Climate change-induced hydroannual dynamics: Unveiling the complex control factor in phytoplankton functional groups succession in river-connected lakes

Global climate change has complex effects on phytoplankton communities and growth by altering hydrological rhythms (e.g., droughts and floods). Predicting and managing aquatic ecosystem health requires in-depth research on the response mechanism of phytoplankton functional groups (FGs) to hydrological dynamic changes. Here, we deployed the structural equation model and preliminarily elucidated the comprehensive driving mechanisms involving water level (WL)-physicochemical parameters-FGs in different hydrological years, which categorized by WL fluctuations from 2004 to 2022 in Dongting Lake, China. The results revealed the groups S1 (cyanobacteria) and Lo (dinoflagellates) were predominant in dry years, while the groups P (diatoms) and MP (cyanobacteria, diatoms) were dominant in normal and wet years. WL and water temperature were the primary driving factors of FGs by altering physicochemical parameters in dry and wet years, while ammonia nitrogen, secchi depth become more essential in normal years. Notably, the effects of nutrients and light on FGs in different hydrological years were influenced by the coupling effects of WL fluctuations (e.g., discharge and duration) and human activities (e.g., pollution discharge and sand mining). Moreover, the low connectivity during droughts and environmental homogeneity during floods intensified the homogenization of lake habitats, leading to regional homogenization in FGs. Overall, we emphasize the direct or indirect effects of WL fluctuations on FGs dynamics and providing insights for formulating effective conservation strategies to control cyanobacterial blooms in river-connected lakes.