Export of Organic Matter and Microbes from the Greenland Ice Sheet: Sources, Composition, and Downstream Implications

Meltwater runoff from the Greenland Ice Sheet (GrIS) has increased by more than 50% in the last 50 years. While considerable uncertainty revolves around the impact this change may have on downstream ecosystems, previous research has suggested that solute and microbial exports from the GrIS are likely to increase with higher freshwater fluxes. We monitored the Watson River, a glacially fed river in West Greenland, over the 2012 and 2015 summers to evaluate the influence increased fluxes may exert on local microbial communities and downstream biogeochemical cycles. Our objectives were to approximate the number of cells exported, characterize cell assemblages, and determine their origin. In 2012, paired microbiological samples were taken sporadically at the Leverett Glacier meltwater portal (at the head of the Watson River) and the Watson River fjord outlet 30 km downstream, to quantify microbial cells and characterize assemblage structure. We found cell concentrations and microbial assemblages to be very similar between locations, despite their distance apart. This suggests that GrIS outlet rivers are "neutral pipes" connecting microbes between glacial and estuarine habitats. We further identified subtle shifts in assemblage structure over the course of the summer melt season (May to August), and hypothesized that this reflects an expanding subglacial drainage network, with waters draining parts of the GrIS bed progressively further inland as the melt season progressed. Meltwaters from the Leverett Glacier portal were again monitored during the 2015 summer to identify the source of exported microbes by sampling during or near outburst events, which flush long-term stored waters from the ice-sheet bed. Using 14C dating, we found that exported suspended sediment-bound carbon becomes progressively older from June to August. This suggests that different reservoirs are tapped as the melt season progresses, which we interpret as originating from a greater distance into the GrIS subglacial environment. While further exploration is necessary to evaluate the long-term consequences of deglaciation, this work provides new and interesting information on glacial exports to downstream ecosystems and insights into their associated biogeochemical cycles