Regional acidification of aquatic habitats has caused severe reductions in biodiversity. Reduced sulfur dioxide emissions over the past several decades have resulted in increased pH and alkalinity in some areas of North America and Europe. However, biological recovery has lagged behind increases in lake pH. We propose that acidification-induced changes in predator assemblages can provide biological resistance to recovery of assemblages in lower trophic levels in lakes. Many recovering lakes remain fishless because of low colonization rates and, therefore, support a high abundance of macroinvertebrate predators that may have a large impact on zooplankton community structure. We assessed the distribution of water beetles in relation to pH and presence/absence of fish in 29 lakes on the Canadian Shield. We found that water beetle assemblage composition was not related to pH. However, the occurrence of fish was significantly negatively correlated with water beetle distribution, particularly for the predaceous diving beetle, Graphoderus liberus. Mesocosm experiments in Swan Lake, a fishless lake recovering from acidification, revealed that larval G. liberus can reduce total zooplankton abundance, species richness, and species diversity. In particular, the densities of 4 taxa (Leptodiaptomus minutus, Diaphanosoma birgei, Bosmina [Bosmina] spp., and calanoid copepodids) were reduced by larval G. liberus predation. The high abundance of G. liberus in the absence of fish and the impact of larval G. liberus predation on crustacean zooplankton in Swan Lake suggest that biological resistance may be an important impediment to the recovery of aquatic food webs, despite increasing pH.
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