The purpose of the study was to examine how forest harvesting affects the accumulation of carbon dioxide (CO2) and methane (CH4) under the ice-cover. Dissolved organic carbon (DOC) was hypothesized to increases post-harvest, this supplementation would in turn lead to an increased accumulation of CO2 and CH4 under the ice-cover. Four small boreal lakes, (two treatment lakes, two reference lakes) were sampled once a year during winter, under a three-year period (2012-2014). The sampling was performed using a headspace equilibration technique. During the three study years (2012-2014), the CO2 concentration increased in both the reference lakes and the harvested lakes (percentage increase; 18 % and 89 % respectively). The mean CH4-concentration in the reference lakes increased during the years 2012-2014, (2, 6, 7.5 μM). While the concentrations in the harvested lakes showed a slightly different pattern, with a peak during year 2013 which latter decreased until year 2014, (6, 12.5, 8.5 μM). The result of this study suggests that harvest in the catchment area increases the accumulation of CO2 and possible CH4 under the ice. Inflow of supersaturated surface water was found to be a more important source to the increased accumulation under ice than DOC, contradicting the previous hypothesis that DOC would be a more important factor controlling the CO2 and CH4 production. Moreover this study indicates that the dimension of the edge-zone left in connection to the inlets can be of crucial importance, possible reducing the amount of supersaturated surface water reaching the lakes.

Freshwater ecosystems have long been neglected as an important part of the global carbon cycle. However, research shows that most of the world’s lakes are net-heterotrophic and consequently emitters of greenhouse gases to the atmosphere. In many boreal and north-temperate lakes, most of the yearly emissions usually occur in spring, shortly after ice-thaw. The aim of this study was to quantify the flux of carbon dioxide (CO2) and methane (CH4) in three boreal lakes, during this annual event. In order to do this, water samples were collected before and after ice-thaw, and the flux was estimated as the difference in mass of carbon between the two sampling occasions. The results showed that the lakes had accumulated high amounts of carbon over the winter, with higher concentrations generally at higher depths. The fluxes during ice-thaw ranged from 234–380 (mean: 302) and -1.15–15.12 (mean: 8.64) mmol m-2 y-1 for CO2 and CH4, respectively. Given their small sizes, the lakes emitted less carbon, per unit area, than expected. This was assumed to be due to the lakes’ rather isolated locations and since the heating of the water rapidly caused them to become highly stratified, thus preventing the wind from releasing deeper stored carbon. Presumably, this holds true for other similar boreal lakes as well, which suggests that attention – in such ecosystems – also needs to be brought to other mixing periods.