ABSTRACT: Sedimentary cladoceran remains as indicators of long-term environmental change in south-central Ontario (Canada) lakes.

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Abstract

Lakes in south-central Ontario, Canada, are being impacted by multiple enironmental stressors, including acidification, eutrophication, climate change, and the introduction of invasive species. The recovery of aquatic biota following restoration efforts is difficult to measure in part due to the absence of long-term datasets. The zooplankton-monitoring program at the Dorset Environmental Science Centre (Ontario Ministry of the Environment) in Ontario, Canada, has collected long-term data on crustacean zooplankton size and taxonomic composition in south-central Ontario lakes at regular time intervals for approximately 30 years. However, there are no data regarding cladoceran assemblage structure prior to the period of anthropogenic disturbance in the area. The chitinized remains of cladocerans have been used in some paleolimnological studies to track long-term environmental changes; however few studies have been done on lakes in south-central Ontario. We present data on fossil cladoceran assemblages in present-day (~2004-05) and pre-industrial (pre-~1850s) sediment samples from 44 lakes in south-central Ontario. Cladoceran remains from both the present-day and pre-industrial sediment samples were well preserved, and were dominated by taxa of the families Bosminidae and Daphniidae. Differences in surface-sediment (~2004-05) cladoceran assemblages between lakes were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified three measured environmental variables as significantly influencing assemblage composition: calcium (Ca), dissolved organic carbon (DOC), and maximum lake depth. There have been two ecologically important changes in the cladoceran assemblages of lakes in this region since pre-industrial times. First, there has been a significant shift from littoral-dominated assemblages in pre-industrial times to pelagic-based assemblages in the present-day sediments of most lakes. Second, there has been a notable increase in the relative abundances of daphniids and a decline in the relative abundances of bosminids in several lakes in this region. Daphniids occurred in low abundances in the surface sediments of five lakes with Ca concentrations below 1.5 mg/L. The distribution of cladocerans in the sediment and their relation to these important environmental variables (Ca, depth, DOC) suggests that there is considerable potential for the use of cladoceran indicators in paleoenvironmental reconstructions, especially in lakes where the Ca and pH signals have become decoupled.

Bibliography of Canadian Geomorphology