Abstract: To explore the dynamics in phytoplankton community structure and functional groups in the Yangshui River that located in the phosphate rock-polluted region of Guizhou Plateau, we investigated phytoplankton and water environment indicators during the dry and wet seasons from 2020 to 2022. Functional group classification, diversity index, evenness index, Spearman correlation analysis, redundancy analysis (RDA), non-metric multidimensional scaling analysis (NMDS), permutational multivariate analysis of variance (PERMANOVA), and stepwise regression analysis were employed to analyze the succession characteristics of phytoplankton community during the dry and wet seasons, as well as the dynamics of phytoplankton functional groups. We further identified the key environmental factors affecting phytoplankton functional groups in Yangshui River. The results showed that: (1) Species composition of phytoplankton in Yangshui River showed significant temporal and spatial variations. 47 species of phytoplankton belonging to 6 phyla were detected. Bacillariophyta accounted for the highest proportion (44.68%), followed by Chlorophyta (31.91%). The total cell abundance of phytoplankton was 1.05×10⁷ cells·L^-1, and the total biomass was 6.55 mg·L^-1. The average cell abundance and biomass increased year by year from 2020 to 2022, being higher in the wet season than in the dry season. (2) 18 phytoplankton functional groups were identified, among which B, D and MP were the common dominant functional groups, and their representative species belonged to Bacillariophyta. In terms of time series, the changes of dominant functional groups from 2020 to 2022 were D/MP/B/S1/X₃ in 2020, MP/D/Y/S1/B in 2021, and MP/D/C/B/P/S1 in 2022. In terms of seasonal differences, MP/D/B/C/S1/X₃ were the dominant functional groups in the dry season, while MP/D/B/S1/Y/P were the dominant ones in the wet season. Spatially, the dominant functional groups shifted from MP/D/B (YS1, upstream)→MP/D/B (YS2)→MP/D/B/S1/Y(YS3, midstream)→MP/Y/C/B/D/S1 (YS4, downstream). (3) The analysis of environmental factors showed that phosphorus was the limiting factor of phytoplankton growth in Yangshui River. The results of RDA analysis showed that the eigenvalues of the first and second ordination axes were 26.82% and 5.76%, respectively. Dissolved oxygen was the main environmental factor affecting the phytoplankton functional groups in Yangshui River. Results of stepwise regression analysis showed that there were significant differences in the adaptation mechanism and ecological needs of different functional groups to dissolved oxygen. During the dry season, the MP functional group had stronger correlation with the B functional group. In the wet season, water temperature and D functional group were significantly correlated with B functional group, while total phosphorus was weakly correlated with B functional group. The ratio of nitrogen to phosphorus may be the key factor for the growth of MP functional group. The results provide an important scientific basis for the long-term management of water resources, and water ecological protection and governance of Yangshui River.

Key words: phytoplankton, functional group, environmental factor, community structure, Yangshui River