Eddy covariance measurements of carbon exchange and latent and sensible heat fluxes over a boreal lake for a full open‐water period

Abstract

Long‐term measurements of sensible and latent heat and carbon dioxide fluxes were performed over a boreal lake in southern Finland using the direct micrometeorological eddy covariance (EC) technique. The water column was sampled weekly for dissolved carbon dioxide, and the CO2 flux was estimated also applying the concentration gradient method. Temperature and oxygen profiles of the lake were measured twice a week. The measurements covered one full open‐water period from April to November 2003, and it is the longest continuous CO2 record ever measured over a lake by EC. The sensible heat flux H was positive, that is, from the lake to the atmosphere, except in May, when it was >0 W/m2 at night and <0 W/m2 in daytime. The latent heat flux dominated clearly over H in spring and summer; that is, the Bowen ratio was less than 1. Higher‐moment turbulence statistics proved to be efficient in detection of frequent nonstationary situations. Applying the statistical criteria for CO2 concentration and vertical wind speed, averaging over a 5‐min period and selecting only the wind direction with longest fetch, we could obtain lake‐representative CO2 fluxes. Footprint analysis based on a closure model revealed that the source areas were relatively short because of the presence of turbulence generated by the surrounding forest, compared to a larger lake with an extended smooth surface. We observed a net CO2 source of 0.2–0.4 μmol m−2 s−1 excluding July, when the flux was closer to zero. The results are consistent with the gradient method, based on more infrequent sampling, and both methods gave the same average flux, 0.2 μmol m−2 s−1, over the whole open‐water period.