Increased grazing rates of microplankton in response to small-scale turbulence

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Abstract
Grazing rates of free-living microplanktonic organisms on bacteria-sized particles, as measured by the decrease of tracer particles over time, increased in response to small-scale turbulence. This was observed in the field, with the natural community, and in the laboratory, with a model protozoan. When ingestion rates were measured at the beginning of laboratory incubations, no statistical difference between treatments was found. Protozoan concentrations increased faster in the turbulence treatment but their cell sizes decreased compared to the non-turbulence treatment. Since direct ingestion rates were not affected by turbulence, the mechanism to explain the difference in tracer decrease remains unresolved. A complex picture of physiological and possibly behavioral changes in the protozoa owing to small-scale turbulence emerged from this study. Some of these changes have been observed before with copepods and might represent a common trend in free-living planktonic organisms. Turbulence could be an important factor in the control of bacterial populations in aquatic systems by means of increasing protozoan grazing rates.

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