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Quantifying the distribution and abundance of ice algae is fundamental for understanding the evolving processes of algal blooms in supraglacial environments, particularly over the Greenland ice sheet, given the role of algal impurities in modulating surface albedo and meltwater production. Field observations of ice algae in Greenland are very limited over space and time. Here we show for the first time the regional variability in algal abundance across the dark zone in southwest Greenland, derived from Sentinel-3 images acquired during the summertime in 2016 and 2017. We demonstrate the capacity of Sentinel-3 imagery to characterize the spatial pattern of algal abundance using the reflectance ratios between 709- and 673-nm bands, highly consistent with field measurements. The estimated algal abundance reveals a significant linear growth pattern of algal population with time after the peak of dark ice presence, shown to be tightly linked to surface runoff and meltwater production.

Plain Language Summary Ice algae bloom plays an important role in enhancing the surface melting of the Greenland ice sheet. It has been challenging to monitor the development of ice algae at a regional scale with temporally frequent observations. In this study, we map the time series of algal abundance over southwest Greenland from space for the first time, by utilizing the advanced capability of Sentinel-3 satellites in detecting chlorophyll-a, a typical pigment generated by algae. The spatial pattern of algal abundance characterized by Sentinel-3 imagery is highly consistent with field measurements. Our analysis reveals the widespread proliferation of algae on bare ice from late July to mid-August, with increasing algal population over time after the peak of surface runoff and meltwater production. Incorporating the algal distribution derived from satellite data into regional climate models is expected to improve the projection of future mass balance over the Greenland ice sheet.