In recent decades, many softwater lakes on the boreal shield have experienced significant reductions in aqueous calcium (Ca) concentrations. These declines are a long-term consequence of acid deposition due to the depletion of base cations from watershed soils. There is concern that in some lakes [Ca] may be falling to levels detrimental to the competitiveness of Ca-rich organisms. By examining the crustacean zooplankton remains preserved in lake sediments, this thesis provides field evidence of reduced [Ca] impacting a Ca-sensitive crustacean zooplankton species (Daphnia pulex). Additionally, a 770 lake data set compiled from several Ontario monitoring programs revealed that 62 % (an increase from 35% in the early 1980s) of the lakes were near or below the laboratory-determined Ca threshold (1.5 mg•L-1) for the growth and survival of D. pulex. To determine whether the 1.5 mg•L-1 Ca threshold could be observed in a spatial survey of crustacean zooplankton sedimentary remains, surface sediments from 36 softwater (Ca range 1-3 mg•L-1) Ontario lakes were analyzed. Significant differences in daphniid abundances across the Ca threshold were present; however, only for the D. longispina species complex, indicating differences in Ca tolerances within daphniid species complexes. Extending the analysis to a comparison of modern-day vs. preindustrial assemblages revealed that in the same 36 lakes there have been large declines (by up to 30%) in daphniid relative abundances since preindustrial times coincident with increases in Ca-poor species (i.e. Holopedium gibberum) irrespective of modern day pH. These findings demonstrate that in natural settings, the competitive disadvantages of Ca limitation may occur at a higher [Ca] than previously suspected. Finally, zooplankton sedimentary remains were analyzed from several “pristine” lakes in northwestern Ontario that have also experienced Ca declines in recent years. Reduced abundances of Ca-sensitive taxa and increases in Ca-insensitive fauna provided further evidence of the impacts of Ca decline independent of acid deposition. Collectively these analyses demonstrate the potential importance of Ca as an environmental stressor in softwater regions, as well as the need for further research in order to make better use of the available information preserved in the sediment record. / Thesis (Ph.D, Biology) -- Queen's University, 2011-03-31 16:17:57.86

Freshwater ecological issues are increasingly being recognized within the context of multiple stressors, even though relatively little is known about the limnological and biological consequences of the interactions between various environmental impacts. Moreover, long-term monitoring data are often lacking. To help address these issues, the overall goal of this thesis was to use paleolimnological approaches to document how multiple environmental stressors have altered limnological communities in south-central Ontario lakes. During the last two centuries, Ontario lakes have been subjected to varying intensities of different environmental impacts, including increases in shoreline residential development, forest clearance and regrowth, the deposition of strong acids via the atmosphere, invasion by non-indigenous species, and climate change. I used multiple paleolimnological approaches to: 1) demonstrate how multiple natural and anthropogenic stressors have affected biological assemblages across lakes in the Muskoka-Haliburton region of south-central Ontario, and 2) reconstruct the limnological histories of four lakes from Algonquin Park that have recorded the near complete extirpation of native crayfish species. In the Muskoka-Haliburton lakes, I assessed the extent of limnological changes that have occurred during the past ~15 years by resampling lakes from an earlier survey, using identical paleolimnological methods. Limnological monitoring data document that, since 1992, the lakes have experienced declines in lakewater calcium and SO4 concentrations, while pH declined marginally; in contrast dissolved organic carbon, silica and Total Kjeldahl Nitrogen increased. Marked regional increases in planktonic diatom taxa, including Cyclotella stelligera, Asterionella formosa and Fragilaria crotonensis, occurred in many lake systems, while colonial scaled chrysophyte algae have undergone a widespread decline in favour of unicellular forms (i.e., Mallomonas spp.), driven by interactions between resource limitation and climate change. In the Algonquin Park study lakes, crustacean zooplankton remains revealed a marked decline in daphniid species with high Ca requirements, in favour of smaller Bosmina spp., while diatom and chrysophyte analysis suggest varying degrees of industrial acidification in the four study lakes. The paleolimnological data suggest that the crayfish decline may have began prior to the long-term monitoring record, likely as a result of declines in pH and lakewater Ca related to atmospheric acid deposition. / Thesis (Ph.D, Biology) -- Queen's University, 2012-09-27 12:53:20.518