Assessing Lake Ecosystem Recovery from Acidification and Responses to Emerging Environmental Stressors: a Paleolimnological Perspective

Abstract

Mining and smelting activities have heavily affected the Sudbury (Ontario, Canada) region since the late-19th century, leading to acidification and metal contamination in many ecosystems. Regulations on restricting acidic emissions were enacted in the 1970s, after which many Sudbury-region lakes recorded increasing pH and decreasing metal concentrations. Meanwhile, other emerging stressors have likely affected these lakes over the past few decades. Here, I revisit lakes in the Sudbury region, half a century after the application of remedial efforts, to assess lake ecosystem recovery from acidification and their responses to newly emerging environmental stressors. First, a canonical correspondence analysis is used to assess the relationships between surface-sediment diatom assemblage structure and environmental variables for 80 lakes. Lakewater pH remains the strongest environmental variable shaping diatom species distributions, and so is used to construct a robust inference model (R2boot = 0.73; RMSEP = 0.32). Additionally, by assessing ecological changes experienced by a subset of these lakes (n = 33, in common with Dixit et al. 2002) over the past few decades, two major trends are identified: an overall increase in diatom-inferred pH and a rise in the relative abundance of planktonic taxa. Further, down-core analyses in dated sediment cores are conducted to assess detailed ecological changes for three historically acidified lakes and two reference systems over the past ~200 years. Despite recording marked chemical recovery, the acidified lakes showed minimal or no evidence of biological recovery, with recent diatom assemblages markedly different from pre-disturbance assemblages, likely due to the legacy effects of acidification and climate warming. Biological recovery is lagging chemical recovery in acidified systems half a century after the reduction of acid deposition, and a return to pre-disturbance biological assemblages may never be achieved due to emerging environmental stressors, especially recently accelerated climate warming.