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

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.