TY - BOOK

T1 - Fluxes of Methane and Carbon Dioxide from a Subarctic Lake

T2 - Seasonality and Significance Within the Landscape

AU - Jammet, Mathilde Manon

N1 - CENPERM[2016]

PY - 2016

Y1 - 2016

N2 - Ongoing climate warming is expected to affect the carbon functioning of subarctic ecosystems.Lakes and wetlands, which are common ecosystems of the high northern latitudes, are of utmostinterest in this context because they exchange large amounts of the climate-forcing gasesmethane (CH4) and carbon dioxide (CO2) with the atmosphere. Yet uncertainties in themagnitude and drivers of these fluxes remain, partly due to a lack of direct observations coveringall seasons of the year, but also because of the diversity in measurement methods that often misscomponents of the transport processes. This prevents in particular accurate estimates of the totalemission of CH4 and CO2 from seasonally ice-covered lakes.This thesis aims to address these spatial and temporal issues to improve quantification andunderstanding of surface-atmosphere exchange of CH4 and CO2 by using the eddy covariancemethod. It is a direct, non-intrusive method which allows an integration of all transport pathwaysof the gases between the ecosystem and the atmosphere. The work took place in a peatlandcomplex of Subarctic Sweden where ecosystem functioning is affected by permafrost thaw. Thefocus was on a shallow lake, which was compared to a waterlogged fen within the samecatchment. These two types of ecosystems are commonly present in the subarctic regions andmay expand in poorly drained lowlands as part of ecosystem shifts induced by climate warming.Two and a half years of measurements revealed clear differences between fen and lake in term ofannual flux cycle. While rates of CH4 and CO2 exchange from the fen were highest during thegrowing season and likely controlled by plant processes, lake fluxes of both CH4 and CO2peaked during the short spring season upon lake ice disappearance and subsequent overturn. Thepresence of an ice lid in winter over the lake surface likely prevents gas exchange with theatmosphere and allows buildup of CH4 in the anoxic bottom. Although the contribution of winterand spring to annual emissions of CH4 and CO2 was significant for both ecosystems, springseason emissions were disproportionally important for the lake annual emissions compared to thelength of the period, as it turned the lake from a small summer CO2 sink into an annual source.Annual inter-annual variability was notable in the magnitude of the CH4 spring release and needsfurther investigation.The high temporal resolution of the flux measurements allowed identifying transport pathwaysof CH4 and CO2 between the lake and the atmosphere during spring and summer. Temperature ofthe surface sediments was a main driver of the seasonality in summer ebullition of CH4. A directlink between breakdown of thermal stratification at ice-out and the release of CH4 and CO2 wasestablished. These results underline the crucial importance of shoulder seasons in the annualcarbon emissions from seasonally frozen lakes.Overall, the lake was an important annual source of carbon to the atmosphere, partiallycompensating the higher, annual sink function of the fen. The lake in focus can be seen as atypical shallow postglacial lake with organic rich sediments, thus may be representative of manylakes across the lowlands of the Arctic and subarctic. Comparison with regional estimates ishowever currently limited by the rarity of ecosystem-scale measurements in northern lakes andthe variability of flux estimates across lake types. This work is a step towards a better assessmentof the importance of inland waters in such landscapes.The thesis also addresses challenges and methodological aspects of EC measurements in lakeenvironments, including flux uncertainty and gap filling of fluxes at the hourly scale.

AB - Ongoing climate warming is expected to affect the carbon functioning of subarctic ecosystems.Lakes and wetlands, which are common ecosystems of the high northern latitudes, are of utmostinterest in this context because they exchange large amounts of the climate-forcing gasesmethane (CH4) and carbon dioxide (CO2) with the atmosphere. Yet uncertainties in themagnitude and drivers of these fluxes remain, partly due to a lack of direct observations coveringall seasons of the year, but also because of the diversity in measurement methods that often misscomponents of the transport processes. This prevents in particular accurate estimates of the totalemission of CH4 and CO2 from seasonally ice-covered lakes.This thesis aims to address these spatial and temporal issues to improve quantification andunderstanding of surface-atmosphere exchange of CH4 and CO2 by using the eddy covariancemethod. It is a direct, non-intrusive method which allows an integration of all transport pathwaysof the gases between the ecosystem and the atmosphere. The work took place in a peatlandcomplex of Subarctic Sweden where ecosystem functioning is affected by permafrost thaw. Thefocus was on a shallow lake, which was compared to a waterlogged fen within the samecatchment. These two types of ecosystems are commonly present in the subarctic regions andmay expand in poorly drained lowlands as part of ecosystem shifts induced by climate warming.Two and a half years of measurements revealed clear differences between fen and lake in term ofannual flux cycle. While rates of CH4 and CO2 exchange from the fen were highest during thegrowing season and likely controlled by plant processes, lake fluxes of both CH4 and CO2peaked during the short spring season upon lake ice disappearance and subsequent overturn. Thepresence of an ice lid in winter over the lake surface likely prevents gas exchange with theatmosphere and allows buildup of CH4 in the anoxic bottom. Although the contribution of winterand spring to annual emissions of CH4 and CO2 was significant for both ecosystems, springseason emissions were disproportionally important for the lake annual emissions compared to thelength of the period, as it turned the lake from a small summer CO2 sink into an annual source.Annual inter-annual variability was notable in the magnitude of the CH4 spring release and needsfurther investigation.The high temporal resolution of the flux measurements allowed identifying transport pathwaysof CH4 and CO2 between the lake and the atmosphere during spring and summer. Temperature ofthe surface sediments was a main driver of the seasonality in summer ebullition of CH4. A directlink between breakdown of thermal stratification at ice-out and the release of CH4 and CO2 wasestablished. These results underline the crucial importance of shoulder seasons in the annualcarbon emissions from seasonally frozen lakes.Overall, the lake was an important annual source of carbon to the atmosphere, partiallycompensating the higher, annual sink function of the fen. The lake in focus can be seen as atypical shallow postglacial lake with organic rich sediments, thus may be representative of manylakes across the lowlands of the Arctic and subarctic. Comparison with regional estimates ishowever currently limited by the rarity of ecosystem-scale measurements in northern lakes andthe variability of flux estimates across lake types. This work is a step towards a better assessmentof the importance of inland waters in such landscapes.The thesis also addresses challenges and methodological aspects of EC measurements in lakeenvironments, including flux uncertainty and gap filling of fluxes at the hourly scale.

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122485170905763

M3 - Ph.D. thesis

BT - Fluxes of Methane and Carbon Dioxide from a Subarctic Lake

PB - Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen

ER -