Shifts in organic matter character and microbial community dynamics from glacier headwaters to downstream rivers in the Canadian Rocky Mountains

Climate change is causing mountain glacial systems to warm rapidly, leading to increased water fluxes from these catchments and concomitant export of glacially-derived sediment and organic matter (OM). Glacial OM represents an aged, but potentially bioavailable carbon pool that is compositionally distinct from OM found in non-glacially sourced waters. Despite this, the composition of riverine OM from glacial headwaters to downstream reaches and its role in structuring microbial communities has yet to be characterized in the Canadian Rockies. Over three summers (2019-2021) samples were collected from four glacially-fed rivers within Banff and Jasper National Parks. Samples were collected before, during and after glacial icemelt along a transect ranging 0-100 km downstream of glacial termini. Organic carbon (OC) isotopes (delta 14C-OC, delta 13C-OC) were used to assess OM age and source, dissolved OM absorbance and fluorescence were used to assess OM character. Microbial community composition was assessed utilizing 16S rRNA gene sequencing.