How Does Extreme Drought Affect Phytoplankton Community Assembly in Aquatic Reserves? A Study from the Confluence of Poyang Lake and Yangtze River, China

Extreme drought events, intensified by climate change, critically threaten aquatic ecosystem stability by restructuring phytoplankton communities. However, the mechanisms underlying drought-driven community assembly remain poorly understood. This study investigated the impacts of extreme drought on phytoplankton community dynamics in the aquatic reserves of Jiujiang City, China, a critical ecotone of the Yangtze River and Poyang Lake. Through multi-temporal sampling (2022–2023) across 12 sites, we integrated taxonomic, functional group, and co-occurrence network analyses with environmental driver assessments. The results revealed that extreme drought significantly reduced phytoplankton species diversity and triggered a functional shift from disturbance-adapted (e.g., MP group) to pollution-tolerant taxa (e.g., W1 group). Deterministic processes dominated community assembly, driven by drought-induced environmental filtering through water temperature, dissolved oxygen, and nutrient fluctuations. Copper emerged as a key stressor, correlating with the abundance of Cryptophyta. Co-occurrence networks, cohesion, and robustness exhibited heightened complexity and stability under extreme drought, emphasizing stress-induced mutualistic interactions. Our findings elucidate how drought reshapes phytoplankton communities via nutrient dynamics and deterministic species interactions, offering critical insights for managing aquatic ecosystems under escalating climatic extremes.