Legacy effects of acidification and climate warming affect recent recovery of diatom assemblages in Sudbury-region lakes (Ontario, Canada).

Smelting activities have strongly affected the Sudbury (Ontario, Canada) region since the late-nineteenth century, leading to acidification and metal contamination of local ecosystems. Regulations restricting acid deposition were enacted in the 1970s, after which many lakes exhibited increasing pH and decreasing metal concentrations. Given the documented chemical recovery and potential emergence of new stressors, we used paleolimnological tools to assess the biological recovery processes in three previously acidified lakes, and also to reconstruct the limnological histories of two reference lakes that were relatively unaffected by acidification. Using subfossil diatoms preserved in lake sediments, we show that assemblages from the acidified lakes have not returned to their pre-disturbance conditions but became dominated by an acidophilous planktonic taxon, Asterionella ralfsii var. americana Körner, during the past few decades. Considering that this taxon has a relatively low pH optimum and is mostly found in thermally stratified waters during summers, we suspect that the legacy effects of acidification and climate warming are the main factors driving its recent prosperity. Our study suggests that biological recovery lags chemical recovery in acid-impacted systems, and a return to pre-disturbance biological assemblages may be impeded by newly emerging environmental stressors. [ABSTRACT FROM AUTHOR]