What Drives Variability in Methane Concentrations of Arctic Ponds?

Abstract:

Arctic ponds are significant sources of methane, but their overall contribution to pan-Arctic methane emissions is still uncertain. Methane concentrations in and fluxes from ponds show large spatiotemporal variability. To better understand this variability, as a step towards upscaling pond methane emissions, we studied 41 ponds in the Lena River Delta, Siberia. We collected multiple water samples in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the geomorphology, vegetation cover as well as on key physical and chemical properties of the ponds and combined them with meteorological data.

The studied ponds fall into three geomorphological categories: polygonal-center ponds, ice-wedge ponds and merged-polygonal ponds. All ponds are supersaturated in methane, but ice-wedge ponds, which feature persistent stratification, exhibit the highest surface water concentrations. The stratification causes surface methane concentrations to mainly depend on wind speed and on the amount of methane accumulated in the hypolimnion. The third, equally important driver in ice-wedge ponds is pond area: The larger the pond, the lower the concentrations. We hypothesize that the distinct features of ice-wedge ponds become less prominent the larger the pond is, and that the behavior of large ice-wedge ponds resembles that of polygonal-center ponds. Contrastingly, in polygonal-center ponds methane surface concentrations are determined by water depth rather than area. The larger merged-polygonal ponds on the other hand show the strongest dependence on area as well as an anticorrelation to water temperature. This hints to a strong influence of oxidation on the surface water methane concentrations in larger ponds. For all pond types we can confirm prior studies, which found that the moss layer is enriched with methane, but do not find that moss cover is a driver of variability in methane between ponds.

These findings underpin the strong variability of methane concentrations in small waterbodies as well as the complexity of the drivers of this variability. No single driver could explain a significant part of the variability over all pond types suggesting that more complex upscaling methods such as process-based modelling are needed.