Recovery in a multiple stressor environment: Using the reference condition approach to examine zooplankton community change along opposing gradients

Abstract

Lakes around the metal smelters of Sudbury, Ontario, Canada offer a unique opportunity to study recovery processes in stressed aquatic ecosystems. Following major reductions in the atmospheric deposition of sulphur and metal particulates, chemical and biological recovery have been observed in many lakes; however, the magnitudes and trajectories of biological recovery are variable. Crustacean zooplankton communities have proven to be particularly valuable in tracking the biological recovery patterns in Sudbury lakes. In a survey of 87 lakes, zooplankton community structure revealed strong gradients related to acidification, metal contamination, trophic status and depth. The study lakes could be divided into four groups related to these gradients: acidified, recovered, urban and reference lakes. Community composition in recovered (to pH >6.0) lakes did not differ from reference lakes for copepods and cladocerans. In contrast, urban lakes subjected to the effects of watershed development as well as atmospheric metal contamination had either copepod or cladoceran communities that differed from reference conditions. Among the acidified lakes, most lakes with pH <5.5 contained species poor copepod communities; however, substantial community recovery was evident in lakes with pH >5.5. Our study illustrates the complexity of biological recovery processes in lakes subjected to multiple environmental stressors. Although substantial zooplankton community recovery clearly occurs as lakes recover from acidification, other stressors including metal contamination and eutrophication in urban environments may greatly affect recovery processes and outcomes. © 2011 The Author. Published by Oxford University Press. All rights reserved.