Benthic macroinvertebrate assemblages and freshwater food webs of beaver-impounded streams in the eastern Canadian Arctic

Abstract

As circumpolar warming facilitates the shrubification of Arctic landscapes, the distribution of North American beavers (Castor canadensis) in Canada has been expanding northward, raising concern in Inuit communities. Though ecosystem engineering by beavers in temperate regions is well-documented, there is limited research that examines the effects of beaver impoundments in the tundra. Freshwater streams in the Arctic support subsistence fish populations and it is currently unclear how flow attenuation by dams will affect the habitat quality or prey resources of resident species. This research assesses differences in the benthic macroinvertebrate diversity and trophic structure of beaver-impounded streams above and below the treeline in Nunavik. Invertebrates and consumer stable isotopes were compared downstream and upstream of dams to characterize changes in assemblage composition, basal resource reliance, and Layman’s food web metrics. Shannon-Weiner diversity and the percentage of lotic invertebrates were lower upstream of beaver dams. Filter-feeders and EPT taxa (Ephemeroptera, Plectoptera, Trichoptera) decreased with variables associated with lentic conditions, such as reduced stream velocity, increased depth, and finer substrates. Geomorphic-driven differences in assemblage composition, without exhibiting changes in richness or abundance, suggest restructuring in response to upstream habitat transformation. In subarctic forest sites, reliance on terrestrially derived carbon in consumer diets was greater upstream of beaver dams but no effect was observed in shrub tundra sites. Additionally, upstream averages of consumer carbon were more enriched and similar to riparian vegetation than epilithic algae. Although a resource shift was observed, overall food web metrics were not affected by beaver dams. Collectively, the findings presented in this study demonstrate that beaver dam effects below the treeline generally resemble the lotic taxa replacement and dietary shifts reported within their historical range, while recently colonised streams above the tree line appear to be marginally less affected.