Shifts in organic matter character and microbial community structure from glacial headwaters to downstream reaches in Canadian Rocky Mountain rivers
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".