Glacier algae phenology on the Qaanaaq Ice Cap (Northwest Greenland)

Glacier algae are relevant factors in the darkening phenomenon of glaciers, especially at the margins of the ice sheets. This study focuses on glacier algae variation during summer seasons in the 2016-2023 period at Qaanaaq Ice Cap, NW Greenland. Based on ice samples and field spectroscopy measurements, an empirical model is proposed to estimate glacier algae abundance from a reflectance ratio (695/687 or 695/681 nm). By applying this method to Sentinel-2 data at high resolution (10 m), through a phenology approach, algae abundance variation was estimated in relation to glaciological parameters and a marked spatial and temporal heterogeneity was found. High algae concentrations were found in the 2019, 2020 and 2023 summer seasons (~1 ×106 cells mL-1 on average) especially at low elevations (< 800 m a.s.l). At the scale of an outlet glacier, strong algal blooms were observed with more than one month of continuous positive air temperature and hiatus of snowfalls. The present research represents one of the first estimations of glacier algae phenology for the high latitudes at this high spatial resolution. These results could set the stage for future research focused on understanding the role of glacier algae at the scale of the Greenland Ice Sheet.