Lakes are large sources of methane, held to be responsible for 18% of the radiative forcing, to the atmosphere. Periods of lake overturn (during fall/winter) are short and therefore difficult to capture with field campaigns but potentially one of the most important periods for methane emissions. We studied methane emissions using four different methods, including eddy covariance measurements, floating chambers, anchored funnels, and boundary model calculations. Whereas the first three methods agreed rather well, boundary model estimates were 5–30 times lower leading to a strong underestimation of methane fluxes from aquatic systems. These results show the importance of ebullition as the most important flux pathway and the need for continuous measurements with a large footprint covering also shallow parts of lakes. Although fluxes were high, on average 4 mmol m–2 d–1 during the overturn period, water column microbial methane oxidation removed 75% of the methane and only 25% of potential emissions were rele...

Methane emissions from a small wind shielded lake determined by eddy covariance, flux chambers, anchored funnels, and boundary model calculations: a comparison.

Aquatic ecosystems are important sources of the greenhouse gas nitrous oxide. Measurements of nitrous oxide concentrations from 321 rivers, lakes and ponds in Canada reveal that some boreal aquatic systems can act as net nitrous oxide sinks.

Nitrous oxide sinks and emissions in boreal aquatic networks in Québec

Fluxes of greenhouse gases at two different aquaculture ponds in the coastal zone of southeastern China

[1] Lakes have been identified as an important source of atmospheric methane. Here the spatiotemporal distribution of dissolved methane was measured in a medium-sized freshwater lake. The data reveal that littoral zones (nearshore, shallow) are the predominant source of methane. Offshore-directed gradients of dissolved methane suggest the transport of methane from the nearshore zone to the pelagic epilimnion. The distribution patterns of epilimnetic methane were highly heterogeneous, independent of the mean lake-wide methane concentration. Consequently, the diffusive flux of methane to the atmosphere strongly varies with location in the lake. A comparison of the diffusive methane flux from different offshore sampling stations indicates that single-point measurements are not necessarily sufficient to estimate lake-wide emissions to the atmosphere accurately. Thus, spatially resolved measurements of methane emissions are needed to improve the reliability of estimates of the methane that lakes contribute to the global methane budget.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/grl.50453

Spatiotemporal distribution patterns of dissolved methane in lakes: How accurate are the current estimations of the diffusive flux path?

Progress in the studies on the greenhouse gas emissions from reservoirs

We investigated greenhouse gas emissions (CO2, CH4, and N2O) from reservoirs located across an altitude gradient in Switzerland. These are the first results of greenhouse gas emissions from reservoirs at high elevations in the Alps. Depth profiles were taken in 11 reservoirs located at different altitudes between the years 2003 and 2006. Diffusive trace gas emissions were calculated using surface gas concentrations, wind speeds and transfer velocities. Additionally, methane entering with the inflowing water and methane loss at the turbine was assessed for a subset of the reservoirs. All reservoirs were emitters of carbon dioxide and methane with an average of 970 ± 340 mg m−2 day−1 (results only from four lowland and one subalpine reservoir) and 0.20 ± 0.15 mg m−2 day−1, respectively. One reservoir (Lake Wohlen) emitted methane at a much higher rate (1.8 ± 0.9 mg m−2 day−1) than the other investigated reservoirs. There was no significant difference in methane emissions across the altitude gradient, but average dissolved methane concentrations decreased with increasing elevation. Only lowland reservoirs were sources for N2O (72 ± 22 μg m−2 day−1), while the subalpine and alpine reservoirs were in equilibrium with atmospheric concentrations. These results indicate reservoirs from subalpine/alpine regions to be only minor contributors of greenhouse gases to the atmosphere compared to other reservoirs.

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T. Diem

Papers

Greenhouse gas emissions (CO2, CH4, and N2O) from several perialpine and alpine hydropower reservoirs by diffusion and loss in turbines