Dual Influence of Rainfall and Water Temperature on Phytoplankton Diversity and Nutrient Dynamics in a Mountainous Riverine Reservoir

The combined effects of anthropogenic activities and climate change, particularly the increasing frequency of extreme rainfall events, continue to pose significant threats to the security of reservoir ecosystems and water quality. Effective prediction and management of aquatic ecosystems require a comprehensive understanding of how environmental factors influence the dynamics of phytoplankton communities. However, the response patterns of phytoplankton community diversity, niche breadth, and cell density to rainfall disturbances in complex mountainous riverine reservoirs remain poorly understood. In this study, we systematically investigated the phytoplankton community structure and its environmental drivers in Zhaoshandu Reservoir (China) via field surveys, morphological identification of samples, and multivariate statistical analyses. Water temperature (WT), rainfall, and phytoplankton cell density in the study area ranged from 11.4 °C to 35.6 °C, from 0 to 72.5 mm, and from 3.33 × 103 to 7.95 × 107 cells/L, respectively. Total phosphorus and total nitrogen concentrations ranged from 0.002 to 0.633 mg/L and from 0.201 to 5.06 mg/L, respectively. Canonical correspondence analysis found that rainfall and WT were the pivotal drivers of phytoplankton density and biomass and were significantly correlated with phytoplankton diversity. Importantly, structural equation modeling revealed that the direct effects of both rainfall and WT on phytoplankton diversity and niche width, as well as the indirect effects of rainfall on ammonium nitrogen concentration, significantly modulated algal density and biomass in Zhaoshandu Reservoir. Our study highlights the role of rainfall as a potential major regulator of phytoplankton communities in this riverine reservoir.