A WHOLE-POND EXPERIMENT TO ASSESS HOW LATE-SUMMER MIXING AFFECTS METHANE DYNAMICS

Ponds emit significant amounts of methane (CH4) into the atmosphere; however, these emissions are very uncertain at regional and global scales. Among the sources of uncertainty are the mechanisms and timing of CH4 emissions. Most temperate ponds stratify in summer, allowing CH4 to accumulate in the hypolimnion between mixing events, especially if those waters become anoxic. When mixing occurs, accumulated CH4 can either be oxidized or emitted to the atmosphere. Summer storms are increasing in frequency and intensity with climate change, potentially increasing the frequency of summertime mixing and altering the fate (oxidation vs. emissions) of CH4.  In this study, we experimentally mixed a dimictic pond over two days in late summer using a water circulator. Before, during, and after induced mixing, we tracked changes in methane concentrations, ebullition, diffusive flux, and oxidation (using stable isotopes). The pond began to re-stratified immediately after the circulator was turned off, but post-mixing stratification was far weaker than pre-mixing stratification and the pond mixed several times  before fall turnover. Methane concentrations declined rapidly during mixing and remained lower than pre-mixing concentrations through the end of the study. Analysis of diffusive flux, ebullitive flux, and oxidation data is underway and will be included in the presentation. Our results will contribute to the understanding of how mixing events impact the fate of pond methane and will help constrain estimates of pond methane emissions.

Presentation Preference: Oral

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

Authors:

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

Pascal Bodmer, Cornell University  (pb577@cornell.edu)

Meredith Holgerson, Cornell University  (meredith.holgerson@cornell.edu)