Effects of drought‐induced acidification on diatom communities in acid‐sensitive Ontario lakes

Abstract

Chemical recovery of many acidified lakes in North America has been delayed or reversed as a result of interactions between climatic variability (alterations between drought and nondrought periods) and previously deposited acids stored in wetlands, but effects of this wetland‐mediated reacidification phenomenon on aquatic biota remain unknown. We compare changes in diatom assemblages in 200‐yr‐long sediment cores from two lakes with similar basin characteristics but different wetland area (4.4% of catchment at Chub Lake, 0% at Blue Chalk Lake) to evaluate the role of wetland‐mediated interactions among acid deposition and climatic variability on algal communities in acid‐sensitive lakes. Diatom assemblages were significantly more variable in Chub Lake than in Blue Chalk Lake. Variance partitioning analysis of approximately annually resolved sedimentary diatom analyses (1977–1997) identified that unique effects of water chemistry, independent of acid deposition and climatic factors, accounted for the greatest amount of variation (25%) in diatom assemblages at Chub Lake. Acid deposition (24%) and climatic factors (22%) accounted for similar, significant amounts of variation in diatom communities. Complex interactions among all three factors, which are attributable to wetland‐mediated drought‐induced reacidification, explained an additional 10% of the variation in diatoms at Chub Lake but only 1% at Blue Chalk Lake. Droughtrelated reacidification effects on water chemistry might thus cause important effects on algal communities in acid sensitive lakes with modest wetland coverage, but not in lakes without wetlands.