Properties of Water - Freshwater Ecology: Concepts and Environmental Applications - page 24

FIGURE 2.9 Patterns of flow behind two differently shaped solid objects at three different
ranges of Reynolds numbers. When the Reynolds number is low, turbulence is minimal. Vor-
tices start to form with increased Reynolds number; vortices and turbulence are more preva-
lent with the cubic object (B and C) than with the streamlined object (E and F). Compare to
Fig. 2.1.
between a microscope slide and a cover slip, they are moving more slowly
than if they were swimming freely. Also, an organism swimming in the in-
terstices of a fine sediment will have a lower velocity than it will in open
water. When viewing rapidly moving protozoa, we can artificially increase
this effect by adding a cellulose solution that further increases the viscos-
ity, slows the organisms, and makes them easier to observe.
Reception of hydromechanical signals by small swimming crustaceans
(copepods) is also related to friction transmitted through water at small
scales. Copepods can sense small moving prey particles and move toward
them, and they can sense predators (larval fish) and move away from them
(Kiørboe and Visser, 1999). Copepods can react to moving predators
slightly less than 1 cm away by making several long “jumps” away from
the source of the hydrodynamic disturbance (Kiørboe et al., 1999). The ac-
tual response distances of the copepods to predators and prey are a func-
tion of velocity and relative size; complex models have been proposed to
describe the effect (Kiørboe and Visser, 1999).