Arrive, survive and thrive: Essential stages in the re-colonization and recovery of zooplankton in urban lakes in Sudbury, Canada

The recovery of lakes from severe, historical acid and metal pollution requires that colonists of extirpated species arrive, survive and subsequently thrive. We employed 40 year records from weekly to monthly crustacean zooplankton samples from Middle and Clearwater lakes near Sudbury, Canada, to identify the main mechanistic bottlenecks in this recovery process. While both lakes now have circum-neutral pH, acidity decreased more rapidly in Middle Lake because of past liming interventions, while Clearwater Lake, being larger and supporting more housing, likely receives more zooplankton colonists than Middle Lake. Community richness increased much faster in Middle Lake than in Clearwater Lake, at 1.6 vs 0.9 species decade-1, respectively. Richness has recovered in Middle Lake, when assessed against a target of 9-16 species collection-1 determined from regional reference lakes, but it has not yet recovered in Clearwater Lake. Species accumulation curves and a metric of annual persistence show that this difference is a product not of greater rates of species introduction into Middle Lake, but rather to their greater annual persistence once introduced. Greater annual persistence was associated with better habitat quality (i.e., lower acid and metal toxicity) in Middle Lake, particularly early in the record, and lower planktivore abundance, more recently. These results support a growing consensus that ecological recovery of zooplankton from acidification and metal pollution does not depend strongly on propagule introduction rates which are adequate, but rather on propagule persistence, in lake-rich, suburban landscapes such as those near Sudbury.