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- Jansen, Joachim, et al.
(författare)
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Drivers of diffusive lake CH4 emissions on daily to multi-year time scales
- 2020
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 17:7, s. 1911-1932
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Tidskriftsartikel (refereegranskat)abstract
- Lakes and reservoirs are important emitters of climate forcing trace gases. Various environmental drivers of the flux, such as temperature and wind speed, have been identified, but their relative importance remains poorly understood. Here we use an extensive field dataset to disentangle physical and biogeochemical controls on the turbulence-driven diffusive flux of methane (CH4) on daily to multi-year timescales. We compare 8 years of floating chamber fluxes from three small, shallow subarctic lakes (2010–2017, n = 1306) with fluxes computed using 9 years of surface water concentration measurements (2009–2017, n = 606) and a small-eddy surface renewal model informed by in situ meteorological observations. Chamber fluxes averaged 6.9 ± 0.3 mg m−2 d−1 and gas transfer velocities (k600) from the chamber-calibrated surface renewal model averaged 4.0 ± 0.1 cm h−1. We find robust (R2 ≥ 0.93, p < 0.01) Arrhenius-type temperature functions of the CH4 flux (Ea' = 0.90 ± 0.14 eV) and of the surface CH4 concentration (Ea' = 0.88 ± 0.09 eV). Chamber derived gas transfer velocities tracked the power-law wind speed relation of the model (k ∝ u3/4). While the flux increased with wind speed, during storm events (U10 ≥ 6.5 m s−1) emissions were reduced by rapid water column degassing. Spectral analysis revealed that on timescales shorter than a month emissions were driven by wind shear, but on longer timescales variations in water temperature governed the flux, suggesting emissions were strongly coupled to production. Our findings suggest that accurate short- and long term projections of lake CH4 emissions can be based on distinct weather- and climate controlled drivers of the flux.
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| 2. |
- Snöälv, Jo, et al.
(författare)
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Flocculation boundaries in the landscape: transformation processes of dissolved organic matter in coastal moorland streams, UK
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- From headwaters to estuaries, organic carbon is transported and transformed. Some carbon may be mineralised and outgassed to the atmosphere, while another fraction is buried in sediments. Organic carbon can end up in the sediment through flocculation. As the dissolved organic matter (DOM) is transported through the landscape, it passes through several flocculation boundaries. Examples of such boundaries are the exposure to suspended minerogenic material derived from soil erosion or weathering processes that causes adsorption or coagulation, and estuarine mixing where riverine DOM encounters a substantial increase in salinity. In addition, both of these processes may occur as river flow causes sediment resuspension in the saline estuary. This study aims to simulate these flocculation boundaries by experimental additions of clay particles, saline mixing, and a combined clay and salt treatment. We performed experiments on water from one peatland pond and eight second and third order streams in a coastal moorland that served as a setting for these processes. Analysis of DOM composition through fluorescence spectroscopy and mass spectrometry showed that flocculation has the potential to modify the composition of DOM, as compounds with a terrestrial signature are preferentially removed by flocculation. This effect could be observed for clay and combined clay and salt mixing, while saline mixing alone caused limited changes in the DOM composition. Contrary to previous studies, combined clay and saline mixing did not lead to more flocculation compared to clay mixing alone. Instead, the added salt desorbed a small amount of organic matter already associated with the clay particles, which confounded the overall flocculation effect. In addition to organic carbon removal by flocculation, we expect that the altered composition of the DOM remaining in solution will also affect its reactivity to other DOM transformation processes.
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