Unexpected large evasion fluxes of carbon dioxide from turbulent streams draining the world’s mountains

Inland waters, including streams and rivers, are active components of the global carbon cycle. Despite the large areal extent of the world’s mountains, the role of mountain streams for global carbon fluxes remains elusive. Using recent insights from gas exchange in turbulent streams, we found that areal CO2 evasion fluxes from mountain streams equal or exceed those reported from tropical and boreal streams, typically regarded as hotspots of aquatic carbon fluxes. At the regional scale of the Swiss Alps, we present evidence that emitted CO2 derives from lithogenic and biogenic sources within the catchment and delivered by the groundwater to the streams. At a global scale, we estimate the CO2 evasion from mountain streams to 167 ± 1.5 Tg C yr−1, which is high given their relatively low areal contribution to the global stream and river networks. Our findings shed new light on mountain streams for global carbon fluxes

Divergent gas transfer velocities of CO₂, CH₄, and N₂O over spatial and temporal gradients in a subtropical estuary

High global uncertainties remain in water-air CO₂, CH₄, and N₂O fluxes from estuaries due to spatial and temporal variability and the poor predictability of the gas transfer velocity (k₆₀₀). This is the first study that directly compares k₆₀₀ of CO₂, CH₄, and N₂O in an estuary with the aim to evaluate the accuracy of using a uniform k₆₀₀ value for estimating water-air fluxes. We calculated 155 k₆₀₀ values from CO₂, CH₄, and N₂O fluxes over spatial (across, along) and temporal (tidal cycle) surveys in the subtropical Maroochy estuary using the floating chamber method. Combined k₆₀₀ values showed a large range over the entire estuary (0.1–198.6 cm h−1) with slightly lower k₆₀₀ in the lower compared to the upper estuary. Overall, temporal variability was greater than spatial variability of k₆₀₀. We found the highest variability of k₆₀₀ between gas species in the lower estuary, whereas the variability was less distinct in the upper estuary. In the Maroochy estuary, k₆₀₀CO₂ (mean 26.4 ± 37.3 cm h−1) was mostly higher than k₆₀₀ CH₄ (mean 10.9 ± 10.6 cm h−1) and k₆₀₀N₂O (mean 9.9 ± 12.3 cm h−1), likely due to chemical and enzymatic enhancements and/or microbial activity in the surface microlayer. We demonstrate that empirical k₆₀₀ models intended for CO₂ may not accurately predict CH₄ and N₂O fluxes in estuaries. Our tested k₆₀₀ models predicted the measured fluxes within an uncertainty range of 5%–40% (over or underestimation), but precise flux estimates should be based on in situ k₆₀₀ of all three gases