Abiotic and biotic regulation of periphyton in recovering acidified lakes

Anthropogenic acidification of lakes alters periphyton growth and taxonomic composition. I tested whether these changes were regulated by abiotic (i.e., resources, acidity) and biotic (i.e., herbivory, competition) conditions, or both. Periphyton was reciprocally transplanted across lakes of low acidity (pH similar to 6.3), moderate acidity (pH similar to 5.6), and high acidity (pH < 5) undergoing recovery from anthropogenic acidification. Transplants were also placed in exclosures that excluded either macrograzers or all grazers and potential algal competitors. Data from 4-wk incubations suggested that periphyton growth and taxonomic composition were primarily regulated by abiotic conditions. Total biomass was significantly lower for periphyton that originated from, or was transferred into, the most acidic lake. Canonical correspondence analysis showed abiotic regulation of periphyton in acidified lakes was best explained by dissolved inorganic carbon availability. Regulation by biotic factors was detectable only in the least acidic lake where macrograzers suppressed loosely attached taxa, but not total biomass, owing to compensatory species replacements. When periphyton was transferred into less acidic lakes, only taxa that were already native to the host lake persisted through the 4-wk incubation, suggesting that algal recolonization during recovery is not impeded by dispersal barriers. My findings suggest that abiotic regulation of epilithic periphyton is superseded by biotic control as acid-sensitive macrograzers recolonize recovering acidified lakes.