Container volume may affect growth rates of ciliates and clearance rates of their microcrustacean predators in microcosm experiments.

We studied the effect of volume in small containers (microcosms) on five common planktonic freshwater ciliates and three zooplankton species, namely Daphnia sp., the calanoid copepod Eudiaptomus sp., and the cyclopoid copepod Cyclops sp. We measured ciliate specific growth rates and their loss rates due to microcrustacean predation in short-term experiments. We hypothesized that container volume ranging from 10 to 200 mL would not affect the activity of our prey and predator species. We found that the response to volume was species-specific; growth rates of three ciliate species were sensitive to volume. However, the volume effect was not unequivocal because different timing of the microcosm experiments (block effects) may have caused random bias due to varying morphological and/or physiological conditions of the ciliates and their predators. For predator clearance rate, the volume effect was insignificant in the filter-feeding Daphnia and Eudiaptomus but was significant for the predatory copepod Cyclops, which was hampered in the smallest experimental containers. Total crustacean clearance rates averaged over all treatments appeared unaffected by predator species, while ciliate species significantly affected the results. Our growth and clearance rates are close to previous findings with the same or similar planktonic prey and predator species.