Widespread nighttime hypoxia revealed in global high-frequency dissolved oxygen data

Riverine and lacustrine hypoxia is an escalating environmental concern, increasing in both frequency and spatial extent. However, hypoxic events occur more frequently at night, yet existing research lacks quantitative analyses of these occurrences and cannot capture sub-daily fluctuations or the true occurrence and intensity of nighttime hypoxia due to insufficient high-frequency monitoring. Using a global compilation of high-frequency dissolved oxygen (DO) and water temperature (WT) records, we show that low-frequency observations substantially underestimate hypoxia, with more than 90% of sites affected and many underestimating hypoxia days by over 50%. On average, daytime DO concentrations were ~4% higher than nighttime values, and sites with larger diel amplitudes exhibited disproportionately high underestimation. Across global rivers, 64% of sites were classified as night-dominated hypoxia, 24% as day-dominated, and 12% as balanced diel hypoxia, highlighting the dominant role of nocturnal oxygen depletion in driving systematic underestimation. Diel DO changes were positively correlated with diel WT changes and varied strongly across major habitat types, with the largest daytime enrichment in Oceanic islands (1.16 ± 2.18 mg L⁻¹) and temperate upland rivers (0.64 ± 0.91 mg L⁻¹), but slightly negative values in large lakes (–0.039 ± 0.67 mg L⁻¹) and temperate coastal rivers (–0.045 ± 0.27 mg L⁻¹). These results emphasize the need for incorporating high-resolution DO monitoring to accurately assess ecosystem stress, guide water quality management, and reveal the true dynamics of oxygen fluctuations in riverine systems.