Zooplankton Communities and Water Chemistry of Sudbury Area Lakes: Changes Related to pH Recovery

Temporal trends in pH, alkalinity, and crustacean zooplankton species richness and community composition were studied in 80 Sudbury area lakes sampled in 1971–76, 1981, and 1988–90. Alkalinity and pH have increased in recent years, but many of the study lakes remain quite acidic (modal pH 4.5–4.9). Species richness has also increased, on average, by 1.9 species/lake but is still low in many acid lakes relative to that in circumneutral lakes. Community composition differed among lakes of pH <5.0, 5.0–5.9, and ≥6.0, with some overlap between these groups because zooplankton recovery has lagged behind chemical recovery. Trajectories of community change in recovering acid lakes varied substantially in the early stages of recovery from highly acidic (pH <5.0) to moderately acidic pH (5.0–5.9). In contrast, trajectories in lakes recovered to pH ≥6.0 converged toward the community structure more typical of circumneutral lakes in the 1970's. This suggests that given sufficient time, zooplankton community structure of recovering lakes may approach that of circumneutral lakes.