such as red tide and algal blooms.

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