Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 45

4. Quantity and quality of light entering water can be altered by the
atmosphere, the surface of the water, and any ice or snow cover over
water.
5. Light is attenuated logarithmically when it enters water. Red light is
absorbed directly by water, and blue light
is transmitted deepest in oligotrophic
lakes. Phytoplankton pigments absorb
red and blue light. Cyanobacteria also
can absorb green light.

Organisms can impart additional colors to
lakes and ponds. Species of water ferns can
be bright purple and reach very high densities
on the surface of certain ponds. Photosynthetic
bacteria can reach high densities, particularly
in saline ponds, and yield purple, brown, yel-
low, or blue appearances. Dissolved metals
can also color ponds. For example, high con-
centrations of copper can lead to metallic blue
ponds or lakes.
The alteration of spectral quality with depth
means that some wavelengths are never pres-
ent in deeper waters, and colors are perceived
differently than under full sunlight. This may be
important in the way that fish are able to per-
ceive color. For example, under full sun, red
fish lures appear red because they absorb
green and blue light and reflect red. However,
red light is not available deep in a lake, so to a
diver or fish a red lure would appear black.
Fish lures appear to be different colors deeper
in oligotrophic lakes than in eutrophic lakes. A
white lure would appear blue under the sur-
face of an oligotrophic lake but green in a eu-
trophic lake because of the predominant color
of light found with depth in each lake. Cer-
tainly, the color patterns of many fish lures are
designed to appeal to anglers rather than fish.
Aquatic ecologists should also remember
these spectral properties in other contexts. For
example, a deep-water fish may be colored
bright red, but this actually may represent cryp-
tic coloration when the fish is deep down be-
cause the red appears black.

QUESTIONS FOR THOUGHT

1. Why would you expect transport
diffusion to occur in a still glass of
water, if evaporation was occurring at
the surface, and how would you keep
such transport diffusion from occurring?
2. Why do aquatic insects in torrential
streams have reduced gills?
3. Calculate the surface area to volume
ratios for a sphere, a cube, and a right
circular cylinder (1 cm high), each with
a volume of 1 cm 3 . Relate these to
diffusion of materials to cells of
different shapes ( r is the radius, h is the
height, S is the surface area, and V is
the volume) for a sphere S r 2 ,

4

r 3 V 4/3 and for a right circular

r 2 h ).
4. Why might large rivers generally have
greater attenuation coefficients than
large lakes?
5. If objects at the surface of a lake are
blue, black, red, or white, what color
would they appear if they were viewed
at 20-m depth in an oligotrophic lake?
6. Use the data plotted in Fig. 3.6 to
demonstrate that you obtain a straight
line if the natural log (ln) of light is
plotted against depth.

cylinder S 2 r h and V