Lakes as a key species pool: multi-trophic biodiversity patterns in the river-lake continuum

Within river-lake continua, phytoplankton, zooplankton, and fish constitute fundamental multi-trophic components sustaining ecosystem functioning. However, a comprehensive knowledge of their multi-trophic biodiversity patterns and associated driving factors is still lacking within this type of system. Using a full-scale basin sampling effort, we compared the multi-trophic biodiversity characteristics of plankton and fish communities across the largest freshwater lake in North China, the Baiyangdian Lake (BYDL). Data were collected over two years at 148 sampling sites across the river and lake areas, from upstream to downstream. A higher Shannon-Wiener diversity index and species richness of phytoplankton and zooplankton were found in the lake compared to the river. The lake served as a key regional species pool, connecting upstream and downstream reaches of the river and promoting substantial potential species migration to the downstream regions revealed by a source-tracking analysis using the fast expectation-maximization microbial source tracking (FEAST) framework. In addition, we found that species turnover contributed more to variations in phytoplankton, zooplankton, and fish community assemblages across these ecosystems, especially in fish communities. A co-occurrence network further revealed a higher complexity but reduced robustness and cohesion in the lake compared to the river. This implies that human-induced disturbances may compromise ecological network stability despite the lake's high environmental heterogeneity. By investigating multi-trophic biodiversity within a river-lake continuum, our results aim to provide comprehensive evidence that lakes serve as a river species pool—a finding critical to sustaining regional biodiversity. • Lake was a potential regional species pool in watershed. • Species turnover primarily drove β-diversity for plankton and fish in each area. • Lake multi-trophic co-occurrence networks are more complex but less stable than river networks.