Long-term dynamics and driving factors of phytoplankton biomass during urbanization, northern China

Understanding the relative impact of nutrients and related environmental variables on phytoplankton biomass changes in urban rivers is important for river management. In this study, we explored the long-term dynamic changes and driving factors of phytoplankton biomass in the process of urbanization from the perspective of time and space, and provided a scientific support to water ecological health assessment and management of urban rivers. From 2012 to 2021, the phytoplankton community composition in Taiyuan section of Fenhe River was found to be dominated by Cyanophyta, Chlorophyta,and Bacillariophyta. Among them, cyanobacteria were the main contributors to the total abundance, green algae ranked second, and diatoms were the least. The biomass of phytoplankton in different groups showed different decreasing trends. Based on 10 environmental factors to explain the changes of three main phytoplankton groups(cyanobacteria, green algae, diatoms), we found that chemical oxygen demand and water temperature had a high explanation rate for the abundance changes of the three main phytoplankton groups, which were important environmental factors affecting their biomass changes. The random forest model was used to analyze the changes in the total biomass of phytoplankton. It was found that the permanganate index had a higher contribution to the total biomass of phytoplankton in time. The contribution of air temperature to the total biomass of phytoplankton was higher in space.