Hydrodynamic and primary production effects on seasonal DO variability in the Danube River

Abstract. Dissolved oxygen (DO) is a fundamental indicator for water quality and ecosystem health, particularly in the context of anthropogenic impacts and climate change. This study presents the first large-scale dataset of DO concentrations combined with its stable oxygen isotope ratios (expressed as δ18ODO), particulate organic carbon concentrations (POC), and respiration / photosynthesis (R / P ratios) from five seasonal campaigns along the entire Danube River in 2023 and 2024. Our findings reveal pronounced seasonal DO driven by temperature, biological activity, and hydrodynamic conditions. During spring and summer, enhanced photosynthesis increased DO up to 0.40 mmol L−1, with δ18ODO values down to +12.1 ‰ and POC up to 0.25 mmol L−1 in two highly productive river sections. Low R / P ratios of 0.1 further indicated strong net autotrophic conditions. Strong correlations between δ18ODO and POC additionally confirm the influence of primary producers (i.e., photosynthetic organisms) in a river section where a reduced slope led to slower flow and lower turbulence. Notably, δ18ODO values were lower than those expected for atmospheric equilibrium (+24.6 ‰ ± 0.4 ‰), a pattern rarely documented in large river systems. In contrast, tributary inflows from the Tisa and Sava rivers diluted biomass and organic material inputs and led to declines in DO and POC. By late summer, intensified respiration reversed photosynthetic signals, led to the lowest DO concentrations down to 0.16 mmol L−1, and raised δ18ODO up to +23.7 ‰, particularly in the Sava River. In fall, DO levels partially recovered despite continued respiration, as indicated by the highest observed δ18ODO values of +25.9 ‰ and the highest R / P ratios of the entire season, reaching 8.9. In winter, oxygen input from the atmosphere became dominant with minimal biological influences. Overall, this study provides new insights into oxygen sources and sinks across the river continuum over several seasons. These new insights underscore the need for continuous DO monitoring, particularly in late summer when oxygen levels can become critically low. Understanding these interactions can help to establish efficient aqueous ecosystem management and conservation strategies in the face of land use and climate change.