The influence of glacier habitat characteristics on the development of snow algal blooms in northern Sweden

Glacier habitats seem inhospitable but contain functioning ecosystems in their ice surfaces and snow cover. The growth of red snow algae, which are widespread across both the Arctic and Antarctica, is impacted by the presence of liquid water, nutrients, sunlight, and positive temperatures. It is, however, unclear where in the landscape snow algal blooms thrive, and how glacier habitat characteristics such as glacier size, surface debris presence and terrain can impact the factors that regulate snow algal bloom formation. This study investigates the distribution of snow algal blooms, and analyses the impact of glacier habitat characteristics on the timing and size of snow algal blooms in northern Sweden. To do this, snow algal blooms were mapped using a red-green spectral index based on Sentinel-2 satellite images, which measured the red light reflected by the carotenoid pigments that red snow algae produce. Glacier habitats were mapped based on orthophotos, and the habitat characteristics of area, elevation, slope, aspect, latitude, surface debris presence, and underlying geology were assigned to each glacier. The relationship between glacier habitat characteristics and snow algal bloom growth was evaluated using generalised linear models.  Spectral index mapping of snow algae shows that snow algal blooms are widespread on glaciers in northern Sweden. The growth of snow algal blooms is spatially heterogeneous, both in terms of area occupied by algal blooms and in terms of algal cell abundance within the algal blooms. On glaciers where snow algal blooms initially emerge early in summer, snow algal blooms larger at the peak of algal growth than on glaciers where blooms initially emerge later in the season. The various snow algal bloom metrics are each impacted by different glacier habitat characteristics. The timing of the start of algal bloom formation is influenced by the glacier area, elevations, slope, and surface debris. The area covered by snow algal blooms is influenced by the same characteristics, in addition to the aspect and surrounding geology. The percentage coverage of glaciers by snow algal blooms is tied to elevation, aspect, latitude, and surrounding geology, while the snow algal cell abundance is impacted by slope and surface debris. Overall, favourable habitats for snow algal bloom growth consist of shallow-sloped glaciers at low elevation that contain surface debris, as these habitats provide increased access to liquid water and nutrients. The complex relationship between snow algal bloom growth and glacier habitat characteristics means that impacts of climate change-induced melt in the future may not be uniform. Earlier onset of melt will likely induce earlier development of snow algal blooms, especially at high elevations, but habitat loss due to glacier shrinkage will likely reduce the size of snow algal blooms, particularly at low elevations.

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