Long term flux measurements of carbon dioxide and methane over a small boreal lake using eddy covariance technique

Advancing our understanding on physical and biogeochemical processes controlling turbulent exchange of energy, carbon dioxide (CO₂), methane (CH₄) and other trace gases across lacustrine systems is crucial in order to improve climate and weather forecast models. Lakes are capable of processing large amounts of organic carbon of terrestrial origin, and their importance in landscape carbon cycle and climate change issues is well recognized. Nevertheless, the amount of CO₂ and CH₄ released into the atmosphere is still uncertain.

Here, we investigate the temporal dynamics of CO₂ and CH₄ exchange using eleven years (for CO₂) and three years (for CH₄) of eddy covariance flux measurements over the Lake Kuivajärvi, a small boreal lake in southern Finland.

The lake ecosystem acted mostly as a net CO₂ source (0.42±1.56 mmol m^-2 s^-1) throughout the ice-free periods and had a relatively high interannual variability when compared to the surrounding forests and wetlands. On average, the lake is a net source of CH₄ (0.63±2.44 nmol m^-2 s^-1), but the measured annual emissions are lower than for CO_2, revealing thatmost of the CH₄ produced at the lake bottom is oxidized in the water column. Carbon dioxide and methane emissions are largely affected by the weather forcing through the effects of wind shear and nocturnal water cooling, which deepens the mixed layer and enhances gas exchange at the air-water interface.