A Paleolimnological Examination of Cladoceran (Branchiopoda, Crustacea) Assemblages in Softwater Nova Scotia (Canada) Lakes and Their Responses to Multiple Stressors
Korosi, Jennifer Barbara
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The Canadian province of Nova Scotia has an abundance of lakes impacted by multiple stressors, including acidification, lakewater calcium (Ca) decline, climate change, and fish introductions. In spite of this, little is known about long-term trends in aquatic biota, especially microfaunal communities that are useful indicators of lake trophy. This thesis helps address this knowledge gap by providing information on the ecological distribution and long-term dynamics of Cladocera in softwater lakes in Nova Scotia using their subfossil remains preserved in lake sediments. Based on a surface sediment survey of 49 lakes, Cladocera in Nova Scotia are mainly structured along gradients of maximum lake depth and dissolved organic carbon (DOC). Daphnia (generally considered sensitive to low pH and [Ca]) were common in the Cape Breton Highlands National Park, but were rare in southwestern Nova Scotia in both modern and pre-industrial sediments, although some lakes exhibited declines in Daphnia since pre-industrial times. The most notable example of this is Lake George (Kings County), where a striking decline in Daphnia, likely a result of declining [Ca], resulted in a marked increase in estimates of past algal production. In Kejimkujik National Park, which has experienced declines in pH and [Ca] since pre-industrial times, significant reductions in chydorid relative abundances were observed, as well as significant increases in the jelly-clad planktonic cladoceran species Holopedium. Cladoceran assemblages in lakes near Yarmouth have also changed significantly since pre-industrial times, but showed no consistent regional trends in the direction of cladoceran species shifts, reflecting the influence of local anthropogenic stressors. An extreme example of this is Trefry Lake, which was treated with copper sulphate in 1938 to remove the native fish community and provide better stocking conditions for brook trout. Copper sulphate treatment resulted in significant and permanent changes to the aquatic food web inferred from a strong decline in the body size of Bosmina. Collectively, these results suggest that many aquatic ecosystems in Nova Scotia are in a state of flux as a result of multiple anthropogenic stressors, and Cladocera subfossils can be effective for inferring the food web implications of these stressors.