Properties of Water - Freshwater Ecology: Concepts and Environmental Applications - page 26

EXAMPLE 2.2.
Calculating the Sinking Rates of Two Spheres through 10 m of Water

There are two spherical objects, one with a radius of 1 m (about the size

of a bacterium) and the other with a radius of 100 m (the radius of a

large spherical alga). Both have a density of 1100 kg m 3 . Assume the den-
sity of water is 998 kg m 3 , gravitational acceleration is 9.8 m s 2 , and
viscosity is 0.001 kg m 1 s 1 . Calculate the time it would take for each
object to sink through 10 m of still water.
First, calculate the velocity of each sphere using Stoke's equation. The
small and large spheres have velocities of 2.2 10 3

m s 1 , respectively. At this velocity it would take the bacteria-sized sphere
1.44 years and the larger sphere 1.26 h to sink 10 m.

10 7 and 2.2

energy it gained against gravity when evaporated by the sun. The genera-
tion of hydroelectric energy illustrates the massive amount of power re-
leased as water flows back to the ocean. The erosive power of water is a
direct effect of this release of potential energy. The Grand Canyon of the
Colorado River is an impressive example of how much geological change
the energy of flowing water can accomplish. I discuss the physics of flow-
ing water and its effects (hydrology) on diffusion in Chapter 3 and the
physiography of rivers and streams in Chapter 5. Gravity plays a further
role in water movement by causing density-induced currents that may be
important in lakes (Chapter 6).
Wind causes surface waves and mixing in lakes, reservoirs, and ponds,
and this water movement can have strong influences on organisms. Much
of the water movement of a passing wave is simply a rotation of individ-
ual parcels of water. As depth increases, this rotation decreases (Fig. 2.11).
Consequently, wind-induced mixing also decreases with depth. How wind
partially controls the stratification and currents in lakes will be discussed
in Chapter 6.
The Coriolis effect can influence large lakes (area greater than 100 km 2 )
by causing rotational currents. The Coriolis effect is a force caused by the
rotation of the earth. Objects moving north or south appear to curve as the
earth rotates under them. This effect is most evident in large-scale ocean cur-
rents, but it can also cause counterclockwise currents in lakes in the North-
ern Hemisphere and clockwise rotation in the Southern Hemisphere.
Finally, organisms can move water on smaller spatial scales as a result
of locomotion or attempts to change the amount of water movement. In
some cases the actions of animals can alter the movement of water through
sediments and increase the exchange of materials between sediments and
the water column. For example, clams and worms that live in sediments
effectively mix the mud and pore water, beavers and humans slow water
in streams by building dams, and humans increase water movement by
channelization. The relationship of temporal and spatial scale and forces
on types of water movement are summarized in Fig. 2.12.