UC Berkeley

Wetlands are the largest natural producer of methane, a potent greenhouse gas. One way in which dissolved methane can be emitted from wetlands is via hydrodynamic transport. This transport pathway includes stirring and bulk motion in the water column as well as diffusion near the air-water interface. It is driven by sources of near-surface turbulence, such as natural convection (e.g. temperature-driven stirring) and forced convection (e.g. wind shear, precipitation, honami). Relative to other methane transport pathways in wetlands, hydrodynamic transport has been understudied and its contributions to total methane flux have been underestimated.

We developed a low-cost, autonomous, and programmable underwater camera to measure water velocity in wetlands. We used this camera in a model wetland to correlate gas exchange across the air-water interface to water-side velocity statistics. Our results found a positive quadratic relationship with a non-zero intercept between the standard deviation of vertical velocity and the normalized gas transfer velocity (k600). This camera was then deployed in Burns Bog outside of Metropolitan Vancouver, British Columbia, Canada in order to estimate the site's methane flux due to hydrodynamic transport. Using water-side velocity statistics to calculate k600, we found that hydrodynamic transport was responsible for approximately 17.2% of total methane flux in Burns Bog; this percentage varied very little throughout the day. Finally, we compare a heat-flux-based method for estimating k600 against our water-velocity-based approach. We found that the heat-flux-based method had consistently lower estimates for k600 and gas flux relative to the water-velocity-based approach. This was because the heat flux data only captured stirring due to natural convection whereas the water-velocity-based approach included both natural and forced convection.