CH4, CO2 and BVOC emissions from three subarctic lakes in Northern Sweden with different chemical properties

Climate warming is stronger in the northern high latitudes than the global average, making ecosystems in the region prone to change with future warming. Approximately 50% of all lakes are located north of 60°N, and lake area is projected to increase from permafrost thaw and meltwater from glaciers, making lakes increasingly important parts of the terrestrial ecosystem carbon budget. We currently lack a process understanding of variations in CO₂ and CH₄ fluxes from high latitude lakes. Lakes are also a source of biogenic volatile organic compounds (BVOCs), and these reactive compounds influence atmospheric oxidation and contribute to aerosol formation. However, to the best of our knowledge, almost no flux measurements of BVOCs have been conducted on lakes and no measurements have been conducted in high latitude lakes, leaving this area largely unexplored but with growing interest due to expanding lake areas in this pristine environment.

This field study aims to quantify the magnitude, composition and variation in emission of CO₂, CH₄ and BVOCs from two small thermokarst lakes at different ages and a riverine lake, all located in subarctic Sweden. Data was collected during a two-week campaign in July-August 2024. For each site, we measured CO₂, CH₄ and BVOCs fluxes using floating chambers, analyzed chemical and physical properties of water samples and collected environmental variables. The lakes had different DOC, NO₃^- and PO₄^3- concentrations as well as pH, despite being located less than one km from each other. The riverine lake is neutral while both thermokarst lakes are acidic. DOC ranged from mean 61.91 mg L^-1 in the younger thermokarst lake to 20.93 mg L^-1 in the vegetation-colonized thermokarst lake and 6.67 mg L^-1 in the riverine lake.

All three lakes had distinguished emission magnitudes of BVOCs. The colonized thermokarst lake showed larger isoprene and hydrocarbon emissions than the other two lakes. Overall, the lake BVOC emissions are of similar or higher magnitude than the surrounding terrestrial ecosystem emissions found in other studies. We found that the younger thermokarst lake had the highest emissions of CH₄. For CO₂, the riverine lake and the colonized lake are small sinks of CO₂ while the younger thermokarst lake is a source. We observed large hourly variation in CO₂ and CH₄ emissions in the three lakes with no clear diel pattern. This could be because of a temporal lag effect between production and consumption in the water and sampling at the surface, as well as relatively stable water temperatures over the diel cycle. Additionally, for the colonized thermokarst lake, photosynthesis was limited because the high vegetation density blocked light from entering the water around the non-see-through chamber.

This study provides quantitative information about emissions of three climate-relevant gases from different high latitude lakes in the growing season. The pioneering sampling of BVOC emissions from lakes paves the way for further exploring reactive volatiles from freshwater systems. Understanding the processes behind these flux variations allows for a more holistic representation of high latitude ecosystems by including diverse freshwater ecosystems, e.g. in regional ecosystem model simulations.