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

Canning, Anna, Wehrli, Bernhard and Körtzinger, Arne (2021) Methane in the Danube Delta: The importance of spatial patterns and diel cycles for atmospheric emission estimates. Open Access Biogeosciences (BG), 18 (12). pp. 3961-3979. DOI 10.5194/bg-18-3961-2021.

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Supplementary data:

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.

Document Type: Article
Funder compliance: info:eu-repo/grantAgreement/EC/H2020/643052
Keywords: Danube Delta; atmosphere; greenhouse gas
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
Main POF Topic: PT6: Marine Life
Refereed: Yes
Open Access Journal?: Yes
Publisher: Copernicus Publications (EGU)
Related URLs:
Projects: C-CASCADES, Digital Earth
Date Deposited: 19 Oct 2020 08:02
Last Modified: 07 Feb 2024 15:41
URI: https://oceanrep.geomar.de/id/eprint/50709

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