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).

中文翻译：

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多瑙河三角洲的横向和大气碳通量的时空变化

河流三角洲及其池塘，河道和季节性淹没地区的马赛克，是陆地-海洋水生生物连续体最后一个大陆上碳交易的热点地区。越来越多的证据表明，河岸湿地在陆地碳向大气的转化和排放中的重要作用。但是，河三角洲的空间异质性很大，这是量化碳排放及其季节性的主要障碍。上游集水区定义了河段的水化学性质，而三角洲湖泊和河道则以局部过程为主，例如水生初级生产，呼吸作用或与湿地的横向交换。为了量化多瑙河三角洲复杂镶嵌图中的碳营业额和排放量，我们在两年内每月在19个地点进行了每月的野战活动，横跨河流，河道和湖泊。在这里，我们报告温室气体通量（CO₂和CH ₄）来自多瑙河三角洲的淡水系统，并提出了其季节性淡水向大气中碳排放的第一个季节性解析估计。此外，我们量化了多瑙河向黑海的横向碳迁移。我们估计三角洲的CO ₂和CH ₄排放量为65 GgC yr ^-1（30-120 GgC yr ^-1，该范围使用25至75观测通量的百分位数），其中约8％以CH _4的形式释放。来自河道，河道和湖泊的CO ₂通量的中值分别为25、93和5.8 mmol m ^-2  d ^-1。CH _4的中位数通量为0.42、2.0和1.5mmol m ^-2  d ^-1。尽管湖泊确实有可能在夏季充当CO _2的汇入区，但它们通常是CH ₄的最大排放者。小通道显示出最大的排放范围，包括相邻湿地维持的CO ₂和CH ₄热点。因此，考虑到其有限的表面积，这些通道对三角洲的排放造成了不成比例的贡献。在横向出口方面，我们估计从多瑙河三角洲到黑海的净出口总量（溶解的无机碳，DIC，溶解的有机碳，DOC和颗粒有机碳，POC的总和）约为160  ±  280 GgC yr ^-1，这仅会使上游河流集水区的碳负荷（8490 ±  240 GgC yr ^-1）略微增加 约2％。尽管三角洲的贡献很小，但三角洲的碳产量（45.6 gC m ^-2  yr ^-1；净出口负荷/表面积）比集水区的河流碳产量（10.6 gC m ^-2  yr- ）高约4倍^{。 1}）。