Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 36

is known to alter diffusion rates in small organisms suspended in water
(Sidebar 3.1). Surface area to volume relationships also explain why large
organisms (more than a few cells wide) have vascular systems. The systems
move liquids through the organisms and thus promote transport diffusion;
otherwise, the relatively low rates of molecular diffusion would lower the
maximum possible metabolic rate. Specialized organs associated with vas-
cular systems such as the gills of fish and aquatic invertebrates have evolved
to increase effective surface area and promote inward diffusion of O 2 and
outward diffusion of CO 2.
Movement of organisms can also increase movement of chemicals, al-
though this movement is often not treated as a diffusion process. Animals
in the benthos can disturb the sediments in which they live (bioturbation)
and increase diffusion of materials by increasing turbulent mixing. Motile
bacteria can absorb and then move organic contaminants through ground-
water sediments more rapidly than expected, given permeability and water
flow. Motile ciliates can increase O 2 transport through sediments up to 10
times above molecular diffusion (Glud and Fenchel, 1999). Migration of
organisms from the bottom to the top of lakes (Horne and Goldman,
1994), migration of fish upstream to spawn (Kline et al., 1990), and emer-
gence of insects from streams (Gray, 1989) can all cause significant move-
ment of nutrients associated with aquatic systems.
The movement of heat can also be viewed as diffusion. An equation
similar to that presented for chemical diffusion (Fick's law) can be used to
describe diffusion of heat. In this case, J is heat flux per unit area, and
( C 1

C 2 ) is replaced by the temperature difference. Transport of hot or
cold water by flow can redistribute heat rapidly. In contrast, heat is trans-
ferred more slowly by molecular collisions in the absence of transport.
Thus, molecular diffusion of heat is considerably slower than transport dif-
fusion. Physical limnologists study distribution and diffusion of heat in
lakes because they are related to stratification and mixing (Chapter 6).
The ideas concerning movement of materials in water as a function of
scale (distance and time) are similar to the concepts for water movement
discussed previously (see Fig. 2.11). On the smallest scales, low Re num-
bers occur, turbulent mixing is not possible, and molecular diffusion dom-
inates. Diffusion rates increase significantly at greater spatial scales because
transport diffusion is likely to override molecular diffusion. Turbulent flow
can occur at scales ranging from rivulets to large rivers and lakes. Finally,
large currents in very large lakes and the hydrological cycle move materi-
als on continental and global scales.

LIGHT AND HEATING OF WATER

The interaction between light and water is important for at least three
reasons: (i) Light is needed for photosynthesis, (ii) organisms with eyes or
light sensors use light as a sensory cue, and (iii) light heats water and ulti-
mately leads to stratification in lakes (see Chapter 6). Feinberg (1969) pro-
vides a basic account of the properties of light and most readers should be
familiar with the common names associated with wavelength ranges of the
electromagnetic spectrum (Table 3.1).