High-frequency O₂-CO₂ records reveal intensity of river metabolism and lateral exchange in the Danube Delta

Abstract. Due to their intense terrestrial-aquatic linkages and intense ecosystem metabolism, many river deltas are aquatic hot spots for carbon dioxide (CO₂) emissions to the atmosphere. A patchwork of wetlands, lakes, channels, and river reaches often complicates the analysis of CO₂ sources such as ecosystem respiration or lateral transfer. Sensing techniques offer the opportunity of measuring the paired CO_2-O₂ concentrations at high temporal resolution for periods from days to months. Such time-series allow quantifying diurnal and seasonal cycles of river metabolism and lateral exchange. This study documents paired O₂-CO₂ measurements at 15-minute time resolution obtained from deployment of sensor packages in river reaches and channels of the Danube Delta in Romania during a total observation period of three years. We show how to combine results from covariance analysis with insights from averaged 24-hour diurnal cycles. 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.