Methane under ice in boreal lakes

English Abstract:

Lakes are important sources of greenhouse gases (GHG) to the atmosphere, emitting both carbon dioxide (CO₂) and methane (CH₄). The extent of ice cover and changes related to mixing of water column strongly influence the seasonal variation in GHG dynamics, and without considering them, the annual estimates of GHG emissions from boreal lakes may be strongly biased. While seasonal variation of CO₂ is relatively well known, the seasonality of CH₄ has not been well described, particularly in northern lakes during the ice cover period. We conducted seasonal measurements of CH₄ and CO₂ concentrations of 13 boreal lakes in Québec over the course of an annual cycle. We observed significant under ice accumulation of CH₄ in seven out of the 13 lakes, whereas CH₄ concentrations and fluxes consistently increased during summer in almost all lakes. Winter CH₄ accumulation has been previously related to the anoxic conditions but most of our lakes remained oxic also in winter and thus the role of oxygen limitation seemed to be minor in these lakes. Lakes accumulating CH₄ under ice were in general larger than non-accumulator lakes (2985±1787 km² and 23±14 km²) and had higher total phosphorous concentration (20.1 µg L^-1 and 12.9 µg L^-1) but there was no difference in the mean depth between the two groups of lakes. Higher nutrient concentrations can increase primary production increasing the amount of organic matter that sediments to the bottom and acts as a source for methanogenesis. In larger lakes the residence time can be longer than in smaller lakes, which can allow more efficient accumulation. Contrary to CH₄, the seasonal CO₂ patterns were similar across the lakes, with all the lakes accumulating CO₂ under ice and having lowest concentrations in the middle of summer. Whereas winter accumulation of CO₂ and subsequent emissions following ice melt represent a major contribution to the annual CO₂ flux, this is not always the case for CH₄, since a significant fraction of lakes did not accumulate CH₄ under the ice and peak CH₄ fluxes occurred in late summer, representing the bulk of annual emissions. Our results collectively suggest that seasonal GHG patterns vary greatly across boreal lakes, and it is therefore important to account for such seasonal heterogeneity to generate reliable annual emissions budgets for boreal lakes.