The Legacy Of Regional Industrial Activity: Is Loon Productivity Still Negatively Affected By Acid Rain?

Metal mining and smelting in Sudbury, Ontario, Canada, began in the 1880s and peaked in the 1950s and 1960s. By the mid-1970s, thousands of lakes had been acidified by sulfur dioxide (SO2) emitted from Sudbury metal smelters. Common loons (Gavia immer) are top predators in lakes across Ontario, and lake acidification has resulted in loss of prey for developing chicks and declines in common loon breeding productivity. Acidifying emissions have declined greatly since the 1960s, but many lakes remain acidic. We evaluated chemical recovery and loon breeding success in 69 single-loon-pair (small), variably acidified lakes near Sudbury using 38 years of loon survey and water chemistry data (1982-2019). Lake pH increased, particularly in lakes that were more acidic and closer to Sudbury, but 65% of lakes that were highly acidic in the 1980s remained below pH 6.0 (where biological damage can occur) in 2019. Furthermore, over half of our study lakes showed hydrochemical properties indicative of acid impairment (e.g., high relative sulfate levels, low base cation concentrations, high aluminum and manganese concentrations). More acidic lakes were associated with lower loon productivity, and overall, loon breeding effort and success declined over four decades on these lakes. The legacy of acid rain continues to negatively impact small lakes near Sudbury, and we hypothesize that loons may be more vulnerable to population declines in areas throughout their range that were directly and heavily degraded due to acidifying emissions.