Assessment of phytoplankton community structure and water quality in the Hongmen Reservoir

To find effective measures to control the water quality of the Hongmen Reservoir, it is necessary to better understand its phytoplankton composition, abundance and spatial and temporal distribution. Samples were collected at three sampling sites in January (dry season), May (wet season) and September (normal season) in 2019. Trophic level and stability status were assessed on the basis of the Shannon diversity index (H), species richness (S) and evenness (J) index. The different relationships between phytoplankton and the concentrations of several physicochemical parameters and the main soluble nutrients were evaluated by statistical tests. The results showed that there were 75 species belonging to seven groups of phytoplankton, including Chlorophyta (44 species), Bacillariophyta (12 species), Cyanophyta (9 species) and others (10 species). The phytoplankton community composition belongs to the Chlorophyta–Bacillariophyta–Cyanobacteria type structure; and Microcystis, Anabaena azotica Ley, Aphanizomenon, Melosira granulata were the main contributors to the dissimilarities in the temporal distributions of their communities. The phytoplankton density ranged from 4.42 × 106 to 8.99 × 106 particles/L, with an average of 6.45 × 106 particles/L, and the biomass was 4.42 × 106 ∼ 8.99 × 106 particles/L, with an average of 6.45 × 106 particles/L. The variation ranges of the Shannon–Wiener index (H′), Margalef index (D) and Pielou evenness index (J) were 2.05 ∼ 2.85, 4.12 ∼ 6.60 and 0.61–0.78, respectively. This research shows that the water in the Hongmen Reservoir is clean and that the pollution level is light. The correlation analysis shows that total phosphorus and dissolved oxygen are the main factors affecting phytoplankton community structure in the Hongmen Reservoir.