Albedo change from snow algae blooms can contribute substantially to snow melt in the North Cascades, USA

Snow algae are ubiquitous in the Pacific Northwest cryosphere in the summer where snowmelt is an important contribution to regional watersheds. However, less attention has been given to biological impurities as drivers of snowmelt compared to inorganic light-absorbing particles. Here we map snow algae near Mt. Baker with a multispectral camera on an uncrewed aerial vehicle using (1) principal components and (2) spectral indexing. The two approaches are tested under differing bloom states and verified with coincident algal pigment and cell count data. During high bloom intensity we found an average instantaneous radiative forcing of 237 W m-2 with a maximum of 360 W m-2. This translated to 1,508 ± 536 m³ of melted snow water equivalent in the 0.1 km² basin. These results demonstrate snow algae contribute to snowmelt at mid-latitudes and the potential for uncrewed autonomous vehicles to map snow algae over expansive areas of the cryosphere.