CO2 Emissions from Northern Lakes : Insights on regulation and spatiotemporal variability across contrasting lakes in Sweden

Sammanfattning: Lakes cover only ~2 % of the global land area, but their connections to the surrounding catchment make them important for the global carbon cycle. A considerable amount of the carbon input to lakes is emitted to the atmosphere as carbon dioxide (CO2) through diffusive flux. This CO2 flux varies with surface water CO2 concentrations (CwCO2) and the transfer velocity of CO2 across the thin boundary layer between surface water and atmosphere (k), which both in turn depend on physical, biological, and chemical factors that interplay with lake and catchment characteristics over various time scales. Comprehensive studies of these interlinkages across lake types are rare, and current assessments of lake CO2 emissions are therefore uncertain. In this thesis, the variability and regulation of lake CO2 fluxes across a latitudinal gradient in Sweden is investigated. The thesis explores how CwCO2 and k regulate lake CO2 fluxes and how spatiotemporal patterns of CO2 fluxes vary within and across lakes.Regulation of CO2 flux at shorter temporal scales (<1 week) was dominated by k. However, the contribution from CwCO2 increased over time making it the dominant factor for seasonal CO2 flux in some lakes. Furthermore, we show that ways of assessing k in lakes may lead to bias, possibly due to inadequate consideration of processes occurring at the upper surface layer of lakes. In the three lakes where daynight variability was studied, we found consistent patterns of higher fluxes of CO2 at daytime during periods where lakes were emitting CO2. Meanwhile, the period of lake water column turnover in autumn was crucial for both day-night variability and total lake CO2 fluxes. Based on the patterns above, we have made recommendations on improved study design for representative measurements of CO2 fluxes in lakes. In addition, we produced models for estimating CO2 flux from combinations of climatic data, satellite imagery and national lake inventory data, i.e., information that is relatively easily available and thus simplify extrapolation of flux estimates to other lakes. Patterns observed across our models suggest strong climate feedbacks, which may lead to increasing CO2 fluxes from lakes at northern latitudes along with precipitation and temperature increases there. Thus, results in this thesis urges forthcoming studies to better account for spatiotemporal variability to improve upon models that can be used for large-scale estimates and future predictions of lake CO2 fluxes.