Methane and carbon dioxide fluxes at high spatiotemporal resolution from a small temperate lake.

Lakes are hotspots for CH and CO effluxes, but their magnitude and underlying drivers are still uncertain due to high spatiotemporal variation within and between lakes. We measured CH and CO fluxes at high temporal (hourly) and spatial resolution (approx. 13 m) using 24 automatic floating chambers equipped with continuously recording sensors that enabled the determination of diffusive and ebullitive gas fluxes. Additionally, we measured potential drivers such as weather patterns, water temperature, and O above the sediment. During five days in autumn 2021, we conducted measurements at 88 sites in a small, shallow eutrophic Danish Lake. CH ebullition was intense (mean 54.8 μmol m h) and showed pronounced spatiotemporal variation. Ebullition rates were highest in deeper, hypoxic water (5-7 m). Diffusive CH fluxes were 4-fold lower (mean 15.0 μmol m h) and spatially less variable than ebullitive fluxes, and significantly lower above hard sediments and submerged macrophyte stands. CO concentration in surface waters was permanently supersaturated at the mid-lake station, and diffusive fluxes (mean 919 μmol m h) tended to be higher from deeper waters and increased with wind speed. To obtain mean whole-lake fluxes within an uncertainty of 20 %, we estimated that 72 sites for CH ebullition, 39 sites for diffusive CH4 fluxes and 27 sites for diffusive CO fluxes would be required. Thus, accurate whole-lake quantification of the dominant ebullitive CH flux requires simultaneous operation of many automated floating chambers. High spatiotemporal variability challenges the identification of essential drivers and current methods for upscaling lake CH and CO fluxes. We successfully overcame this challenge by using automatic floating chambers, which offer continuous CH and CO flux measurements at high temporal resolution and, thus, are an improvement over existing approaches.