Methylmercury bioaccumulation and biomagnification in streams within forested catchments defoliated by spruce budworm

Though outbreaks of defoliating insects are a widespread, natural disturbance in Canadian boreal forests and their magnitude and duration are increasing in recent years, we have little understanding of the impacts on stream ecosystems. Herein we examined the fate of mercury (Hg), a toxic element affected by other landscape-scale forest disturbances, in twelve stream food webs in the Gaspé Peninsula (Québec, Canada) that ranged in their severity of watershed defoliation from spruce budworm (Choristoneura fumiferana). Basal food sources (coarse and fine particulate organic matter, and biofilms), several macroinvertebrate taxa, and fish (brook trout [Salvelinus fontinalis] and slimy sculpin [Cottus cognatus]) were sampled in 2019 and 2020 and analyzed for stable isotopes of C and N, methylmercury (MeHg) or total Hg (THg, fish only). Hierarchical partitioning models were used to identify relative importance among landscape and local water quality variables, and they explained 76 and 65 % of variation in brook trout THg and carnivorous invertebrate (Chloroperlidae, Rhyacophila, Parapsyche) MeHg levels, respectively, with dissolved organic carbon (DOC) as the main driver increasing biotic mercury levels. Trophic magnification slopes (TMS) (calculated as log₁₀Hg vs. δ¹⁵N) ranged from 0.27 to 0.38 across all watersheds but were not related to defoliation severity or DOC concentrations. Collectively, these findings suggest that local measures of water quality are more important drivers of Hg bioaccumulation in stream biota than landscape disturbances caused by forest defoliation by spruce budworm.