Gas properties of winter lake ice in Northern Sweden: implication for carbon gas release

This paper describes gas composition, total gas content and bubbles characteristics in winter lake ice for four adjacent lakes in a discontinuous permafrost area. Our gas mixing ratios for O-2, N-2, CO2, and CH4 suggest that gas exchange occurs between the bubbles and the water before entrapment in the ice. Comparison between lakes enabled us to identify 2 major "bubbling events" shown to be related to a regional drop of atmospheric pressure. Further comparison demonstrates that winter lake gas content is strongly dependent on hydrological connections: according to their closed/open status with regards to water exchange, lakes build up more or less greenhouse gases (GHG) in their water and ice cover during the winter, and release it during spring melt. These discrepancies between lakes need to be taken into account when establishing a budget for permafrost regions. Our analysis allows us to present a new classification of bubbles, according to their gas properties. Our methane emission budgets (from 6.52 10(-5) to 12.7 mg CH4 m(-2) d(-1) at 4 different lakes) for the three months of winter ice cover is complementary to other budget estimates, as our approach encompasses inter-and intra-lake variability. Most available studies on boreal lakes have focused on quantifying GHG emissions from sediment by means of various systems collecting gases at the lake surface, and this mainly during the summer "open water" period. Only few of these have looked at the gas enclosed in the winter ice-cover itself. Our approach enables us to integrate, for the first time, the history of winter gas emission for this type of lakes.