Nutrient and organic carbon input and abundance in supraglacial environments

Supraglacial environments are vital to glacial and ice sheet systems as they collect atmospheric and windblown inputs, are the locus of meltwater production and are a key nutrient source to englacial, subglacial and downstream environments. They have also been found to host abundant and diverse microbial communities focused in three main habitats; snow packs, bare ice and cryoconite holes. Yet due to the harsh conditions of this environment, microbial activity was considered of little global importance, particularly to biogeochemical cycles. Recently, large scale blooms of supraglacial microbial communities, in areas such as the Greenland Ice Sheet Dark Zone, an annually reoccurring area of decreasing surface albedo due to large scale algal blooms along the west-coast, have begun to change this assumption. In an effort to enhance the current understanding of nutrient cycling in supraglacial environments and the influence of resident microbial communities, comprehensive datasets of snow, ice and meltwater from multiple ablation seasons were collected from the Greenland Ice Sheet Dark Zone. A snow incubation experiment was also conducted to investigate microbial nutrient cycling under simulated polar winter conditions. The data indicate microbially mediated nutrient cycling in supraglacial environments during both the polar winter and main growth season, having implications for nutrients released from the snow pack during spring thaw and nutrients later exported from the supraglacial environment. Results from the spring thaw on the Greenland Ice Sheet (GrIS) show the snow pack to be an important source of nitrogen to the supraglacial environment, which may act as a stimulant for the annual algal bloom initiation. After the development of the algal bloom, during the peak of the growth season, the data reveal bare ice surfaces to be areas of high dissolved organic matter accumulation as glacier algae cycle nutrients from the inorganic to the organic phase more efficiently then heterotrophs remineralize the organic matter. Glacier algae blooms also depict a potential retention ability for maintaining dissolved organic nutrients at the surface. As a result, nutrient export to downstream environments is likely to be pulsed rather than constant, as well as rich in labile, dissolved organic nutrients, potentially impacting chemoheterotroph and chemoorganotroph community activity in subglacial environments. Further, data from early ice melt during spring thaw on the GrIS indicate dissolved organic nutrients retained in the ice surface over winter from the previous glacier algal bloom, provide a further nutrient source for the following season algal bloom. As the GrIS is the largest contributor to cryospheric sea level rise, and glacier algae blooms have been shown to significantly increase melt rates on the GrIS, it is imperative to better understand how they utilize and cycle nutrients in these surface ice habitats, as their expansion has critical implications for global sea level rise.