Abstract
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 modelling and predictions. Here we took a fully contained sensor set-up to obtain measurements of CH4, O2 and auxiliary parameters, installed on a houseboat for accessibility, to assess and analyse surface water concentrations within the Danube Delta, Romania. During three expeditions in different seasons, we transected a ∼ 400 km route with concentration mapping and two 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. Calculated diffusive CH4 fluxes for the overall delta yielded an average of 49 ± 61 µmol m−2 h−1, corresponding to an extrapolated annual flux of 0.43 ± 0.53 mol m−2 yr−1. The dataset was split into three different subsystems – lakes, rivers and channels – with channels showing the highest variability. We found overlapping CH4 concentrations throughout each subsystem, with large inflows coming from reed beds and channels into the lakes. Seasonal variability and water flow direction also influenced the overall dynamics in each region. 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 lake diel cycle showed a clear linear trend with an O2:CH4 molar ratio of -50:1 during the phase of nocturnal convection, with the two water stratified bodies mixing during the night, suggesting daily vertical stratification allowing for macrophytes to create a temporal oxycline due to a lack of light and movement between the stems as previously suggested, and potentially incurring an uncertainty range of a factor of 4.5. Our data illustrate the importance of high-resolution spatio-temporal data collection throughout the entire delta and the increased need for diel cycles in different habitats to improve the concentration and emission estimates from wetland systems.
Highlights
Methane (CH4) is one of the most relevant anthropogenic greenhouse gases following carbon dioxide (CO2), with an estimated global emission rate of 572 Tg CH4 yr−1 for the decade 2003–2012 (Saunois et al, 2020)
The overall Danube Delta surface waters were a source of CH4, at a mean concentration of 1.7 ± 1.93 μmol L−1 and calculated aquatic emission to the atmosphere of 0.43 ± 0.53 mol m−2 yr−1
This is comparable to concentrations and a diffusive flux mean of other systems of this type and size
Summary
Methane (CH4) is one of the most relevant anthropogenic greenhouse gases following carbon dioxide (CO2), with an estimated global emission rate of 572 Tg CH4 yr−1 for the decade 2003–2012 (Saunois et al, 2020). We have seen an accelerated increase from 1775 ppb in 2006 to 1850 ppb in 2017, and over a 100-year interval, CH4 is 34 times more potent as a greenhouse gas than CO2 when including climate carbon feedbacks (28 times without feedbacks: Myhre et al, 2013; Schubert and Wehrli, 2019). This continued increase has the potential to reverse any progress made for climate mitigation by reducing CO2 emissions (Nisbet et al, 2019). Inland waters are known to have a significant CH4 source strength and, have seen an increase in attention (see Abril and Borges, 2005; Panneer Selvam et al, 2014; Richey et al, 2002; Wang et al, 2009; Melton et al, 2013; Zhang et al, 2018)
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