Coordinated methane flux measurements from northern lakes by the SITES Water program – open data and learning examples

Abstract. Lakes represent one of the main sources of methane (CH₄) to the atmosphere, contributing roughly 3–25 % of the total global yearly emissions. While the scientific interest in understanding and modelling these emissions is increasing rapidly, methodologically consistent long-term flux measurement programs of integrated lake CH₄ emissions are largely missing. Here we present results from a systematic and comparable spatiotemporal multi-lake CH₄ flux program initiated by the Swedish Infrastructure for Ecosystem Science (SITES). Five lakes distributed across Sweden covering a latitudinal gradient from 68° N to 57° N, including the arctic subalpine, boreal and north temperate zones, were monitored during 2016–2022 in ways that captured variability in space and time within lakes and that allowed between-lake comparisons. The data includes 2375 unique CH₄ flux measurements (incl. total CH₄ fluxes, diffusive fluxes and surface water concentrations) along with other common physical and chemical lake variables. This paper presents these data (Swedish Infrastructure for Ecosystem Science, 2026, https://doi.org/10.23700/05ax-st65), underlying methodological priorities and procedures, and provides examples of what can be learned from such a coordinated sampling program. Briefly, the data show the need for appropriate consideration of ebullition and space-time integration to properly represent whole lake CH₄ fluxes. The data further show that both diffusive and total CH₄ fluxes have an apparent temperature dependency, which can be modulated by water depth in ways that differ among lakes. Also, yearly open water CH₄ flux estimates were consistently shaped by temperature in all lakes, but the strength of this response differed between lakes; increases of whole season mean water temperature between 0.5 and 3.2 °C corresponded to increases in CH₄ fluxes ranging from 16 to 74 %. Finally, we propose that temperature-normalized CH₄ fluxes should be used in lake emission inter-comparisons.