Hydrodynamic controls on persistent high algal biomass in stratified reservoirs: Retention and nutrient supply in recirculation zones

Algal blooms in reservoirs are occurring with increasing frequency, especially under global warming and eutrophication. Hydrodynamic conditions are recognized as key factors in algal proliferation by altering flow velocities and material transport processes; however, the underlying mechanisms remain poorly understood. In this study, we employed the CE-QUAL-W2 model (2019–2023) to investigate how hydrodynamics sustains high algal biomass in a deep channel-type reservoir, utilizing density-current classification statistics and flux analysis. The riverine zone was strongly affected by runoff events, whereas the photic zone in the transitional and lacustrine zones maintains consistently high chlorophyll-a (Chl- a ) concentrations. This spatial pattern was associated with a persistent recirculation zone extending over the transition-lacustrine reach, where low flow velocity limited direct flushing. Interflow and underflow collectively accounted for nearly 80% of the inflow volume and transported through mid-depth or deep pathways, thereby separating most external nutrient loading from the surface algal hotspot.. Flux analysis of this hotspot indicated that vertical transport is the primary nutrient source, with dissolved phosphorus (PO 4 -P) fluxes of approximately 6, 15, and 57 kg d −1 under overflow, interflow, and underflow conditions, respectively. Under underflow conditions, vertical nutrient supply exceeded internal regeneration by more than 7 times. The generalized additive model ( n = 1826) further indicated that the vertical PO₄-P flux increased nonlinearly when the inflow discharge approached approximately 100 m 3 s −1 , thereby partially offsetting dilution or washout within the hotspot under high-flow conditions. Overall, these results suggest that recirculation zones provide stable retention conditions conducive to algal proliferation and aggregation, while also serving as the primary pathway for nutrient supply, thereby offering a mechanistic explanation for the persistence of algal hotspots under hydrologically active conditions.