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 concentration combined with its stable oxygen isotope ratios (expressed as δ¹⁸O_DO), 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 dynamics driven by temperature, biological activity and hydrodynamic conditions. During spring and summer, enhanced photosynthesis increased DO up to 0.40 mmol/L with δ¹⁸O_DO values down to +12.1 ‰ and POC up to 0.25 mmol/L in two highly productive river sections. The strong correlation between δ¹⁸O_DO and POC further confirms the dominant influence of primary producers (i.e., photosynthetic organisms) in a river section where a reduced slope led to slower flow and lower turbulence. Notably, δ¹⁸O_DO values were significantly 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 and raised δ¹⁸O_DO up to +23.7 ‰, particularly in the Sava River. In fall, DO levels partially recovered despite continued respiration, while in winter, oxygen input from the atmosphere became the dominant control with minimal biological influences. Overall, this study provides new insights into dynamic interplays between oxygen sources and sinks across the river continuum over 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 environmental and climate change.