ABSTRACT: The copepod Acartia tonsa exhibits 2 different feeding modes: when feeding on small phytoplankton cells it sets up a feeding current and acts as a suspension feeder; when feeding on motile prey it acts as an ambush feeder. We examined experimentally the effects of small-scale turbulence on feeding rates in these 2 modes. The different feeding behaviours were triggered by offering the copepods diatoms Thalassiosira weissflogii and ciliates Strombidium sulcatum, respectively. Turbulence at 5 different intensities (energy dissipation rate, epslion, between 4 x 10^-3 and 3.7 x 10¹ cm² s^-3) was generated by an oscillating grid. In ambush feeding mode, low (realistic) intensities of turbulence (epslion = 10^-3 to 10^-2 cm² s^-3) enhanced clearance rates by up to a factor of 4 above those observed in calm water. Higher intensities of turbulence (epslion = 10^-1 to 10¹ cm² s^-3) resulted in a depression of clearance rates, although the rates were still significantly higher than those observed in calm water. The depression of clearance rates at high turbulence intensities was due partly to a decline in capture success, but mainly to a decrease in reactive distance, because turbulence interferes with prey perception by disturbing the hydrodynamical signal generated by motile prey. The negative effects were evident only at turbulence intensities exceeding those normally encountered by A. tonsa in its natural habitat. In suspension feeding mode, low intensities of ambient turbulence (epslion = 10^-3 to 10^-2 cm² s^-3) had negligible effects on clearance rates, while at higher turbulence intensities (epslion = 10^-1 to 10¹ cm² s^-3) we observed a negative effect (depression of clearance rate). The negative effects become evidentwhen ambient turbulent fluid shear approaches the maximum shear rate of the copepod's feeding current, and we hypothesize that at these intensities the feeding current is eroded. Again the negative effects were observed only at turbulence intensities higher than those typically experienced by A. tonsa in the sea. The differential response to turbulence of the 2 feeding behaviours, including the negative effects, were accurately predicted by encounter rate and feeding behaviour models proposed by Kiørboe &AMP Saiz (1995; Mar Ecol Prog Ser 122:135-145). Because feeding behaviour is specific to the prey (phytoplankton vs motile prey), and because ambush-mode feeding is much more dependent on turbulence than suspension-mode feeding, our findingssuggest that prey selection in A. tonsa may be partly governed by turbulence in the ocean. This may explain why microzooplankton at times dominates the diet of A. tonsa and other copepods, even though it is numerically scarce relative to phytoplankton in the environment.