The recovery of acid‐damaged zooplankton communities in Canadian Lakes: the relative importance of abiotic, biotic and spatial variables

Summary 1. Acidification has damaged biota in thousands of lakes and streams throughout eastern North America. Fortunately, reduced emissions of sulphur dioxide and nitrogen oxides beginning in the 1960s have allowed pH levels in many affected systems to increase. Determining the extent of biological and pH recovery in these systems is necessary to assess the success of emissions reductions programmes. 2. Although there have been promising signs of biological recovery in many systems, recovery has occurred more slowly than expected for some taxa. Past studies with crustacean zooplankton indicate that a mixture of local abiotic variables, biotic variables and dispersal processes may influence the structure of recovering communities. However, most studies have been unable to determine the relative importance of these three groups of variables. 3. We assessed chemical and biological recovery of acid‐damaged lakes in Killarney Park, Ontario. In addition, we assessed the relative importance of local abiotic variables, biotic variables and dispersal processes for structuring recovering communities. We collected zooplankton community data, abiotic and biotic data from 45 Killarney Park lakes. To assess the recovery of zooplankton communities, we compared zooplankton data collected in 2005 to a survey conducted for the same lakes in 1972–73 using several univariate measures of community structure, as well as multivariate methods based on relative species abundances. To determine the factors influencing the structure of recovering zooplankton communities, we used hierarchical partitioning for univariate measures and spatial modelling and variation partitioning techniques for multivariate analyses. 4. Our survey revealed significant pH increases for the majority of sampled lakes but univariate measures of community structure, such as species richness and diversity, indicated that only minor changes have occurred in many acid‐damaged lakes. Hierarchical partitioning identified several variables that may influence our univariate measures of recovery, including pH, dissolved organic carbon (DOC) levels, fish presence/absence, lake surface area and lake elevation. 5. Multivariate methods revealed a shift in communities through time towards a structure more typical of neutral lakes, providing some evidence for recovery. Variation partitioning suggested that the structure of recovering copepod communities was influenced most by dispersal processes and abiotic variables, while biotic (Chaoborus densities, fish presence/absence) and abiotic variables were more important for cladoceran zooplankton. 6. Our results indicate that the recovery of zooplankton communities in Killarney Park is not yet complete, despite decades of emission reductions. The importance of variables related to acidification, such as pH and DOC, indicates that further chemical recovery may be necessary. The differing importance of abiotic, biotic and dispersal processes for structuring copepod versus cladoceran zooplankton might indicate that different management approaches and expectations for recovery are needed for these groups.