Department of Physics, Chemistry and Biology (IFM)

Lakes play an important role in the global carbon cycle but the mechanisms that control global evasion of CO₂ are not well known. Here seasonal, diel and spatial variations in CO₂ and the driving mechanisms for the variation, was investigated from May to September in a small Swedish forest lake using CO₂ sensors. Comparative measurements were done in an adjacent forest lake, more similar to a wetland. Respiration was dominating in both lakes, which strongly regulated CO₂ flux and partial pressure of CO₂ (pCO₂). This resulted in the lakes functioning as carbon sources, emitting substantial amounts of CO₂ into the atmosphere. There was a seasonal variation with higher flux and pCO₂ at the end of the growing season when stratification was released. The diel variation showed an increase in pCO₂ during the dark period as a result of respiration. As for the spatial variation, there was a tendency for higher concentrations of pCO₂ close to the lake inlets. This suggests that terrestrially derived carbon was transported into the lake. The pCO₂ correlated negatively with water temperature and O₂ concentration, and positively with air pressure. The flux was as well negatively correlated with O₂, but correlated positively to air pressure and wind speed. Explanations for this can be that warmer waters stimulated the primary production which consumed CO₂, and that the temperature naturally dropped towards the autumn. More unstable, windy weather conditions with lower air pressure favored flux release, suggesting a decreased pCO₂ concentration in the water.