Abstract: Spatial and temporal variation of phytoplankton community can reflect ecosystem stability, which is of great significance in maintaining the function of mangrove ecosystem and biodiversity conservation. In this study, community composition of phytoplankton was investigated at four sampling points in different seasons to evaluate phytoplankton diversity and water trophic status. A total of 52 phytoplankton species were identified, belonging to five phyla and 36 genera. The community structure of phytoplankton showed temporal and spatial variations. Phytoplankton density ranged from 0.27×10⁴ to 25.75×10⁴ cells·L^-1, with an average annual density of 5.06×10⁴ cells·^-1. As one of the dominant species, Synedra sp. was recorded in all seasons. Shannon diversity index, Margalef richness index and Pielou evenness index ranged from 0.44 to 2.38, 0.38 to 2.01, and 0.18 to 0.96, respectively. The eutrophication level of water quality in the mangrove restoration area of Haicang Bay was at the state of moderate eutrophication according to the community structure and diversity index of phytoplankton. As the impacts of human activity may turn into a potential source for the eutrophication, attention should be paid to enhancing monitoring frequency to prevent algae bloom.

Key words: phytoplankton community, spatial and temporal variation, diversity index, trophic status, mangrove restoration area.