Description: Waterbodies in the arctic permafrost zone are considered a major source of the greenhouse gas methane (CH4). Spatial extrapolation of these CH4 fluxes to a region or the circum-Arctic, however, are still associated with large uncertainties. Here, we address this issue by using a combination of airborne CH 4 flux measurements and waterbody mapping based on TerraSAR-X and Sentinel-1 data across two study areas (1000 km2) in the Mackenzie Delta, Canada. Our results indicate that permafrost waterbodies, even if they seem to be strong emitters on an individual basis, do not necessarily translate into significant CH 4 emission hot spots on a regional scale. Our results show inconsistent patterns in the correlations between waterbody types and the CH 4 flux in the two study areas and across different spatial resolutions. Technical advances enabling the determination of the CH4 flux of individual waterbodies across a region provide a prospective direction to improve our understanding.

Description: In the past decades the Arctic has experienced stronger temperature increases than any other region globally. Shifts in hydrological regimes and accelerated permafrost thawing have been observed and are likely to increase mobilization of organic carbon and its transport through rivers into the Arctic Ocean. In order to better quantify changes to the carbon cycle, Arctic rivers such as the Lena River in Siberia need to be monitored closely. Since 2018, a sampling program provides frequent in situ observations of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) of the Lena River. Here, we utilize this ground truth dataset and aim to test the potential of frequent satellite observations to spatially and temporally complement and expand these observations. We explored all available overpasses (~3250) of the Ocean and Land Colour Instrument (OLCI) on Sentinel-3 within the ice-free periods (May – October) for four years (2018 to 2021) to develop a new retrieval scheme to derive concentrations of DOC. OLCI observations with a spatial resolution of ~300 m were corrected for atmospheric effects using the Polymer algorithm. The results of this study show that using this new retrieval, remotely sensed DOC concentrations agree well with in situ DOC concentrations (MAPD=10.89%, RMSE=1.55 mg L−1, r²=0.92, n=489). The high revisit frequency and wide swath of OLCI allow it to capture the entire range of DOC concentrations and their seasonal variability. Estimated satellite-derived DOC export fluxes integrated over the ice-free periods of 2018 to 2021 show a high interannual variability and agree well with flux estimates from in situ data (RMSD=0.186 Tg C, MAPD=4.05%). In addition, 10-day OLCI composites covering the entire Lena River catchment revealed increasing DOC concentration and local sources of DOC along the Lena from south to north. We conclude that moderate resolution satellite imagers such as OLCI are very capable of observing DOC concentrations in large/wide rivers such as the Lena River despite the relatively coarse spatial resolution. The global coverage of remote sensing offers the expansion to more rivers in order to improve our understanding of the land-ocean carbon fluxes in a changing climate.