Seasonal shifts in assembly dynamics of phytoplankton communities in a humans-affected river in NE China

Identifying seasonal shift in phytoplankton community is essential for understanding the significance of eutrophication and finding biological indicators of ecological health of a lotic system. Phytoplankton communities, as well as the seasonal changes in the Ashi River Basin (ASRB) of Heilongjiang Province were investigated from April 2018 to January 2019. A survey in April (spring), July (summer), October (autumn), and January (winter) at 16 sampling sites was conducted. The composition, abundance, and biodiversity indices of phytoplankton were studied and 127 taxa of phytoplankton were identified. Among them, Bacillariophyta dominated the phytoplankton communities in the whole year. There were significant spatio-temporal changes in the structures of the phytoplankton communities during the study period. Trophic state index (TSI) show that the nutritional status of the ASRB was at mesotrophic-middle eutrophic levels. Redundancy analysis (RDA) revealed that total nitrogen (TN), water temperature (WT), oxidation reduction potential (ORP), pH, and dissolved oxygen (DO) were the critical factors in the dynamic phytoplankton community structure. The multivariate regression tree (MRT) analysis showed that Chlamydomonas microsphaerella Pascher et Jahoda, Melosira granulata (Ehrenberg) Ralfs, Merismopedia tenuissima Lemmermann, and Asterionella formosa Hassall were valuable indicators in the determination of water quality in ASRB. Our findings provide a scientific basis for water quality protection and management at basin scale.