Shifts in organic matter character and microbial community structure from glacial headwaters to downstream reaches in Canadian Rocky Mountain rivers

Abstract. Climate change is causing mountain glacial systems to warm rapidly, leading to increased water fluxes 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) we collected samples before, during and after glacial ice melt along stream transects ranging 0 – 100 km downstream of glacial termini on the eastern slopes of the Canadian Rocky Mountains. We quantified dissolved and particulate organic carbon (DOC, POC) concentration, and used isotopes (Δ¹⁴C-OC, δ¹³C-OC) and dissolved OM (DOM) absorbance and fluorescence to assess OM age, source, and character. Environmental data were combined with microbial 16S rRNA gene sequencing to assess controls on microbial community composition. From glacial headwaters to downstream reaches, OM showed a clear transition from being aged and protein-like with an apparent microbial source to being relatively younger and humic-like. Indicator microbial species for headwater sites included chemolithoautotrophs and taxa known to harbour adaptations to cold temperatures and nutrient-poor conditions, suggesting a role for glacial seeding of microbial taxa to headwaters of this connected riverine gradient. However, environmental conditions (such as deuterium excess, an indicator of water source; water temperature; POC concentration; and protein-like DOM) could only significantly explain ~9 % of the observed variation in microbial community structure. This finding, paired with the identification of a ubiquitous core microbial community that comprised a small proportion of all identified amplicon sequence variants (ASVs), but was present in large relative abundance at all sites, suggests that mass effects largely overcome species sorting to enable a connected microbial community along this strong environmental gradient. Thus, with a loss of novel glacial and microbial inputs with climate change, our findings suggest consequent changes in OC cycling and microbial community structure may lead to complex ecosystem responses across the evolving mountain-to-downstream continuum in small, glacierized systems.