Abstract: To clarify the impact of ecological water replenishment on phytoplankton community of the Yongding River (Beijing section), we analyzed the phytoplankton community dynamics during different water supplementation periods and their relationship with environmental factors. In 2023, a survey of phytoplankton communities and water environment was conducted in the mountain gorge and plain sections of the Yongding River during the pre-supplementation (June, August) and post-supplementation (September, October) periods. Phytoplankton samples and water samples were collected from eight sites. The results showed that ecological water replenishment significantly altered water physicochemical characteristics and the structure of phytoplankton community. During the post-supplementation period, water depth and chlorophyll a concentration were higher, while water temperature, pH, and total organic carbon concentration were higher during the pre-supplementation period. The mountain gorge section had higher pH and total nitrogen concentration than the plain section, while the plain section had relatively higher water temperature and chemical oxygen demand. A total of 144 phytoplankton species were recorded, mainly belonging to the Chlorophyta, Bacillariophyta, and Cyanophyta phyla. The highest species number occurred in September, while the lowest was in June. The phytoplankton density and biomass reached their maximum in September and August, respectively. Ecological water replenishment significantly affected phytoplankton diversity, with the highest biodiversity index in August and the highest evenness index in October. Canonical correspondence analysis showed that water temperature, total organic carbon, total nitrogen, and chlorophyll a concentration were key factors influencing the distribution of phytoplankton community. This study reveals the impacts of ecological water replenishment on phytoplankton community structure and provides important scientific evidence for the ecological restoration and management of the Yongding River.

Key words: Yongding River, ecological water replenishment, phytoplankton, community structure, environmental factor