Environmental Engineering Reference
In-Depth Information
in the red and blue wavelengths used by most photosynthetic organisms.
Cyanobacteria use their specialized pigments to harvest green light and
transfer it to chlorophyll a for photosynthesis. Dense populations of
cyanobacteria are sometimes found well below the surface of lakes (Fig.
3.10). The algae that contain chlorophyll re-
move blue and red light in the water column
above, but enough green light is transmitted
to depth to allow photosynthesis for organ-
isms adapted to use this wavelength range.
Sidebar 3.2.
Why Are Lakes the Color They Are, and
How Are Colored Fish Lures and Other
Objects Perceived Under Water?
Several factors are involved in determining
how an observer perceives lake color. The
color of a lake can tell us much about the bio-
logical status. Very unproductive (oligotrophic)
lakes are a deep blue color because pure wa-
ter transmits blue and absorbs green and red
light. The blue light goes more deeply into the
lake, but some of it is scattered by suspended
particle material and returns to the surface.
Scattered light that is blue is more likely to
leave the lake than scattered green or red light.
In productive (eutrophic) lakes, blue and red
wavelengths are absorbed by algal pigments,
and green wavelengths travel relatively fur-
ther. The probability is greater that green light
will be scattered back out of the lake rather
than red or blue light, giving highly productive
lakes their green color.
Dissolved organic materials such as tannin
and lignin can impart a brown color to a lake,
pond, or stream. Observation of such lakes may
reveal little suspended material, but the dis-
solved material absorbs all wavelengths of
light, yielding a brownish color. Sediments can
also color lakes. A lake that is very turbid with
reddish clay will look red. High-altitude lakes
near glaciers often contain very fine sediment
particles (glacial flour) created by glacial ac-
tion. These particles can lend a milky appear-
ance to otherwise unproductive blue lakes.
SUMMARY
1. Diffusion of materials and heat in water
can be described by Fick's law. In this
relationship, diffusion flux is related
positively to a diffusion constant, the
concentration gradient, and the
reciprocal of distance between the
diffusion source and location to which
it is diffusing.
2. Diffusion constants can be affected by
many factors. The most important is
that moving masses of water result in
diffusion constants many orders of
magnitude higher than those for
molecular diffusion in still water.
Factors that increase diffusion constants
also include increased temperature and
bioturbation. Sediment properties, such
as affinity of the diffusing molecules for
the sediments and low permeability, can
be associated with low diffusion
constants.
3. Diffusion properties can constrain
natural selection with respect to
relationships among chemicals and
organisms and body shape of organisms.
This includes uptake of nutrients and
interactions between organisms that are
mediated by chemicals.
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