Spatiotemporal changes in sedimentary chironomid assemblages across a gradient of lake water pH and metal contamination from the Sudbury (Ontario, Canada) metal-smelting region

Base metal smelting in Sudbury (Ontario, Canada) resulted in marked environmental degradation starting in the late nineteenth century. The environmental legacy of intensive industrial activity remains evident in the region despite remediation efforts and recovery. Even though chironomids (Diptera: Chironomidae) are important trophic intermediates, relatively little is known about their response to base metal smelting contaminants. We examined the spatial distribution of recently deposited (i.e., modern) chironomid assemblages in 64 lakes, which we found was most related to lake depth. Further, we compared pre-1880 assemblages to recent sedimentary assemblages with a 32-lake subset. The influence of past contamination was apparent from the replacement of acid- and metal-sensitive taxa with tolerant taxa and early colonizers. In clear lakes, littoral taxa experienced the most change. Chironomid diversity appears lower in recent sediments compared to pre-impact sediments, which is in part related to the persistence of tolerant communities and the lack of recovery of acid-/metal-sensitive taxa, but may also be related to changes in biotic stressors. Spatial trends in chironomid assemblages can inform modern studies of biodiversity recovery in metal smelting regions and strengthen future inferences of how smelting emissions have influenced fish communities and deep-water habitat in the context of multiple modern stressors.