Summer heatwaves increased surface dissolved oxygen through algal photosynthetic compensation in a eutrophic lake

Dissolved oxygen (DO) serves as the material basis for biogeochemical cycling and maintaining aquatic ecosystem health. Under the threat of global warming, the decline in DO driven by increasing temperature and lake heatwaves results in profound negative impact on aquatic ecosystems. However, many studies have overlooked the replenishing effects of heatwaves on oxygen through promoting algal blooms and their photosynthesis. Therefore, the comprehensive mechanism of increasing temperature, especially the heatwaves, on DO remains unclear. In this study, we combined the high-frequency water quality data derived from hyperspectral proximal sensing during 2022–2024 and interpretable modeling approaches to clarify the comprehensive mechanism of summer heatwaves on DO dynamics in Lake Taihu. The results showed a counterintuitive 21.7% increase in surface DO concentrations during summer heatwaves (6.67 ± 2.67 mg/L) compared to non-heatwave periods (5.48 ± 1.54 mg/L) in Lake Taihu. Integrating Random Forest (RF) modeling with SHAP analysis, we quantified that Chlorophyll-a (Chla) contributed 54.6% to DO variability, effectively counteracting the negative influence of thermal deoxygenation. Structural equation modeling (SEM) further revealed that heatwaves amplified the positive coupling between Chla, dissolved organic carbon (DOC), and DO. Crucially, despite the rise in surface DO, the probability of bottom-layer hypoxia at 7:00 increased by 226.9%, exacerbating the ecological risks associated with bottom oxygen depletion. Our results demonstrate that with a doubling of Chla concentrations during heatwaves, a potent photosynthetic compensation was triggered. This effect was sufficient to counteract the oxygen losses from decreased solubility and accelerated DOC mineralization, ultimately leading to the observed rise in surface DO. The findings provide mechanistic insights into the interactions between climate change and eutrophication, underscoring the urgent need for adaptive management strategies that account for oxygen modulating. • Summer heatwaves raised surface DO by 1.19 mg/L in eutrophic Lake Taihu. • Algal photosynthesis and respiration dominate DO dynamics in summer heatwaves. • Despite rising daytime surface DO, bottom hypoxia at 7:00 increased significantly.