Methane emission from lakes: biophysical drivers impacts beyond measurement uncertainties

Abstract:

For freshwater ecosystems, lakes are the largest contributors of CH₄ emissions to atmosphere and may offset the terrestrial land sink. Strong uncertainties remain concerning the driving factors of such emissions. In this study, we compiled literature data on CH₄ emission in lakes and we collected ancillary data related to methodology, climate, landscape or lake characteristic. We used studies measuring one or more emission pathways from open waters (i.e. diffusive, ebullitive and storage fluxes), all over the world. Relationships between CH₄ emissions, environmental variables, lake characteristics and methodological approaches were investigated through stepwise linear regression and regression trees analysis. Our analysis showed that diffusive flux was the most frequently estimated assuming that others were negligible and considering it as a good surrogate for the total flux. However, when the three forms of open water fluxes were estimated, assuming that diffusive flux is a good surrogate for the total flux would have lead to a 600% underestimation. Differences in environmental variables related to climatic conditions both at regional scale (climatic zone), or at local scale (air temperature, precipitation), induced significant differences in CH₄ estimations with higher emissions when temperature was high. Surrounding landscapes were also a significant driver with higher emissions where lakes were located within anthropized areas (urban zones or croplands). We also observed that DOC concentration was positively related to total, diffusive and storage CH₄ pathway emissions. Finally we could highlight that methodological approaches related to differences in estimation methods influenced significantly all the pathway emissions that present different methods to obtain estimation values (total, diffusive and ebulitive fluxes). We conclude that the most estimated pathway of CH₄ emission from open waters is the less important contributor to total flux from lakes inducing great underestimation of the total emission at regional or global scale. Besides environmental and lake characteristics variables, the methods used to estimate CH₄ fluxes should also be considered as a significant source of the great differences observed in estimation fluxes between lakes.