Abstract
Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations.
Highlights
Inland freshwater systems such as lakes act as a net source of natural carbon to the atmosphere
(2014) we identified the origin of these fluxes as non-local processes
The findings were supported by the quadrant analysis, which related the non-local processes to flux contributions from above
Summary
Inland freshwater systems such as lakes act as a net source of natural carbon to the atmosphere. The magnitude of the carbon contribution from lakes to the atmosphere is comparable to the open ocean carbon sink (e.g., Cole et al 2007; Tranvik et al 2009; Raymond et al 2013). To study the lake’s contribution to the carbon cycle, various methods have been used. Eddycovariance flux measurements of carbon dioxide (CO2), and more recently methane (CH4), have gained popularity (e.g., Eugster et al 2003; Huotari et al 2011; Podgrajsek et al 2014, 2015; Erkkilä et al 2018; Morin et al 2018). The eddy-covariance method gives a direct measure of net gas exchange across the air–water interface as long as the lake is within From Jammet et al (2017), it remains unclear whether the fluxes were due to a physical evasion of CO2 through snow over the lake or due to lateral advection of land-emitted CO2
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