Long-term stability of cladoceran assemblages in small, shallow, South-central Ontario lakes subjected to multiple stressors
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Shallow lakes in Muskoka-Haliburton have been largely ignored in previous limnological and paleolimnological studies, as many are considered to be less desirable for cottage development and other cultural activities. Nonetheless, shallow lakes offer important habitat for many animals. 30 oligo- and mesotrophic, shallow lakes were chosen for a paleolimnological study to assess the impacts of multiple stressors on cladoceran invertebrates. Fossil cladoceran remains preserved in the sediment samples of the study lakes were used to evaluate the nature and magnitude of any changes in assemblages from both modern and pre-industrial times. Relationships between present-day assemblages and key environmental variables were investigated using redundancy analysis, which identified that lake area (p<0.01) and Secchi depth (p<0.05) were significant predictors of assemblage composition in the shallow lakes. Secchi depth was not correlated to water clarity measures, as it usually is, but rather to macrophyte abundance. The modern-day assemblages were compared to the pre-industrial assemblages using the snapshot “top-bottom” paleolimnological approach. The top and bottom assemblages were compared using an ANOSIM which was not significant (p=0.2), confirming that modern assemblages were similar to pre-industrial assemblages in shallow lakes. Full core analysis from three shallow lakes also showed only subtle changes in littoral assemblage composition through time, further supporting results from the top/bottom study. The changes recorded were mainly in the pelagic taxa, with the littoral taxa relatively very stable through time. These small, shallow lakes were then compared and combined with a deep lake set from the same region. The results show some striking differences between shallow and deep lakes. For example, pelagic taxa appear to be driving changes within assemblages across a depth gradient, with most of the changes in the deeper lakes. Water chemistry has an increasingly important role in structuring cladoceran assemblage as lakes become deeper, although lake morphometry does play an important role in defining cladoceran assemblages in all the study lakes. Cladoceran assemblages in shallow lakes appear to be more stable than deeper lakes, despite being exposed to the same regional stressors, such as acidification, calcium decline, and climate change.