Small ciliated larvae of Dendraster provide a useful model organism for studying the interactive effects of temperature and viscosity because: (1) mechanisms of ciliary propulsion involve viscous forces ( Sleigh and Blake, 1977), (2) larvae can be tethered in place for measurements of water movement ( Emlet, 1990), (3) metabolic responses to temperature change have been examined ( McEdward, 1984, 1985), (4) effects of temperature on water movement are likely to have consequences for feeding and life-history characteristics related to feeding development ( Strathmann, 1971, 1985), and (5) these effects can be quantified and easily related to body form ( Hart, 1991). We measured swimming activity and water movement in larvae of the sand dollar Dendraster excentricus (Eschscholtz). 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. For all of these mechanisms, the key underlying control on groundwater movement is the viscous resistance resulting from the interaction of the fluid with solid surfaces in the aquifer (grain edges or fracture walls).The small size and slow movement of aquatic, microscopic organisms means that the viscosity of water has a predominant influence on their motion. Tortuosity is a measure of how far fluid must go to “circumnavigate” its way around particles: higher tortuosity indicates that water must go farther to get to its destination (a more tortuous path). But what controls their magnitude? The main factors are grain size and shape, sorting, porosity (degree of compaction or fracture aperture), particle orientation or alignment that affects the tortuosity of the flow path, and cementation. ![]() So…that’s how we define permeability and hydraulic conductivity. “thinner”), it will flow more easily through the aquifer. So even for the same aquifer, the hydraulic conductivity goes up if it is warmer! This makes some sense – if the water is less viscous (i.e. It is also important in considering the effects of temperature, because water viscosity decreases with increasing temperature: it’s less than half as viscous at 90° than at 32° F. ![]() water – whether you are thinking about an oil reservoir or contamination of groundwater by a gasoline spill). ![]() This is important for comparing different fluids (say, oil vs. More viscous fluids will flow more slowly through the same rock than less viscous ones. More specifically, it is the viscosity and density of the fluid that matter.
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