Crustacean zooplankton sedimentary assemblages and the calcium concentration of softwater Ontario lakes
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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.