Propagating effects of permafrost thaw slumps on microbial communities throughout Arctic stream networks

Permafrost thaw in northwestern Canada has accelerated in recent decades due to rapid warming and increased precipitation. The degradation of ice-rich permafrost transforms landscapes, triggering widespread mass-wasting features known as thaw slumps. Across the Peel Plateau in the Northwest Territories (Canada), these thaw slumps mobilize substantial volumes of materials rich in organic matter, nutrients, and other solutes. The lateral transport and deposition of thawed materials into streams shift hydrological, sedimentary, and geochemical regimes, with cascading effects on biogeochemical cycles, food webs, and water quality. Microorganisms play a pivotal role in mediating the processes that regulate ecosystem structure and function. Yet, the responses of microbial communities within slump-affected stream networks remain poorly understood.  

To address this knowledge gap, we conducted a two-year (2022-2023) catchment-scale investigation in the 1,100 km² Stony Creek watershed on the Peel Plateau. Our study tracked microbial community responses along the slump-stream continuum, including rill water runoff from seven thaw slumps, impacted headwaters, major tributaries, and a transect along the Stony Creek mainstem. We characterized microbial communities using 16S rRNA amplicon sequencing and combined these data with stream physicochemical properties, dissolved (DOC) and particulate organic carbon (POC) composition, and landscape metrics to identify drivers of microbial community response. We also assessed microbial functional potential and biomass production using marker gene-based functional predictions (PICRUSt2) and tritiated leucine incorporation experiments. 

Connectivity between thaw slumps and streams produced pronounced shifts in microbial community composition. The extent of community divergence downstream of rill water inflow covaried strongly with the change in sediment loading and associated particulate organic matter. With stronger slump influence, microbial communities became progressively more associated with anoxic conditions and a reduced, low-energy carbon pool, reflecting a transition to a system dominated by POC. Further downstream, microbial community patterns in major tributaries and along the mainstem became increasingly mixed, suggesting that as larger areas of the catchment were integrated and in-stream processes became more complex in higher-order stream segments, the direct disturbance signal from thaw slumps became less dominant in shaping community structure. Despite this, biomass production was strongly correlated with DOC throughout the mainstem transect, even though DOC represented a small fraction of the total carbon pool relative to POC.  

Through this work, we aim to document how microbial communities transform as thaw slump materials are transported through fluvial networks. Establishing these patterns is critical for understanding biogeochemical cycling in thermokarst-affected landscapes, where terrestrial-aquatic connectivity is pronounced. Collectively, our findings provide a baseline for microbial community dynamics in this region and complement comprehensive geochemical and carbon cycling data. As climate change and permafrost thaw intensify in Arctic ecosystems, understanding the role of microbial communities becomes increasingly important. By characterizing stream microbial diversity, predicting functional profiles, and microbial activity, we advance our understanding of the mechanisms driving biogeochemical changes and their broader impacts on food webs, water quality, and greenhouse gas exchange.