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<p>River systems form critical interfaces in the terrestrial-atmospheric carbon cycle, yet basin-scale constraints on the spatial and seasonal controls of inorganic carbon dynamics and CO<sub>2</sub> exchange remain limited. We present a multi-seasonal longitudinal assessment of dissolved inorganic carbon (DIC), its stable isotopes (<i>δ</i><sup>13</sup>C<sub>DIC</sub>), and aqueous CO<sub>2</sub> partial pressure (<i>p</i>CO<sub>2(aq)</sub>) along the entire Danube River. Five sampling campaigns captured seasonal variability along the main stem and major tributaries of this geologically diverse basin. DIC comprised &gt; 90% of total carbon, showing limited longitudinal and seasonal variability (0.3 to 5.3 mmol L<sup>− 1</sup>), with highest concentrations in the upper Danube due to groundwater inputs and carbonate weathering, followed by downstream homogenization. In contrast, <i>δ</i><sup>13</sup>C<sub>DIC</sub> increased downstream from − 13.3 to -8.7‰, indicating cumulative CO<sub>2</sub> degassing, with deviations linked to tributary inflows and biological activity. C<sub>4</sub> plant inputs may have contributed up to 27.2% to the <i>δ</i><sup>13</sup>C<sub>DIC</sub>. River <i>p</i>CO<sub>2(aq)</sub> ranged from ∼260 to 3,770 µatm, predominantly supersaturated, with CO<sub>2</sub> fluxes of 0.975–1.993 Gg C d⁻¹, confirming the Danube as a persistent CO<sub>2</sub> source. Near-equilibrium conditions in spring and summer coincided with enriched <i>δ</i><sup>13</sup>C<sub>DIC</sub>, consistent with episodic photosynthetic uptake. These results reveal how groundwater inputs, in-stream photosynthesis, and air-water exchange regulate inorganic carbon dynamics and CO<sub>2</sub> emissions in large rivers.</p>

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Geological, land use and biological influences on carbon cycling and CO2 degassing in the Danube River

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年6月26日

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  • Jan Maier,
  • Johannes A. C. Barth

摘要

River systems form critical interfaces in the terrestrial-atmospheric carbon cycle, yet basin-scale constraints on the spatial and seasonal controls of inorganic carbon dynamics and CO2 exchange remain limited. We present a multi-seasonal longitudinal assessment of dissolved inorganic carbon (DIC), its stable isotopes (δ13CDIC), and aqueous CO2 partial pressure (pCO2(aq)) along the entire Danube River. Five sampling campaigns captured seasonal variability along the main stem and major tributaries of this geologically diverse basin. DIC comprised > 90% of total carbon, showing limited longitudinal and seasonal variability (0.3 to 5.3 mmol L− 1), with highest concentrations in the upper Danube due to groundwater inputs and carbonate weathering, followed by downstream homogenization. In contrast, δ13CDIC increased downstream from − 13.3 to -8.7‰, indicating cumulative CO2 degassing, with deviations linked to tributary inflows and biological activity. C4 plant inputs may have contributed up to 27.2% to the δ13CDIC. River pCO2(aq) ranged from ∼260 to 3,770 µatm, predominantly supersaturated, with CO2 fluxes of 0.975–1.993 Gg C d⁻¹, confirming the Danube as a persistent CO2 source. Near-equilibrium conditions in spring and summer coincided with enriched δ13CDIC, consistent with episodic photosynthetic uptake. These results reveal how groundwater inputs, in-stream photosynthesis, and air-water exchange regulate inorganic carbon dynamics and CO2 emissions in large rivers.