Effects of changes in acidity on aquatic insects in rocky littoral habitats of lakes near Sudbury, Ontario

Benthic aquatic insects were collected from rocky nearshore areas (<1 m deep) of 17 lakes near Sudbury, Ontario, with a pH range of 4.7-7.3 and a size range of less than 10 ha to over 10,000 ha. These insect communities were composed of taxa common to lake soft-sediments and streams. Direct and indirect effects of lake acidity appeared to be major controls on the structure of these communities, implying that several factors may be involved in restructuring during acidification or recovery. Declines in abundances of several taxa of Ephemeroptera at pH below 5.5 were attributable to acid toxicity, while increases in the abundances of Odonata and Diptera at pH below 5.5 were associated with the absence of fish predators and other indirect effects of acidity. The communities of two experimentally neutralized lakes restructured rapidly within 5 years, approaching but not achieving community structures typical of our near-neutral survey lakes. Neutralization led to recolonization or increased abundance of the acid-sensitive mayfly, Stenacron interpunctatum , and the dragonfly, Boyeria grafiana; however, recolonization by other taxa expected to be present in near-neutral lakes (Stenonema femoratum, Eurylophella, and Basiaeshna janata) was not observed. Consistent with results for the survey lakes, declines in the abundances of the dragonflies Aeshna interrupta, Aeshna eremita, and Leucorrhinia glacialis in the neutralized lakes were associated with reintroductions of Salvelinus fontinalis (aurora trout) and increased fish predation pressure, while reduced abundances of the dipterans Ceratopogonidae, Psectrocladius, and Stackelbergina may be related to indirect effects of acidity other than fish predation. Although community composition varied greatly across the acidity gradient, total species richness and abundance were not correlated with lake chemistry or number of fish species.