Phytoplankton Seasonal Variation and Water Quality Assessment Using Bioindicators in the Dong Tranh and Long Tau Rivers, Vietnam

Abstract Phytoplankton communities were investigated as bioindicators of water quality at ten sampling sites along the Dong Tranh and Long Tau rivers during the dry and rainy seasons of 2023. Phytoplankton samples were analyzed in terms of taxonomic composition, species diversity, and abundance. Water quality was further evaluated using two widely applied biological indices: Palmer’s Algal Genus Pollution Index and the Shannon–Weaver Diversity Index (H ′). A total of 107 phytoplankton species belonging to 70 genera, 54 families, 41 orders, 10 classes, and 6 divisions were identified, with diatoms representing the most diverse group. Phytoplankton abundance ranged from 2210 to 48 513 cells L–1 during the rainy season and from 921 to 15 159 cells L–1 during the dry season. Diatoms dominated during the rainy season, whereas cyanobacteria prevailed in the dry season. A distinct seasonal shift in functional groups was observed: group P dominated the rainy season, reflecting eutrophic conditions and high mixing, whereas groups MP, S1, and S2 prevailed during the dry season, indicating organically enriched waters. Biological assessments showed H' values ranging from 1.89 ± 0.28 to 2.71 ± 0.23 and Palmer scores from 15.20 ± 0.94 to 15.60 ± 1.19, classifying the rivers as moderately to slightly polluted. Overall, water quality was relatively better during the dry season, while the rainy season experienced greater impacts from nutrient loading and hydrological disturbances.