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.