Seasonal Dynamics of Dissolved Oxygen in the Danube River: The Role of Primary Producers and Slope

Dissolved oxygen (DO) is one of the most fundamental health indicators of aqueous ecosystems. Beyond climate change and habitat modifications, excessive nutrient and pollutant inputs can significantly degrade this critical water quality parameter. Eutrophication, for instance, can drive harmful algal blooms, oxygen depletion and biodiversity loss.

In our study, we present seasonal data on DO concentrations and its oxygen stable isotope ratios (δ¹⁸O_DO*) from five campaigns along the Danube River in 2023 and 2024, complemented by particulate organic carbon (POC) data as a biomass indicator. Our results highlight dynamic seasonal patterns. Photosynthesis dominated in spring and summer, while respiration and atmospheric equilibration prevailed in fall and winter. Notable hotspots were identified in the middle and lower Danube, with DO peaks of 0.35 mmol/L and 0.40 mmol/L, accompanied by δ¹⁸O_DO* enrichments of +9.8 ‰* and +12.5 ‰* and POC concentrations of 0.25 mmol/L and 0.24 mmol/L. These regions, characterized by reduced river gradient und resulting lower flow velocities and turbulence, suggest enhanced primary producer activity. Notably, nutrient levels remained low, with nitrate under 0.29 mmol/L and phosphate largely undetectable, indicating minimal anthropogenic influence—likely due to environmental improvements and reduced industrial impacts in the catchment. All DO levels were within safe ecological ranges (> 0.06 mmol/L), ruling out hypoxia or harmful algal blooms.