Flow regulation capacity determines reservoir phytoplankton community assembly: threshold, mechanisms, and implications

Dam construction changes lotic rivers to lentic reservoirs with divergent hydraulic residence time (HRT), altering phytoplankton communities. However, how community assembly shifts along HRT gradients and the underlying mechanisms remain unclear. Here, we investigated phytoplankton communities from 21 reservoirs spanning a broad HRT range (0.21-94.65 d) in China, quantifying assembly patterns and disentangling the roles of the regional species pool and local habitat in shaping community assembly. The modified stochasticity ratio (MST) declined non-linearly with increasing HRT and a breakpoint at 10 d was identified. Reservoirs with HRT < 10 d had MST > 0.5, whereas those with HRT > 10 d showed an MST < 0.5, indicating a shift from stochastic- to deterministic-dominated assembly across this threshold. For reservoirs with HRT < 10 d, observed β-γ relationships matched null expectations, consistent with a stronger influence of the regional species pool; stochastic processes were dominated by drift and dispersal. Among reservoirs > 10 d HRT, observed β-γ relationships deviated from null expectations, indicating stronger local habitat filtering. Nutrients (nitrogen and phosphorus) explained ∼33% of habitat effects, while water temperature explained ∼13%. Deterministic assembly was characterized by strong species sorting and niche partitioning. By linking HRT to phytoplankton assembly timescales, our findings identified an operationally relevant 10-day HRT threshold, highlighting that strategic manipulation of HRT can steer phytoplankton assembly and support ecosystem resilience in dammed rivers.