Microorganisms are flushed from the Greenland Ice Sheet (GrIS) where they may contribute towards the nutrient cycling and community compositions of downstream ecosystems. We investigate meltwater microbial assemblages as they exit the GrIS from a large outlet glacier, and as they enter a downstream river delta during the record melt year of 2012. Prokaryotic abundance, flux and community composition was studied, and factors affecting community structures were statistically considered. The mean concentration of cells exiting the ice sheet was 8.30 × 104 cells mL-1 and we estimate that ∼1.02 × 1021 cells were transported to the downstream fjord in 2012, equivalent to 30.95 Mg of carbon. Prokaryotic microbial assemblages were dominated by Proteobacteria, Bacteroidetes, and Actinobacteria. Cell concentrations and community compositions were stable throughout the sample period, and were statistically similar at both sample sites. Based on our observations, we argue that the subglacial environment is the primary source of the river-transported microbiota, and that cell export from the GrIS is dependent on discharge. We hypothesise that the release of subglacial microbiota to downstream ecosystems will increase as freshwater flux from the GrIS rises in a warming world.

Bristol Glaciology Centre School of Geographical Sciences University of Bristol University Road BS8 1SS Bristol UK

Center for Permafrost University of Copenhagen Øster Voldgade 10 DK 1350 Copenhagen Denmark

Department of Ecology Faculty of Science Charles University Viničná 7 128 43 Prague Czech Republic

Department of Environmental Science Aarhus University Frederiksborgvej 399 DK 4000 Roskilde Denmark

Department of Geochemistry Geological Survey of Denmark and Greenland Øster Voldgade 10 DK 1350 Copenhagen Denmark

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