Abstract
While it has been recently recognized that freshwater ecosystems may significantly offset the terrestrial carbon sink through emissions of carbon dioxide (CO2) and methane (CH4), empirical data on the magnitude of these sources are still scarce, in particular in temperate regions. In this study, we measured the near-surface dissolved concentrations of CH4 and CO2 from 40 lakes in the Alpine area to estimate their potential for greenhouse gas (GHG) emissions. We hypothesized (1) a temperature-driven gradient of dissolved gas concentrations in terms of elevation and latitude of the lakes and (2) that lower concentrations would be measured in man-made reservoirs compared to natural lakes. Average CH4 and CO2 surface dissolved concentrations amounted to 1.10 ± 1.30 and 36.23 ± 31.15 µmol L−1, respectively. All the lakes, except for one, were supersaturated, exceeding ambient atmospheric CH4 and CO2 concentrations by a factor of 400 ± 424 and 2.43 ± 2.29, respectively. Consistent with our hypothesis, we found lower surface dissolved GHG concentrations in man-made reservoirs compared to natural lakes, which was shown to be related to their greater depth. Even though temperature is known to affect multiple physico-chemical and biological processes governing the strength of the uptake, release and conversion of CH4 and CO2, and temperature is inversely related to elevation, no relationship between dissolved GHG concentrations and elevation could be determined. This is believed to be the result of the overriding importance of lake depth for near-surface CH4 concentrations and the lack of explanatory variables related to lake carbon cycling. Overall, this study suggests that lakes in the Alpine region act as sources of CO2 and CH4 to the atmosphere and that further research should be carried out to quantify the actual GHG emissions from Alpine freshwater bodies and how these are affected by ongoing changes in climate and land use.