The impacts of permafrost thaw slumps on Arctic stream ecology

Climate change is intensifying permafrost thaw across terrain of the circumpolar Arctic. Increased temperatures and rainfall at northern latitudes have resulted in the expansion of thermokarst features such as retrogressive thaw slumps, which make previously frozen debris available for transport into nearby river systems. In the Peel Plateau, Northwest Territories, the mobilization of these materials exposes rivers to a large quantity of sediment and solutes that may act as a chronic, press disturbance, potentially altering stream food webs. Here, I examine if: 1) thaw slumps have driven changes to the physical and biological properties of slump impacted streams since an initial sampling campaign during 2010-2014, 2) thaw slump derived carbon is incorporated into stream food webs, and 3) thaw slump presence changes stream food web structure. I found that thaw slumps acted as press disturbances on streams, where slump-driven pressures on invertebrate communities remained consistent between 2010-2014 and 2021. High total suspended solids (TSS) persisted in impacted streams resulting in continually low invertebrate abundance in slump affected watersheds. However, changes to community composition through increased chironomid diversity were evident in the most recent sampling campaign, a trend that was reflected in increased taxonomic diversity in 2021 relative to previous assessments. All invertebrate metrics measured throughout 2010-2014 and 2021 were driven by the number of slumps upstream, where abundance decreased in response to large numbers of slumps, and diversity increased. Thaw slumps also altered food web structure in impacted streams through the addition of previously frozen organic carbon (C) into watercourses. As slump impact increased, consumers relied more on slump-derived C as a dietary resource. Sites with intermediate impact exhibited the broadest range of resource use, as consumers used food sources typical of both unimpacted and impacted food webs. Finally, thaw slumps affected food web structure by reducing trophic redundancy, but increasing trophic diversity, likely reflecting differences in microbial communities between levels of impact. Overall, thaw slumps cause a variety of impacts on the physical and biotic aspects of streams that will continue to affect ecosystems as climate change progresses.