Concentrations of CO2 and CH4 in water columns of two stratified boreal lakes during a year of atypical summer precipitation

摘要

We measured CO2 and CH4 concentrations throughout the water columns of two boreal lakes with contrasting trophic status and water color during a wet summer. Previous work suggested that rainfall was important for carbon gas evasion. During the stratified period, precipitation generated unexpected variabilities in CO2, CH4, and DOC concentrations below the euphotic zone, especially in the metalimnion. The DOC concentrations after the rains rose to 22 and 10 mg L-1 from the initial 13 and 8 mg L-1, in the humic and clear-water lakes respectively, simultaneously with an increase in carbon gas concentrations. In both lakes, the water column was stable, suggesting that the high gas concentrations were not due to transport from hypolimnia rich in carbon gases. The high concentrations of CH4, which can only be produced in anoxic conditions, in the oxic metalimnion and epilimnion in comparison to the hypolimnetic concentrations indicated that a considerable proportion of the pelagic CH4 originated from the catchment and/or the littoral zone. Thus, as a consequence of high levels of precipitation, carbon gas concentrations during summer stratification can increase, which can have overall importance in annual carbon budgets.