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The boreal biome contains the world’s largest number of lakes that are sensitive to anthropogenic acidification. Thousands of poorly buffered Canadian boreal lakes (Fig. 1) were acidified during the 20 century by acid deposition generated from industrial activity (1). Since 1970, some boreal lakes have shown chemical and biological recovery because of reductions in acidic sulfur oxide emissions by Canadian industries (2–6). However, other lakes have shown limited recovery, while many continue to acidify (3). Therefore, evidence of resilience, i.e. recovery rate to anthropogenic acidification in boreal lake ecosystems is required to assess the need for further abatement of emissions of sulfur and nitrogen oxides. Anthropogenic acidification of boreal lakes impacts several environmental variables (7). Increased acidity results in declines in dissolved organic carbon (DOC) and base cations (e.g. calcium), and elevated aluminum, concentrations in boreal lakes. As a consequence, biota experience increased exposure to DNAdamaging UV-B (280–320 nm wavelength) radiation and metal toxicity because DOC is the primary attenuator of ultraviolet (UV; 280–400 nm wavelength) radiation and organic chelator of metals in boreal lakes (8–11). Lake acidification also lowers the concentration of dissolved inorganic carbon (DIC), which limits rock-associated, i.e. epilithic, algae (12). Consequently, anthropogenic acidification impairs biodiversity and certain ecosystem processes in boreal lakes (7, 13). Algal assemblages may closely track environmental changes in lakes owing to their wide dispersal potentials and relatively fast growth rates (14–16). In particular, taxonomic composition can be a more sensitive indicator than primary production because compensatory species dynamics make aggregate community functions more resistant against the impacts of acidification (14, 17). For example, taxonomic shifts in epilithon (15, 18, 19), phytoplankton (5, 19, 20), and diatom assemblages (21, 22) are more often detected than are changes in total algal biomass in recovering acidified lakes. Nevertheless, taxonomic resilience will depend on recruitment from local (within-lake) and regional Resilience of Epilithic Algal Assemblages in Atmospherically and Experimentally Acidified Boreal Lakes Rolf D. Vinebrooke, Mark D. Graham, David L. Findlay, and Michael A. Turner