Chemical transport by methane ebullition in a freshwater lake

Abstract

Methane bubbling from lakes contributes significantly to atmospheric methane levels, and methane is second only to carbon dioxide in global warming potential. Microorganisms in aquatic sediments produce methane while consuming organic matter, and the majority of this methane is released via bubbling. Bubbles dissolve as they rise, and the fraction of original methane that dissolves versus escapes to the atmosphere is strongly influenced by bubble size. While bubble sizes are critical to methane fate, traditional methods of measuring bubbles sizes in situ are resource intensive (i.e. sonar or video cameras). In this work we design, build, and deploy a fleet of novel optical bubble size sensors capable of measuring methane bubbles in situ for long periods of time. Data from our field campaign on Upper Mystic Lake, MA illuminate spatial differences in bubble size distributions and provide an estimate of the contribution from methane bubble dissolution to dissolved methane accumulation. These results improve our understanding of processes governing the emission of this important greenhouse gas. In addition to transporting gas, bubbles effectively transport particles in water columns. This process has been used extensively in industry since the 1900s to separate chemicals of interest from bulk solutions. While bubbles also transport particulate matter in marine systems, to date very little work has focused on the possibility that methane bubbles transport particles in freshwater systems. We use laboratory and field experiments on Upper Mystic Lake to show that bubbles can transport arsenic-containing sediment particles to the surface of the lake from depths exceeding 15 m. While we estimate that arsenic transport is insignificant at the relatively modest methane bubbling levels in Upper Mystic Lake, other water bodies experience an order of magnitude more ebullition and bubbling may therefore constitute a significant contaminant flux in these systems. Furthermore, bubbles may also transport organisms (or pathogens) from the sediment to the water surface.