LAUSR

Abstract. River deltas with their mosaic of ponds, channels and seasonally inundated areas act as the last continental hotspots of carbon turnover along the land-ocean aquatic continuum. There is increasing evidence for the important role of riparian wetlands in the transformation and emission of terrestrial carbon to the atmosphere. The considerable spatial heterogeneity of river deltas, however, forms a major obstacle for quantifying carbon emissions and their seasonality. While river reaches crossing the delta can serve as reference systems, delta lakes are often dominated by aquatic production and channels act as collection systems for carbon exported from adjacent wetlands. In order to quantify carbon turnover and emissions in the complex mosaic of the Danube Delta, we conducted monthly field campaigns over two years at 19 sites spanning river reaches, channels and lakes. Here we report greenhouse gas fluxes (CO2 and CH4) from the freshwater systems of the Danube Delta and present the first seasonally resolved estimates of its freshwater carbon emissions to the atmosphere. Furthermore, we quantify the lateral carbon transport of the Danube River to the Black Sea. We estimate the delta’s CO2 and CH4 emissions to be 65 GgC yr−1, of which about 8 % are released as CH4. The median CO2 fluxes from river branches, channels and lakes are 25, 93 and 5.8 mmol m−2 yr−1, respectively. Median total CH4 fluxes amount to 0.42, 2.0 and 1.5 mmol m−2 yr−1. While lakes do have the potential to act as CO2 sinks in summer, they are generally the largest emitters of CH4. Small channels showed the largest range in emissions including a CO2 and CH4 hotspot sustained by adjacent wetlands. The channels thereby contribute disproportionately to the delta’s emissions considering their limited surface area. In terms of lateral export, we estimate the net export of the Danube Delta to the Black Sea to about 160 GgC yr−1, which only marginally increases the carbon load from the upstream river catchment (8490 GgC yr−1) by about 2 %. While this contribution of the delta seems small, deltaic carbon yield (45.6 gC m−2 yr−1, net export load/surface area) is about 4-fold higher than the riverine carbon yield from the catchment (10.6 gC m−2 yr−1).