Optical diversity of thaw ponds in discontinuous permafrost: A model system for water color analysis

Permafrost thaw ponds result from the irregular melting and erosion of frozen soils, and they are active sites of greenhouse gas emissions to the atmosphere throughout the circumpolar North. In the discontinuous permafrost region of Nunavik, Canada, thaw ponds show pronounced differences in color even among nearby ponds, ranging from white to green, brown and black. To quantify this optical variation and to determine its underlying controlling mechanisms, we studied the apparent and inherent optical properties and limnological characteristics of the ponds. The pond colors were well separated on a color coordinate diagram, with axis values determined from above-water spectral reflectance measurements. Our analyses of optical properties and their empirical relationships with optically active substances showed that the differences in color could entirely be attributed to variations in the concentration of two optically active substances: dissolved organic carbon, which was a major contributor to spectral absorption, and nonalgal suspended particulate matter, which contributed to spectral scattering as well as absorption. The latter component was dominated by small sized particles that had unusually high mass-specific absorption and scattering properties. Analysis of high spatial resolution, multispectral satellite imagery of these ponds showed that these two optically important constituents could be estimated by multivariate modeling. The results indicate that remote sensing surveys will provide valuable synoptic observations of permafrost thaw ponds across the vast subarctic region, and may allow scaling up of local greenhouse gas flux measurements to regional and circumpolar scales.