To partition the effects of temperature and viscosity, we used a simple technique for artificially altering seawater viscosity. Environmental variation in viscosity due to temperature fluctuations could lead to temperature responses or adaptations that are nonphysiological. These changes in viscosity may cause substantial reductions or increases in swimming and feeding rates that are biologically relevant. If uncorrected for effects of viscosity, temperature coefficients such as Q 10 values can overestimate the influence of temperature on the physiological processes that underlie the generation of motion at small spatial scales. The physical effects of viscosity can therefore make up a large component of the effect of temperature on activity of microscopic organisms. 40% of the decrease in swimming speed and 55% of the decrease in water movement were accounted for by increases in viscosity alone. Over an environmentally relevant, 10-degree drop in water temperature (22 to 12☌), swimming speed was reduced by approximately 40% and water movement was reduced by 35%. To partition these effects, we artificially altered seawater viscosity and, at two temperatures, we measured swimming speed and water movement by larvae of the sand dollar Dendraster excentricus. Because water viscosity is physically coupled to temperature, changes in temperature can influence the activity of microscopic organisms through both physiological and physical means. Temperature, through its effects on physiological processes, also influences motion. The small size and slow movement of aquatic, microscopic organisms means that the viscosity of water has a predominant influence on their motion.
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