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Glacier algae residing within the surface ice of glaciers and ice sheets play globally significant roles in biogeochemical cycling, albedo feedbacks, and melt of the world’s cryosphere. Here, we present an assessment of the macro-nutrient stoichiometry of glacier algal assemblages from the southwestern Greenland Ice Sheet (GrIS) margin, where widespread glacier algal blooms proliferate during summer melt seasons. Samples taken during the mid-2019 ablation season revealed overall lower cellular carbon (C), nitrogen (N), and phosphorus (P) content than predicted by standard microalgal cellular content:biovolume relationships, and elevated C:N and C:P ratios in all cases, with an overall estimated C:N:P of 1,997:73:1. We interpret lower cellular macro-nutrient content and elevated C:N and C:P ratios to reflect adaptation of glacier algal assemblages to their characteristic oligotrophic surface ice environment. Such lower macro-nutrient requirements would aid the proliferation of blooms across the nutrient poor cryosphere in a warming world. Up-scaling of our observations indicated the potential for glacier algal assemblages to accumulate ∼ 29 kg C km2 and ∼ 1.2 kg N km2 within our marginal surface ice location by the mid-ablation period (early August), confirming previous modeling estimates. While the long-term fate of glacier algal autochthonous production within surface ice remains unconstrained, data presented here provide insight into the possible quality of dissolved organic matter that may be released by assemblages into the surface ice environment.

The surface (supraglacial) environment of the Greenland Ice Sheet (GrIS) is an active site for the storage, transformation and transport of carbon, which is driven by extremely high levels of solar radiation throughout the ablation season. Within the south west of the GrIS, blooms of Streptophyte micro-algae (hereafter glacier algae) at abundances of ~ 105 cell mL−1 dominate primary production in the surface ice and provide dissolved organic matter (DOM) to the heterotrophic bacterial community. Glacier algae contain photoprotective secondary phenolic pigment that comprises a large proportion of the cell (~ 4 % of the dry weight) and could represent a substantial, additional carbon source for the heterotrophic community. The transformation and degradation of DOM by solar radiation (photodegradation) and heterotrophic communities (biodegradation) represent two crucial controls on DOM composition and quantity; however, the influence of these processes within the surface ice is yet to be constrained. This study therefore assessed responses in the composition and quantity of two carbon sources (glacier algae secondary pigment and surface ice DOM) following exposure to UV, PAR, UV+PAR (photodegradation) and subsequent incubation with bacterial communities isolated from the ambient environment (biodegradation). Our results indicate that exposure to predominantly UV radiation altered the composition of glacier algal pigment and surface ice DOM; however, the quantity of DOM remained constant. Biodegradation caused the greatest changes to both DOM composition and quantity, particularly in surface ice DOM. Secondary pigment extracted from glacier algae was not a highly bioavailable source of carbon and did not support significant growth of surface ice heterotrophic bacterial communities. Conversely, low molecular weight compounds in surface ice DOM were rapidly utilised by heterotrophic bacteria supporting between a 3 and 9-fold increase in bacterial abundance over a 30-day incubation. We found that photodegradation of glacier algal pigment and surface ice DOM did not influence heterotrophic consumption. Photodegradation and biodegradation of DOM in the surface ice habitat are likely intimately linked and act as fundamental controls on the composition and quantity of DOM exported to downstream environments.