Abstract
Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Previous studies have shown that lakes can be 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. Most previous lake flux studies are based on chamber measurements, by using the eddy covariance method, 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 show that both methane fluxes (FCH4) and carbon dioxide fluxes (FCO2) experience high nighttime fluxes for a large part of the data set (largest median FCH4night ≈ 13 nmol m2 s-1 and smallest median FCH4day ≈ 4.0 nmol m-2 s-1, largest median FCO2night ≈ 0.2 μmol m2 s-1 and smallest median FCO2day ≈ 0.02 μmol m-2 s-1, with larger variability during night). For the diel cycle of the CH4 fluxes it is suggested that water side convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to water side convection. If gas fluxes are not measured during nighttime, when water side convection normally is generated, periods of potential high gas flux might be missed and estimations of the total amount of gas released from lakes to the atmosphere will be biased.
Highlights
Gas fluxes of carbon dioxide (CO2) and methane (CH4) are important parts in the global carbon cycle
Analyses of the daily changes of CH4 and CO2 fluxes (FCH4 and FCO2) from Lake Tämnaren revealed that both FCH4 and FCO2 were substantially higher during nighttime compared to during daytime on average
The diffusive gas flux of CO2 over the air-water surface is driven by the difference of partial pressure between the water and air (ΔpCO2), and the efficiency of the gas transfer, the transfer velocity (k): FCO2 = Ko ⋅ k ⋅ ΔpCO2, where Ko is a gas specific solubility constant
Summary
Gas fluxes of carbon dioxide (CO2) and methane (CH4) are important parts in the global carbon cycle. The global carbon cycle describes how carbon is stored, processed, and transported between different domains such as the ocean, fossil fuel reservoirs, land, and atmosphere. Freshwater systems, including lakes and rivers, have long been seen as only a funnel transporting carbon from the land domain to the ocean, without processing the carbon. To obtain knowledge on processes affecting the gas fluxes of CO2 and CH4 over the lake surface, scientists have started to use the eddy covariance (EC) technique for lake gas flux studies. The EC technique, which is frequently used for land and ocean studies, measures the fluxes continuously without much labor. Instruments measuring the turbulent fluctuations of three different wind components and the turbulent fluctuations of the gas concentration are needed when using the EC technique
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