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Because seawater temperature is correlated with viscosity, temperature changes may impact small zooplankton through a mechanical pathway, separately from any thermally-induced effects on metabolism. We evaluated both viscous and thermal effects on copepod feeding in experiments where viscosity was manipulated separately from temperature using a non-toxic polymer. Two copepod species, Acartia tonsa and Parvocalanus crassirostris, feeding on two monoalgal diets (a diatom and a dinoflagellate) were compared. At constant temperature, increase in viscosity nearly always reduced feeding; at constant viscosity, changes in temperature had no effect on feeding. The effects of viscosity and temperature were more pronounced for the diatom than the flagellate prey. Overall, reductions in zooplankton feeding at cold temperatures can be explained primarily by the mechanical effect of viscosity. Q10 values for copepod feeding (1.0–7.9), calculated from the present data and from the literature, were generally higher and more variable than Q10 values from the literature for copepod respiration (1.5–3.1) indicating that, at cold temperatures, feeding is more dramatically suppressed than metabolism. We conclude that (i) high viscosity may inhibit copepod feeding, and (ii) this viscous effect on feeding (rather than a thermal effect on metabolism) may influence the cold-temperature bounds of zooplankton populations.