Investigation of local carbon fluxes from lakes to the atmosphere from eddy covariance observations and the erroneous contribution from non-local processes

Inland freshwater bodies such as lakes provide the largest contribution of natural carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. This method allows to estimate the fluxes from local source processes. The difficulty of such local observations is that large-scale non-local processes as for example entrainment or advection can add erroneous contributions to the eddy covariance flux estimations. Scalar quantities from the free-atmosphere above the measurement tower can be entrained into the boundary layer and to the measurement site. Also, the lake is surrounded by land, from where scalar characteristics can be horizontally advected to the measurement site.

During four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden, we found unexpected fluxes of carbon dioxide when the lake was entirely covered with ice. We investigated these unexpected fluxes using a statistical approach, which is based on surface-layer data (van de Boer et al., 2014). This approach reveals that non-local processes produce these erroneous fluxes. Additionally, we found that the strength and occurrence of the non-local processes depend on the fetch (distance between the instrumented tower and upwind shore) and the prevailing wind speed. The combination of fetch and wind speed was defined as the time over water. The smaller the time over water, the higher the contribution of the non-local processes.

We present how we corrected the contribution of the non-local processes to the estimations from the eddy covariance flux tower, not only for the ice-covered periods, but also the periods when the lake was free of ice. We present this analysis approach as an example for Lake Erken and propose that it can be extended to other lakes. This can potentially increase our understanding of the carbon exchange between lakes and the atmosphere.