Lake Fluxes of Methane and Carbon Dioxide
Abstract: Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Recent studies have shown that lakes, although they cover a small area of the globe, can be very important natural sources of atmospheric CH4 and CO2. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange.By using the eddy covariance method for lake flux studies, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes.Eddy covariance measurements at a Swedish lake revealed a diel cycle in the fluxes of both CH4 and CO2, with higher fluxes during nighttime than daytime. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to waterside convection. For the diel cycle of CH4 flux it was suggested that waterside convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition.Simultaneous flux measurements of CH4 and CO2 have been presented using both the eddy covariance method and the floating chambers method of which the latter is the traditional measuring method for lake fluxes. For CO2 the two methods agreed well during some periods but differed considerably during others. Disagreement between the methods might be due to horizontal heterogeneity in partial pressure of CO2 in the lake. The methods agreed better for the CH4 flux measurements. However, it is clear that due to the discontinuous nature of the floating chambers, this method will likely miss important high flux events.The main conclusions of this thesis are:1) the two gas flux methods are not directly comparable and should be seen as supplementary to each other2) waterside convection enhances the fluxes of both CH4 and CO2 over the water-air surface. If gas flux measurements are not conducted during nighttime, potential high flux periods might be missed and estimates of the total amount of gas released from lakes to the atmosphere may be biased.
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