Eutrophication-driven widespread surface dissolved oxygen enrichment in Chinese lakes: A warming-vulnerable transient state?

Dissolved oxygen (DO) is a critical determinant of lake ecosystem function and services. The interaction between climate warming and anthropogenic nutrient inputs exerts opposing effects on lake DO—warming decreases gas solubility (DOsat), while nutrient enrichment enhances net ecosystem production through eutrophication-driven primary production, potentially leading to transient DO enrichment. This creates a vulnerable state that may shift under future climate-nutrient interactions, which limits accurate predictions of lake DO dynamics. Here, we calculated ΔDO (= DO – DOsat) in the surface waters of 99 freshwater lakes in China (66 subtropical and 33 temperate), spanning a broad trophic gradient (TN: 0.1–8.3 mg·L⁻¹; TP: 5.4–256 μg·L⁻¹). Using a combined process-based and data-driven modeling approach, we examined DO dynamics in relation to water temperature, primary productivity, and water quality. Despite rapid lake warming (0.38 [0.19–0.48]°C·decade⁻¹) and a corresponding decline in DOsat (-0.06 [-0.18 – -0.02] mg·L⁻¹·yr⁻¹), we observed widespread DO enrichment from 2006 to 2018, with an average ΔDO increase of 0.07 [-0.15–0.42] mg·L⁻¹·yr⁻¹. This enrichment was most pronounced in eutrophic lakes, driven by enhanced primary production. However, future projections suggest that current DO enrichment trends may be a transient state and reverse due to a disproportionate increase in DO consumption relative to production, driven by asymmetric responses to warming, potentially leading to deoxygenation. While nutrient management is widely regarded as an effective strategy against eutrophication, our findings underscore the need to carefully assess its coupled effects on lake DO before implementation. Given the accelerating warming of water bodies, developing nutrient management strategies informed by lake DO levels should be a priority.