CO2-O2 time series Danube Delta 2016-2018

The time series reveal two orders of magnitude variability in daily CO2 fluctuations along the main Danube reaches with typical maxima in the early morning and minima in the afternoon. The amplitude of monthly averaged daily cycles was 2-2.5 times larger in delta channels compared to the river reaches and the parameter for metabolic intensity reacted on average four times more sensitive to changes in water temperature and cloud cover within the delta compared to the main river.

Inflow of O2-depleted and CO2-rich wetland water to the downstream river stations was most intense during spring floods with estimated mixing rations of up to 5-20% depending on the station. Stoichiometry of O2-CO2 changes pointed to strong contributions of methane oxidation and/or nitrification in 40% of the analysed summer data, which was also evident from the frequently observed oxygen deficiency. During June-July, a monitored lake system and an adjacent channel showed clear evidence for calcite precipitation and a switch to photosynthetic uptake of HCO3-.

The study illustrates the benefits of a combining covariance analysis with 24-hours averaging for identifying the timing, intensity and type of processes in river metabolism and lateral exchange