Methane emission from aquatic environments have been reported to be highly variable but significant enough to be largely responsible for the uncertainty in estimates of methane emissions from natural sources. Surface emissions correspond to the net balance between production and consumption processes and include several biotic and abiotic mechanisms such as diffusion and lateral transport of sediment CH₄, demethylation of organic compounds in oxic environments, photosynthesis, ebullition, and transport by plants and organisms. Nevertheless, the contribution of individual mechanisms to accumulations and emission of methane is not well constrained. Additionally, there are large discrepancies between top-down and bottom-up approaches to evaluate the role of lakes in closing the global methane budget.

In deeper water, at the oxic-anoxic interfaces, be it in the water column, close to the sediment or within the sediment column, aerobic methane oxidizers consume significant amounts of the methane produced in anoxic environments. In the oxic surface layers of oceans and lakes, this process is less understood. For example, contradicting data exists regarding the inhibitory effect of light and oxygen on the methane oxidation process. As methane emissions are a result of production, transport, and consumption, it is essential to gain a better understanding of the organisms playing a role in the process as well as the different rates of production and consumption.

In this session we welcome contributions addressing methane production, consumption, transport, and emissions from different aquatic bodies. Studies may relate to microbial communities involved in CH₄ cycling, quantification of classical and novel processes and pathways, large-scale spatio/temporal studies, isotopic characterization of novel methane sources, as well as novel methodological approaches to quantify methane emissions from water. Of particular interest are methane studies in oxic environments.

Lead Organizer: Mina Bizic, Leibniz Institute of Freshwater Ecology and Inland Fisheries (mbizic@igb-berlin.de)

Co-organizers:

Sofia Baliña, Radboud University (sofiabalinia@gmail.com)

Paul del Giorgio, Université du Québec à Montréal (del_giorgio.paul@uqam.ca)

Tonya DelSontro, University of Waterloo (tonya.delsontro@uwaterloo.ca)

Yves Prairie, Université du Québec à Montréal (prairie.yves@uqam.ca)

Presentations

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05:30 PM

A WHOLE ECOSYSTEM EXPERIMENT TO ASSESS HOW WATER COLUMN MIXING FREQUENCY IMPACTS METHANE CONCENTRATIONS AND EMISSIONS (7800)

Primary Presenter: Kathy Stenehjem, Cornell University (kjs297@cornell.edu)

The frequency of mixing events within ponds may dramatically impact methane production. While stratified, it’s common for pond bottom waters to become anoxic, creating favorable conditions for anaerobic methanogenic archaea to produce methane.  Alternatively, when ponds mix frequently, oxygen is generally high throughout the water column so methane production is less likely. Although ponds have been recognized as a significant source of methane to the atmosphere, emissions estimates vary widely, potentially due to these mixing related differences between ponds. In this study, we manipulated pond mixing regimes in experimental ponds to quantify impacts on methane emissions. All ponds were identical in surface area (30 x 30 m) and depth (1.9 m) and were naturally intermittently mixing (e.g., mixing every few weeks). We altered the mixing dynamics of three ponds using underwater propellers to create daily mixing conditions. From March - October 2023, we collected weekly samples to track surface and bottom methane concentrations, ebullitive flux, diffusive flux, and water chemistry. Thermistor lines at the center of each pond continuously tracked mixing dynamics. Both methane concentrations and emissions increased as mixing frequency decreased. The results of this study indicate that mixing frequency can explain some of the variability in pond greenhouse gas emissions.

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05:30 PM

Summer 2023 Greenhouse Gas Fluxes at Cedar Bog Lake, MN (7920)

Primary Presenter: William Brown, University of Minnesota (brow6589@umn.edu)

Freshwaters are significant natural sources of greenhouse gasses (CO2, CH4, and N2O) to the atmosphere. Small freshwater bodies like Cedar Bog Lake, where Ray Lindeman did his classic work on the ‘Trophic dynamic concept in ecology’,  are increasingly being recognized as having large greenhouse gas emissions compared to their small size. Measured emissions can vary by orders of magnitude. To assess trends in greenhouse gas emissions at Cedar Bog Lake, concentrations and emissions of CO2, CH4, and N2O were measured roughly weekly over a two-month period from the middle of July until the middle of September in 2023. Over the two-month sampling period, Cedar Bog Lake was a net source of CH4 (mean 775.19 [se 556.00] mg CH4 m-2 d-1). CO2 fluxes varied more between being a source and a sink but was overall a source over the sampling period (mean 411.60 [se 696.29] mg CO2 m-2 d-1). N2O concentrations were measured in situ but average fluxes were not calculated since there was no appreciable difference between sample flux (floating chamber measurement) and in headspace concentrations at depth between the sample and the atmosphere. Both CH4 and CO2 concentrations were highest at the deepest measured depth (only 1.2 m) and concentrations peaked in mid-August which corresponded to the hottest part of the summer and the lowest concentration of O2 at the bottom. In general, there was a high degree of both spatial (within lake) and seasonal heterogeneity in the floating chamber measurements used to calculate the greenhouse gas fluxes from Cedar Bog Lake.

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