Elevated metal concentrations inhibit biological recovery of Cladocera in previously acidified boreal lakes

Summary Lakes near S udbury, C anada, have been exposed to intense acidification and metal contamination from nearby mining and smelting operations. Although lakewater p H improved substantially following the implementation of emission controls in the late 1960s, biological recovery continues to lag behind chemical recovery. We assessed the current state of biological recovery (relative to pre‐impact times) using multiproxy palaeolimnological records from two nearby lake districts ( S udbury and K illarney), both impacted by acidification but having experienced differences in metal contamination due to their respective distances from smelters. Twentieth century cladoceran shifts were most pronounced in the acidified and metal‐contaminated S udbury lakes, with assemblage changes tracking industrial activity. C hydorus brevilabris increased markedly in dominance, largely at the expense of B osmina spp. In contrast, the K illarney lakes, with similar changes in p H but lower C u and N i inputs than the S udbury lakes, experienced minimal changes within their sedimentary cladoceran assemblages. These regional differences in cladoceran impact and recovery patterns are best explained by varying levels of C u and N i contamination, with concentrations of these metals still exceeding provincial water quality guidelines in the S udbury lakes. Biological recovery in these systems appears to be inhibited by persistent high metal levels. Increased lake primary production and coincident shifts in cladoceran assemblages over the past approximately 40 years in all the study lakes suggest that climate impacts may be gaining prominence as drivers of ecological change, and therefore a return to the biotic structure of the pre‐smelter era is unlikely.