The ecological impacts of lakewater calcium decline on softwater boreal ecosystems

Abstract

In recent decades, marked declines in calcium (Ca) concentrations have been noted in many softwater boreal lakes, and are believed to be a long-term consequence of acid deposition as well as other stressors (such as timber harvesting). Reduced Ca availability may act as a potent environmental stressor. Investigations of the direct ecological impacts of lower Ca concentrations in freshwater systems have largely focused on Ca-rich members of the Cladocera; however, a growing body of work, spanning several scientific fields, suggests Ca decline will have profound direct and indirect consequences for aquatic ecosystems. Here, we synthesize recent laboratory analyses and field surveys to provide an overview of these consequences, while highlighting paleolimnological investigations that provide some long-term perspective on the phenomenon. However, considerable questions remain regarding ‘baseline’ or pre-impact conditions, due to the accelerated leaching of Ca from watershed soils during the period of anthropogenic influence. Furthermore, catchment-specific differences in both leaching rates and the initial size of the Ca pool in watershed soils complicate determination of the eventual endpoints of the declines. Despite these uncertainties, persistent low Ca concentrations are anticipated to impede biological recovery from lake acidification, and that ongoing declines will have cascading effects throughout aquatic ecosystems due to the loss of vulnerable taxa. To better understand how reduced Ca availability will continue to change affected surface waters and how these changes will interact with other environmental stressors will require continued investigation of the declines from multiple scientific perspectives.