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

Lakes are hotspots for CH4 and CO2 effluxes, but their magnitude and underlying drivers are still uncertain due to high spatiotemporal variation within and between lakes. We measured CH4 and CO2 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, wemeasured potential drivers such as weather patterns, water temperature, and O-2 above the sediment. During five days in autumn 2021, we conducted measurements at 88 sites in a small, shallow eutrophic Danish Lake. CH4 ebullition was intense (mean 54.8 mu mol m(-2) h(-1)) and showed pronounced spatiotemporal variation. Ebullition rates were highest in deeper, hypoxicwater (5-7m). Diffusive CH4 fluxes were 4-fold lower (mean 15.0 mu mol m(-2) h(-1)) and spatially less variable than ebullitive fluxes, and significantly lower above hard sediments and submergedmacrophyte stands. CO2 concentration in surface waters was permanently supersaturated at the mid-lake station, and diffusive fluxes (mean 919 mu mol m(-2) h(-1)) 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 CH4 ebullition, 39 sites for diffusive CH4 fluxes and 27 sites for diffusive CO2 fluxes would be required. Thus, accurate whole-lake quantification of the dominant ebullitive CH4 flux requires simultaneous operation of many automated floating chambers. High spatiotemporal variability challenges the identification of essential drivers and current methods for upscaling lake CH4 and CO2 fluxes. We successfully overcame this challenge by using automatic floating chambers, which offer continuous CH4 and CO2 flux measurements at high temporal resolution and, thus, are an improvement over existing approaches.

Automated summary

Lakes are important sources of CH4 and CO2 emissions, but the magnitude and drivers of these emissions are still uncertain due to high spatiotemporal variability. In this study, CH4 and CO2 fluxes were measured at high temporal and spatial resolution using automatic floating chambers equipped with continuous sensors. The results showed intense CH4 ebullition with pronounced spatiotemporal variation, while diffusive CH4 fluxes were lower and less variable. CO2 concentration in surface waters was consistently supersaturated, and diffusive fluxes tended to be higher from deeper waters and increased with wind speed. To accurately quantify whole-lake fluxes, a large number of automated floating chambers would be required. The use of automatic floating chambers provides continuous measurements at high temporal resolution, improving current approaches.