Department Surface Waters - Research and Management

River deltas, with their mosaic of ponds, channels and seasonally inundated areas, act as the last continental hot spots 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. The water chemistry in the river reaches is defined by the upstream catchment, whereas delta lakes and channels are dominated by local processes such as aquatic primary production, respiration or lateral exchange with the wetlands. In order to quantify carbon turnover and emissions in the complex mosaic of the Danube Delta, we conducted monthly field campaigns over 2 years at 19 sites spanning river reaches, channels and lakes. Here we report on the 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 (30–120 GgC yr^-1, a range calculated using 25 to 75 percentiles of observed fluxes), 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 d^-1, respectively. Median total CH₄ fluxes amount to 0.42, 2.0 and 1.5 mmol m^-2 d^-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 hot spot sustained by adjacent wetlands. Thereby, the channels contribute disproportionately to the delta's emissions, considering their limited surface area. In terms of lateral export, we estimate the net total export (the sum of dissolved inorganic carbon, DIC, dissolved organic carbon, DOC, and particulate organic carbon, POC) from the Danube Delta to the Black Sea to be about 160 ± 280 GgC yr^-1, which only marginally increases the carbon load from the upstream river catchment (8490 ± 240 GgC yr^-1) by about 2 %. While this contribution from the delta seems small, deltaic carbon yield (45.6 gC m^-2 yr^-1; net export load/surface area) is about 4 times higher than the riverine carbon yield from the catchment (10.6 gC m^-2 yr^-1).

Global estimates see river deltas and estuaries contributing about equally to CO₂ and CH₄ emissions as lakes and reservoirs, despite a factor 6 smaller surface area. Assessing the horizontal gradients in dissolved gas concentrations from large river reaches to connecting canals and wetland lakes remains a challenge in many deltaic systems. To elucidate the processes affecting local CO₂ and CH₄ concentrations in the Romanian part of the Danube Delta, we mapped dissolved O₂, N₂, He and Ar using a portable gas-equilibration membrane-inlet mass spectrometer (GE-MIMS), along with CO₂, CH₄, water temperature and conductivity. We measured the concentrations along the aquatic continuum from a small houseboat during two campaigns, in spring and autumn, to capture different hydrological and plant growth conditions. Delta-scale concentration patterns were comparably stable across seasons. Small connecting channels were highly influenced by the riparian wetland, which was strongest in the eastern part of the biosphere reserve. These sites represented the delta’s CO₂ and CH₄ hotspots and showed clear signs of excess air, i.e., supersaturation of dissolved noble gases with respect to air-saturated water. As the adjacent wetland was permanently inundated, this signal was likely caused by root aeration of Phragmites australis, as opposed to traditional excess air formation via water table fluctuations in the unsaturated zone. The special vegetation setting with reed growing on floating peat coincided with the highest CO₂ and CH₄ concentrations (>700 μmol/L CO₂ and 13 μmol/L CH₄, respectively) observed in an adjacent channel. Shallow lakes, on the other hand, were major sites of photosynthetic production with O₂ oversaturation reaching up to 150% in spring. The observed deficit in non-reactive gases (He, Ar and N₂) indicated that the lakes were affected by O₂ ebullition from macrophytes. According to our estimations, this ebullitive flux decreased O₂ concentrations by up to 2 mg/L. This study highlights the effect of plant-mediated gas transfer on dissolved gas concentrations and supports recent studies stressing the need to account for ebullitive gas exchange when assessing metabolism parameters from O₂ in shallow, productive settings.