In recent years, the Yitong River in Changchun City has been polluted by human activities to a certain extent, and therefore, it is necessary to monitor its water quality. Phytoplankton are sensitive to environmental changes and are an ideal biological indicator for environmental monitoring. This study studied the phytoplankton distribution in eight sampling sites of the Changchun section of the Yitong River from April to September 2019. The aim was to investigate the relationships between phytoplankton community structure's temporal and spatial distribution characteristics and water quality in the Changchun section of the Yitong River. Environmental factors such as the total nitrogen (TN), total phosphorus (TP), water temperature (WT), ammonia nitrogen (NH 3 -N), chemical oxygen demand (COD Cr ), permanganate index (COD Mn ), and chl-a were measured and assessed. The results identified 161 species of phytoplankton belonging to eight phyla among eight sampling sites of the Changchun section of the Yitong River, including 67 species of Chlorophyta, 11 species of Cyanobacteria, and 51 species of Bacillariophyta. The percentage of Cyanobacteria phytoplankton abundance in the urban area was higher than that in the reservoir and the wet regions. From the temporal perspective, the total phytoplankton density and biomass peaked in summer, with the highest values recorded in June, followed by July. The study showed that TN, TP, and chl-a were the main environmental factors affecting the phytoplankton community structure in the Changchun section of the Yitong River.

To explore how environmental factors affected the plankton structure in the Yitong River, we surveyed the water environmental factors and plankton population in different seasons. The results showed high total nitrogen concentrations in Yitong River throughout the year, while the total phosphorus, water temperature (WT), and chemical oxygen demand in summer were significantly higher than those in other seasons (p < 0.05), and the dissolved oxygen (DO) concentrations and TN/TP ratio were significantly lower (p < 0.01) than those in other seasons. There was no significant seasonal change in other environmental factors. Cyanophyta, Chlorophyta, and Bacillariophyta were the main phytoplankton phylum, while Protozoa and Rotifera were the main zooplankton phylum. The abundance and biomass of zooplankton and phytoplankton in the summer were higher than those in other seasons. Non-Metric Multidimensional scaling methods demonstrated obvious seasonal variation of phytoplankton in summer compared to spring and winter, while the seasonal variation of the zooplankton community was not obvious. The results of the redundancy analysis showed that WT, DO and nitrate nitrogen were the main environmental factors affecting phytoplankton abundance. In contrast to environmental factors, phytoplankton was the main factor driving the seasonal variation of the zooplankton community structure. Cyanophyta were positively correlated with the changes in the plankton community.

The Yitong River Basin is the main northern urban river basin in China, and its ecological health has become an important issue of public interest. In 2021, the plankton community and main water quality parameters of the Yitong River were monitored quarterly. An evaluation system was established based on the Planktonic Index of Biotic Integrity (P–IBI) to assess the spatial and temporal variability in river health. To verify the feasibility of the P–IBI, its assessment results were compared with those of the water quality index (WQI), and the main factors affecting the P–IBI in different seasons were analyzed using correlation analysis between P–IBI metrics and environmental factors. Seasonal comparisons showed that most sampling sites exceeded the “moderate” level in spring and autumn, followed by winter. The worst assessment results occurred in summer. Urban river reaches were more polluted than reaches in rural areas. Regarding driving factors, hydraulic effects on the P–IBI in spring and autumn were significantly stronger than other environmental factors. In summer and winter, special attention should be paid to the input of nitrogen compounds and organic pollutants. Trends based on the P–IBI and the WQI differed in winter, but site patterns were consistent between both indices. The results provide a theoretical basis and data support for the health maintenance of the Yitong River.

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