17 Oct 2020

17 Oct 2020

Review status: a revised version of this preprint is currently under review for the journal BG.

Methane in the Danube Delta: The importance of spatial patterns and diel cycles for atmospheric emission estimates

Anna Canning1, Bernhard Wehrli2,3, and Arne Körtzinger1,4 Anna Canning et al.
  • 1GEOMAR Helmholtz-Zentrum für Ozeanforschung, Kiel, Schleswig-Holstein, Germany
  • 2Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Zürich, 8092, Switzerland
  • 3Eawag, Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, 6047, Switzerland
  • 4Christian-Albrechts-Universität zu Kiel, Kiel, Schleswig-Holstein, Germany

Abstract. Methane (CH4) is one of the substantial greenhouse gases in our atmosphere and its concentration has increased by ~ 4 % over the last decade. Although sources driving these increases are not well constrained, one potential contribution comes from wetlands, which are usually intertwined with rivers, channels and lakes, creating a considerable need to acquire higher resolution data to facilitate 5 modelling and predictions. Here we took a fully contained sensor set-up to obtain measurements of CO2, CH4, O2 and auxiliary parameters, installed on a houseboat for accessibility, to assess and analyse surface water concentrations within the Danube Delta, Romania. Over 3 seasons, we transected a ~ 400 km route with concentration mapping and additional stations for monitoring diel cycles. Overall, the delta was a source for CH4 throughout all seasons, with concentrations ranging between 0.113–15.6 μmol L−1. The dataset was split into three different subsystems; lakes, rivers and channels, with channels 10 showing the highest variability. We found large to extreme diel cycles in both the lakes and channels, with concentrations varying by an order of magnitude between these two systems. The observed strong diel cycle within the lake suggests daily vertical stratification allowing for macrophytes to create a temporal oxycline due to lack of light and movement between the stems as previously suggested. While throughout the day, there was a consistent overall surface concentration of CH4 at around 0.4 μmol L−1, there was a clear linear trend with an O2:CH4 molar ratio of −50:1 during the phase of nocturnal convection 15 with the two water stratified bodies mixing during the night. Daily spot sampling techniques and neglecting such diel cycles reducing the estimated average methane concentrations by 25 % and increase by 3.3 % for channels and lakes, respectively. On an individual lake basis, spot sampling can potentially incur an uncertainty range of a factor of 4.5. Analyses also included a ‘hot spot’, with a 10-fold stronger methane increase (4–15.6 μmol L−1) overnight compared to the lake, with an almost immediate and extreme decrease in CH4 following sunrise. Calculated diffusive CH4 fluxes for the overall delta yielded an 20 average of 49 ± 61 μmol m−2 h−1 corresponding to an extrapolated annual flux of 0.43 ± 0.53 μmol m−2 yr−1. Our data illustrate the importance of collecting information on diel cycles in different habitats to improve the emission estimates from wetland systems.

Anna Canning et al.

Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

Anna Canning et al.

Anna Canning et al.


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Short summary
Inland waters, are usually not well restrained in terms of greenhouse gas measurements. One of these regions is the Danube River Delta, Romania. Therefore, we measured continuously with sensors to collect high resolution data for CO2, CH4 and O2, throughout the Danube Delta. We found significant variation over the day and night and between regions, for all concentrations. This leads to the implications of spot sampling just during the day, which could potentially lead to vast underestimations.