Opposing Effects of Climate and Permafrost Thaw on CH 4 and CO 2 Emissions From Northern Lakes

Abstract

Small, organic‐rich lakes are important sources of methane (CH 4 ) and carbon dioxide (CO 2 ) to the atmosphere, yet the sensitivity of emissions to climate warming is poorly constrained and potentially influenced by permafrost thaw. Here, we monitored emissions from 20 peatland lakes across a 1,600 km permafrost transect in boreal western Canada. Contrary to expectations, we observed a shift from source to sink of CO 2 for lakes warmer regions, driven by greater primary productivity associated with greater hydrological connectivity to lakes and nutrient availability in the absence of permafrost. Conversely, an 8‐fold increase in CH 4 emissions in warmer regions was associated with water temperature and shifts in microbial communities and dominant anaerobic processes. Our results suggest that the net radiative forcing from altered greenhouse gas emissions of northern peatland lakes this century will be dominated by increasing CH 4 emissions and only partially offset by reduced CO 2 emissions. Methane (CH 4 ) and carbon dioxide (CO 2 ) emissions from small lakes are globally significant, yet highly uncertain and our understanding of how CH 4 and CO 2 emissions from small lakes in the north will change with warming and permafrost (i.e., frozen ground) is not well known. Here, we measured emissions from 20 lakes across a 1,600 km climate and permafrost gradient. Our results show that CH 4 and CO 2 emissions followed opposing trends along a north‐to‐south gradient. We show that increasing CH 4 emissions are strongly associated with warmer temperatures while decreasing CO 2 exchange is linked to shifts in hydrology and within‐lake primary productivity brought on by the absence of permafrost. Our results indicate small northern lakes have the potential to be greater sources of CH 4 but also greater CO 2 sinks. Lake methane emissions increased moving south along a climate and permafrost gradient and were linked with increasing water temperatures Lake carbon dioxide exchange decreased moving south and was driven by greater hydrological connectivity and nutrient availability Net radiative forcing from the lakes will increase due to increased methane emissions, despite decreasing carbon dioxide emissions