Carbon Cycling and Nutrient Storage in Supraglacial Environments on the western margin of the Greenland Ice sheet

During summer, exposed ice on the surface of glaciers becomes weathered by solar radiation, creating a ~1 m water-saturated layer of porous ice referred to as the weathering crust aquifer. Here we present results from a hydrological, biogeochemical, and microbiological study of supraglacial waters from the Greenland Ice Sheet margin in proximity to Kangerlussuaq. Through comparisons with supraglacial streams with relatively shorter water residence times, we demonstrate the weathering crust aquifer contains a distinct geochemical composition and unique community of microorganisms that is actively cycling carbon and nutrients. This evidence includes changes in the abundance of organic nutrients and compositional changes in the fluorescent dissolved organic matter properties over the melt season. Sources of organic matter production include photosynthesis, which was indicated by changes in the natural abundance of δ¹³C in inorganic carbon as well as in experiments where added ¹³CO₂ was incorporated into biomass. Our results show there were relatively high dissolved concentrations of solute in the weathering crust aquifer in relation to supraglacial streams, implying water-sediment interactions occurring in the weathering crust affected the meltwater chemistry. Solutes enriched in the meltwater included trace elements (e.g., Zn, Ni, and Cu) and phosphorus, the latter of which could be due to the presence of apatite, an easily weatherable phosphorus-bearing mineral. Processes affecting the availability of phosphorus in supraglacial waters are significant considering results from nutrient addition experiments that demonstrated the supraglacial phototrophic communities were phosphorus limited. Cell and Chlorophyll a concentrations initially increased with progression of the melt season but decreased the late season, suggesting that much of the new biomass accumulating in the weathering crust aquifer during the summer months was subsequently transported downstream with meltwater. The study site is a component of a ~3000 km²  supraglacial catchment estimated to store ~0.5 km³ of meltwater per season that is discharged to the Akuliarusiarsuup Kuua, highlighting the scale of hydrological and biogeochemical processes influencing ecosystems near the ice sheet margin. Consequently, a better understanding of microbial processes cycling carbon and contributing to nutrient availability in the weathering crust aquifer is needed to decipher the biogeochemical effects on Arctic supraglacial and proglacial systems that are undergoing rapid changes.