Paleoecotoxicology of smelting-affected lakes near Sudbury (Ontario, Canada)
For over a century, Sudbury (Ontario, Canada) has been a major copper-nickel metal smelter centre. Emissions of acidifying compounds and metal particulates beginning in the 1880s damaged thousands of lakes and watersheds in the region. Emission controls introduced in the 1960s and 1970s greatly affected water chemistry. Despite evidence of chemical recovery, comparatively little is known about long-term changes in aquatic biota of affected lakes. Here, I use three paleolimnological approaches to assess spatial and long-term temporal trends in cladocerans (Branchiopoda: Cladocera) and chironomids (Diptera: Chironomidae). First, a survey of >60 lakes revealed that recent cladoceran assemblages were related to lakewater specific conductance in combination with nutrients, alkalinity, and lake depth, whereas recent chironomid assemblages are structured primarily by lake depth and secondarily by copper and calcium concentrations. Second, a comparison of recent and pre-industrial sediments using a “top-bottom approach” demonstrated that both cladoceran and chironomid assemblages changed modestly. Acid- and metal-tolerant taxa were more common in recent sediments. In cladoceran assemblages, the Daphnia longispina complex was partially replaced by the D. pulex complex, which was more tolerant of low-calcium conditions in our focal lakes; further modern zooplankton monitoring may refine this interpretation by determining which species are represented in the morphological species complexes in sediment records. For both indicators, early colonist taxa were more common in recent sediments. I also found that recent cladoceran assemblages clustered spatially, which may be the result of historical contamination and modern stressors. Finally, I examined four lakes with varying environmental histories to compare recovery trajectories, establish baseline conditions, and assess biotic changes during peak emissions. Lake acidification and metal contamination altered all proxies in acid-sensitive lakes. The most consistent changes in invertebrate assemblages were the loss of littoral/sub-littoral taxa during acidification, the replacement of acid-/metal-sensitive taxa with tolerant assemblages, and the lack of recovery in recent sediments. Overall, these results suggest that cladoceran and chironomid assemblages responded to historical smelting activities, with limited biological recovery to pre-industrial conditions. Placing recovery trajectories in the context of modern environmental stressors, such as recent climatic change, will continue to be important for future mitigation efforts.