Spatio-temporal variations in lateral and atmospheric carbon fluxes from the Danube Delta

[摘要] 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 (CO 2 and CH 4 ) 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 CO 2 and CH 4 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 CH 4 . The median CO 2 fluxes from river branches, channels and lakes are 25, 93 and 5.8 mmol m −2  d −1 , respectively. Median total CH 4 fluxes amount to 0.42, 2.0 and 1.5 mmol m −2  d −1 . While lakes do have the potential to act as CO 2 sinks in summer, they are generally the largest emitters of CH 4 . Small channels showed the largest range in emissions, including a CO 2 and CH 4 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 ).

[学科分类] 大气科学