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 utmost
interest in this context because they exchange large amounts of the climate-forcing gases
methane (CH4) and carbon dioxide (CO2) with the atmosphere. Yet uncertainties in the
magnitude and drivers of these fluxes remain, partly due to a lack of direct observations covering
all seasons of the year, but also because of the diversity in measurement methods that often miss
components of the transport processes. This prevents in particular accurate estimates of the total
emission of CH4 and CO2 from seasonally ice-covered lakes.
This thesis aims to address these spatial and temporal issues to improve quantification and
understanding of surface-atmosphere exchange of CH4 and CO2 by using the eddy covariance
method. It is a direct, non-intrusive method which allows an integration of all transport pathways
of the gases between the ecosystem and the atmosphere. The work took place in a peatland
complex of Subarctic Sweden where ecosystem functioning is affected by permafrost thaw. The
focus was on a shallow lake, which was compared to a waterlogged fen within the same
catchment. These two types of ecosystems are commonly present in the subarctic regions and
may 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 of
annual flux cycle. While rates of CH4 and CO2 exchange from the fen were highest during the
growing season and likely controlled by plant processes, lake fluxes of both CH4 and CO2
peaked during the short spring season upon lake ice disappearance and subsequent overturn. The
presence of an ice lid in winter over the lake surface likely prevents gas exchange with the
atmosphere and allows buildup of CH4 in the anoxic bottom. Although the contribution of winter
and spring to annual emissions of CH4 and CO2 was significant for both ecosystems, spring
season emissions were disproportionally important for the lake annual emissions compared to the
length 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 needs
further investigation.
The high temporal resolution of the flux measurements allowed identifying transport pathways
of CH4 and CO2 between the lake and the atmosphere during spring and summer. Temperature of
the surface sediments was a main driver of the seasonality in summer ebullition of CH4. A direct
link between breakdown of thermal stratification at ice-out and the release of CH4 and CO2 was
established. These results underline the crucial importance of shoulder seasons in the annual
carbon emissions from seasonally frozen lakes.
Overall, the lake was an important annual source of carbon to the atmosphere, partially
compensating the higher, annual sink function of the fen. The lake in focus can be seen as a
typical shallow postglacial lake with organic rich sediments, thus may be representative of many
lakes across the lowlands of the Arctic and subarctic. Comparison with regional estimates is
however currently limited by the rarity of ecosystem-scale measurements in northern lakes and
the variability of flux estimates across lake types. This work is a step towards a better assessment
of the importance of inland waters in such landscapes.
The thesis also addresses challenges and methodological aspects of EC measurements in lake
environments, including flux uncertainty and gap filling of fluxes at the hourly scale.