Tracing the origin and fate/behaviour of nutrients and water in the Danube River using stable isotope techniques

Within the frame of the Danube River Survey 2025 we will especially look at the source and fate of nutrients (namely carbon, phosphate, sulphate and nitrate) and water in the Danube river using stable isotope techniques. As a basic but important parameter we will determine both hydrogen and oxygen isotope composition of the river water (2H-H2O and 18O-H2O) to define the contribution of various sources of water as well as processes such as evaporation using a mixing model approach. This information is the basis for interpreting the fate and behaviour of nutrients in the river system. Among the nutrients, we will investigate the carbon isotope composition of dissolved inorganic carbon (13C-DIC), which will give the possibility to assess the source of dissolved CO2, especially estimating the contribution from organic material degradation. The oxygen isotope composition of phosphate (18O-PO4) will be used to draw conclusion on its source but also to identify the microbial turnover of phosphate in the river system. The later is done by comparing 18O-PO4 with the 18O-H2O of the river water. The sulphur and oxygen stable isotope composition of sulphate (34S-SO4, 18O-SO4) is used to identify its source but most importantly to define its biochemical turnover in the river system. Last but not least the oxygen and nitrogen isotope composition of nitrate (18O-NO3 and 15N-NO3) will help to define nitrogen sources and quantify processes in the nitrogen turnover. The parameters are interlinked and will give a full picture on the sources and the prevailing biochemical processes such as organic matter degradation, sulphate reduction, denitrification, and biotic phosphate turnover. The stable isotope results will be interpreted in the light of previous surveys (1988, 2008 and 2019), where stable isotopes of water and nitrate had been monitored.