Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
While it is not the most abundant greenhouse gas, a significant portion of the greenhouse effect is
caused by methane. The amount of methane in the atmosphere is increasing, indicating that there is a
continuous source of methane to the atmosphere. One significant source of methane is freshwater
lakes, even though they cover only a small portion of the Earth’s surface. Because of this, it is
important to monitor methane fluxes from lakes in order to understand the processes which affect the
magnitude of these fluxes. Methane is produced in the sediment at the bottom of the lake, and
transported through the water by ebullition, diffusive flux, storage flux, or plant mediated emission.
This study looked to examine the amount of methane transmitted to the atmosphere by these processes
on Lake Erken in eastern Sweden.
Using the eddy covariance method, we can study the methane flux with good spatial and temporal
resolution. Regular sampling of lake water, both from the surface and depths of 5 and 10 meters, also
helps us to understand the amount of methane dissolved in the lake. These measurements can help us
to better understand the transfer velocity, or the efficiency of the exchange between water and air, as
well as the amount of methane transported from lakes to the atmosphere.
Water sampling showed that there is very little variation in methane concentration between different
parts of the lake. Concentrations at four surface locations are nearly identical. These surface
measurements are also similar to concentrations at different depths. Over time, the concentrations
generally stayed the same, with isolated high and low concentration events.
The amount of methane emitted by the lake was studied with the lake divided into a shallow water
area, and a deep water area. The magnitude of fluxes from both areas was very similar, but the area of
shallow water had a higher total flux. The fluxes were well correlated with wind speed; higher fluxes
coming during times with higher wind speed. This relates well to the transfer velocity theory, and the
bulk flux approximation. However, there was no clear diurnal cycle in methane fluxes. The fluxes
during the night were similar to daytime fluxes. Atmospheric pressure also had an impact on fluxes,
with greater fluxes coming at times of lower pressure.
A large seasonal variation was clear. More methane escaped the water in autumn and winter than in
spring or summer. This is due in part to the fluxes from when the lake freezes over/thaws and the
water in the lake turns over, bringing methane rich water from the lake’s bottom to the surface. As
expected, the waterside concentration of methane also had a strong correlation with the fluxes.
The main conclusions of this study are:
1) Methane fluxes are variable with wind speed, waterside concentrations, and the seasons
2) Water depth and diurnal cycles do not affect methane fluxes as strongly.
Keywords: methane, transfer velocity, flux, waterside concentration, eddy covariance
methane, transfer velocity, flux, waterside concentration, eddy covariance