Properties of Water - Freshwater Ecology: Concepts and Environmental Applications - page 21

TABLE 2.3 Contrasting Effects of Scale and Reynolds Number
on Aquatic Organisms

Parameter Small organism ( 100 m) Large organism ( 1 cm)

Re Low High

Viscosity ( F u ) High Low

Inertia ( F i ) Low High

Flow Laminar or none Turbulent

Body shape Variable Streamlined

Diffusion Molecular Transport (eddy)

Particle sinking rates Low High

Relative energy requirement High Low

for motility

On larger spatial scales water flow can be either laminar or turbulent.
Laminar flow is characterized by flow paths in the water that are primar-
ily unidirectional. Turbulent flow is characterized by eddies, where the flow
is not as unidirectional. Turbulent flow (mixing) decreases at small scales
because viscosity dampens out turbulence as the Reynolds number de-
creases (below a value of approximately 1). Many methods have been used
to measure water velocity (Method 2.1).
Surfaces interact with flowing water. Flow slows and becomes laminar
near solid surfaces. The equation for F v indicates that viscous force in-
creases closer to surfaces (as spatial scale decreases). Thus, friction with the
solid surface is transmitted more efficiently through the solution (Fig. 2.7),
water flows more slowly (Fig. 2.7B), and flow becomes more laminar at
the bottom and sides of stream channels and pipes (Fig. 2.7A). The outer
edge of the region where water changes from laminar to turbulent flow is
called the flow boundary layer .
The thickness of the flow boundary layer decreases with increased wa-
ter velocity, decreased roughness of the surface, the decreased distance
from the upstream edge of an object, and decreased size of the object
(Fig. 2.8). The reader should understand that the flow boundary layer is
not a sharp, well-defined line below which no turbulence occurs, even
though it is convenient to conceptualize the layer in such a fashion. Rather,
the outer edge of the boundary layer represents a transitional zone between
fully turbulent flow and laminar flow. These relationships have many prac-
tical aspects. For example, algal growth can have significant influences on
hydrodynamic conditions several centimeters from the bottom and edge of
a stream (Nikora et al., 1997), a region utilized by many aquatic animals.
Organisms can find refuge from high flows in cracks in rocks.
The effects of scale on flow or movement through water have sub-
stantial practical implications. Very small objects experience laminar flow,
and larger ones experience turbulent flow. Large organisms that swim
through the water benefit from being streamlined (shaped to avoid turbu-
lence); small organisms (bacteria sized) do not need to streamline (Fig. 2.9).
The breakpoint at which streamlining becomes useful is approximately
1 mm (depending on velocity). When a large organism moves through wa-
ter, turbulent flow acts opposite to the motion of the organism and tends