Abstract: To reveal the characteristics of phytoplankton communities and their ecological adaptation mechanisms in the coral reef areas of the Nansha Islands, a survey was conducted at 29 stations across 7 reefs (MJ, RA, TX, XB, HT, WF, and NE) in the region from April to June 2024, analyzing the species composition, dominant species distribution, diversity characteristics of phytoplankton, and their relationships with environmental factors. The results showed that a total of 106 phytoplankton species belonging to 51 genera and 5 phyla were identified in the survey area, among which Bacillariophyta (38.68%) and Pyrrhophyta (57.55%) were the main groups. The main dominant species included Trichodesmium thiebautii, Pinnularia spp., Scrippsiella trochoidea, Cochlodinium polykrikoides, Pleurosigma spp., Bacteriastrum spp., and Synedra spp. The average values of the Simpson's dominance index (C), Shannon-Wiener diversity index (H′), Pielou's evenness index (J′), and Margalef's richness index(d) of the phytoplankton community were 0.35±0.19, 2.43±0.79, 0.59±0.20, and 2.00±0.56 respectively. Among environmental factors, temperature (29.60~31.58℃) and salinity (33.50~34.24‰) showed small variations; nitrate was not detected; and there was no significant regional difference in the concentrations of other nutrients (ANOVA, P＞0.05). Correlation analysis revealed that only salinity showed a significant positive correlation with the abundances of Pyrrhophyta and Scrippsiella trochoidea (P<0.05). This study provides important basic data for understanding the community dynamics of primary producers in the coral reef ecosystem of the Nansha Islands and their responses to environmental changes.

Key words: Phytoplankton, Community structure, Environmental factors, Nansha Islands