Associations of oxygen availability with eukaryotic plankton assembly and coalescence in river–lake ecotones

Oxygen depletion threatens the productivity, biodiversity, and functioning of aquatic ecosystems; however, ecological responses of freshwater plankton, particularly community assembly and intertrophic interactions, remain poorly understood. Herein, we investigated how oxygen availability is associated with the abundance of functional populations, diversity, assembly processes, and interactions of phyto- and zooplanktonic communities in river–lake ecotones. Declining dissolved oxygen (DO) was associated with reduced diversity and pronounced taxonomic and compositional shift in eukaryotic plankton. Despite this, plankton exhibited broader niche breadths under lower oxygen levels, indicating an adaptive expansion of ecological strategies. Assembly processes diverged by trophic level: phytoplankton communities were predominantly governed by stochastic processes across all oxygen levels, whereas zooplankton shifted toward deterministic assembly at intermediate DO (~8 mg/L). Co-occurrence network analysis further revealed that positive, potentially cooperative interactions between phyto- and zooplankton weakened with oxygen depletion, while network complexity and intertrophic connectivity peaked at intermediate DO. Notably, dispersal limitation was the lowest, and cross-group community coalescence was the strongest at intermediate DO, suggesting a system-specific optimal oxygen window that supports community stability and functional integration. Together, these results demonstrate that oxygen availability is strongly associated with variation in plankton diversity, assembly, and interaction structure. This work identifies oxygen-dependent thresholds in freshwater ecosystems and provides a framework for understanding and managing plankton dynamics under ongoing deoxygenation. • Oxygen availability is associated with plankton diversity and composition in river–lake ecotones. • Low oxygen reduces diversity but expands plankton niche breadth. • Phytoplankton assembly remains stochastic across oxygen gradients. • Zooplankton shifts to deterministic assembly at ~8 mg/L dissolved oxygen. • Network complexity and coalescence peak at intermediate oxygen levels.