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Daphnia and Bosmina fragments were identified and measured in the surface sediments of 42 lakes in the Muskoka-Haliburton region of Ontario, Canada, in an attempt to identify environmental factors that may influence cladoceran body size. Specifically, pecten length on Daphnia post-abdominal claws, antennule length on Bosmina headshields, and carapace and mucro lengths of Bosmina carapaces were measured. These measurements were then compared to limnological variables previously identified as possibly influencing cladoceran size, including dissolved organic carbon (DOC), total phosphorus (TP), pH, calcium (Ca), Chaoborus density, and fish presence/absence. Cladoceran size displayed a linear relationship to TP, with larger Bosmina and Daphnia present in lakes with lower nutrient levels. We suspect that, as larger individuals are more efficient grazers, they may competitively exclude smaller individuals when nutrients are limiting in these lakes. Bosmina mucro length and cladoceran community size structure displayed a step response to DOC, with mean size significantly smaller when DOC concentrations were higher than 5.89 mg L-1. Daphnia pecten length displayed a negative linear relationship to DOC above a concentration of 4.90 mg l-1. Reduced predation pressure from gape-limited macroinvertebrate predators, such as Chaoborus, may have influenced these relationships. DOC was also highly correlated to TP in these lakes, and size trends might be responding to the TP gradient rather than the DOC gradient. Mean cladoceran body size in acidic lakes (pH < 6.0) was significantly smaller than mean body size in circumneutral lakes (pH > 6.0). There was no relationship between size structure and Ca concentrations, attributed to a narrow Ca gradient in these lakes. Predation effects were examined using limited Chaoborus density and fish presence/absence data. Although there were no significant relationships between cladoceran size and Chaoborus density, some significant relationships between size variables and fish predation were identified. The sensitivity of size variables to certain environmental gradients suggests that size structure has the potential to be an effective paleolimnological tool.
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