An Empirical Model to Predict in situ Grazing Rates of Diaptomus minutus Lilljeborg on Small Algal Particles

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Abstract
Grazing rates of Diaptomus minutus were measured in situ in six lakes in south-central Ontario [Canada] using the tracer species Chlorella vulgaris and Scenedesmus ovalis. An empirical model was constructed to predict grazing rate as a function of the relative proportion of small (< 10 .mu.m) to large (10-30 .mu.m) algal particles in the nanoplankton. It accurately predicted diaptomid grazing rates for three lakes in an independent lake set; however, it produced overestimates for two other lakes. There was also a significant positive correlation between grazing rate and the biomass concentration of small algae in the lakes. Difference in ambient lake temperature did not contribute significantly towards explained variation in grazing rate; prosome length was also a poor predictor of grazing rate. In parallel experiments, diaptomid grazing rates were higher when Pediastrum was used than when Scenedesmus was used as the tracer; in a similar set of experiments, grazing rates were higher when Chlamydomonas was used than when Scenedesmus was used.