Thermal stratification onset increases phytoplankton resource use efficiency by promoting photosynthetic activity in a subtropical reservoir.

Phytoplankton resource use efficiency (RUE), expressed as biomass per unit nutrient (RUETN, RUETP) integrates phytoplankton productivity and nutrient exploitation in aquatic ecosystems, yet how thermal stratification affects RUETN, RUETP, and the underlying photosynthetic mechanisms remain unclear. We used five years of monitoring data (2015-2020; n = 171) from an oligotrophic subtropical reservoir and divided the data into three thermal stratification periods: non-stratified (NS, n = 66), weakly stratified (WS, n = 44), and strongly stratified (SS, n = 61). We quantified the relationship between RUETN, RUETP, and the photosynthetic capacity of phytoplankton communities. RUETN and RUETP were significantly lower during NS (deep mixing, Zmix:Zeu = 4.7) than during WS and SS, when Zmix:Zeu fell to 3.6. This transition coincided with a recovery of Chlorophyta photosynthetic capacity (Fv/Fm-Gr) from 0.09 under deep mixing to 0.20-0.30 once stratification set in, and Fv/Fm-Gr was positively correlated with both RUETN (R = 0.29) and RUETP (R = 0.37). The proportion of Chlorophyta nearly doubled across this transition (NS 25.5% to SS 47.2%). Bacillariophyta Fv/Fm (Fv/Fm-Br) showed no significant period difference, and Cyanobacteria Fv/Fm (Fv/Fm-Bl) remained near zero across NS, WS, and SS. PLS-PM revealed that stratification elevated RUE via surface TN and TP reductions during WS and through additional direct physical effects during SS. Water level decline prolonged the light-suppressed state during the NS period by reducing Zeu. Our results indicated that the onset of thermal stratification is a critical condition for the recovery of Chlorophyta photosynthetic capacity, with Zmix governing RUE transitions between stratification states and Zeu modulating RUE within each state. Overall, our study provides evidence that water level drawdown management may serve as a supplementary measure to external nutrient loading reduction for controlling algal biomass during the winter mixing period.