| 26 Nov 2025
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 CO2-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.
This is a very nice dataset and the manuscript is well-written. Being biased to my own work and that of my co-authors, I have two comments to the authors that can improve their interpretation of the data:
1) The more accurate way to get inference on look at O2-CO2 curves is as O2-DIC curves. This is especially true in high alkalinity systems like you have. Temperature and carbonate-equilibria have a large effect on the interpretation of these curves. For more detail you can see:
a) all the analysis Diamond, J. S., Truong, A. N., Abril, G., Bertuzzo, E., Chanudet, V., Lamouroux, R., & Moatar, F. (2025). Inorganic carbon dynamics and their relation to autotrophic community regime shift over three decades in a large, alkaline river. Limnology and Oceanography, 70(5), 1122-1136.
b) a bit hidden, but seen Supplementary Figure S20 in: Diamond, J. S., & Bertuzzo, E. (2025). A coupled O2‐CO2 model for joint estimation of stream metabolism, O‐C stoichiometry, and inorganic carbon fluxes. Journal of Geophysical Research: Biogeosciences, 130(4), e2024JG008401.
2) You claim in the abstract and in the discussion that there is clear evidence of calcite precipitation and autotrophic HCO3 uptake. I do not agree that this is clearly shown in the results. I recommend reading in detail (and the supplementary information) our paper (a) above, which elaborates a means to test for calcite precipitation and HCO3 uptake using data that you already have.
Finally, while it may not be an objective of the paper, it would seem to be a natural extension of this work to estimate metabolism and air-water CO2 flux using your data.
Regards,
Jake Diamond