Environmental Engineering Reference
In-Depth Information
potential is an estimate of the relative concentration of the available
electrons in the environment. The O
2
concentration is a primary
determinant of redox potential.
4. Potential energy drives chemical and biochemical processes. It is
present when the redox potential of the reactants is very different
from the redox potential of the surrounding environment. A
chemical reaction will release energy if the products of the reaction
have less potential energy than do the reactants.
5. A chemical reaction will not occur spontaneously if the activation
energy is too large, regardless of the potential energy that will be
released. Organisms can promote chemical reactions by lowering the
activation energy with enzymes. In the case of reactions that go
against potential energy, another source of energy must be present. For
example, organisms can shunt chemical energy into a chemical
reaction. Photosynthetic organisms
convert light into chemical energy and
use this chemical energy to drive
carbon fixation (convert CO
2
to sugar).
6. The presence of O
2
is very important
because of its role in redox and
respiration. The concentration of O
2
varies with space and time in aquatic
environments. Habitats with O
2
are
called oxic or aerobic, and those
without O
2
called anoxic or
anaerobic.
7. Photosynthesis produces O
2
in aquatic
environments, and aerobic respiration
consumes it.
8. Photosynthetic rates are controlled
mainly by the amount of light,
temperature, and nutrient availability.
Organisms are able to acclimate to
low light. High light (especially UV)
can cause photoinhibition, and
adaptations such as special protective
pigments can be used as protection.
9. The balance of photosynthesis and
respiration, the degree of contact with
the atmosphere, and transport
processes determine the actual O
2
concentration.
10. Some common anoxic habitats include
the hypolimnia of eutrophic lakes;
organic-rich sediments in lakes,
streams, and wetlands; organic-rich
groundwaters; digestive tracts of
animals; decaying vegetation in water;
and particles with high rates of
associated microbial activity.
Fish kills from anoxia are relatively common
in many areas. For example, in the state of Mis-
souri, from 1970 to 1979 there were more than
40 known winter kills and at least 100,000 fish
deaths. During the same period, there were
about 20 summer kills resulting in the death of
more than 200,000 fish (Meyer, 1990). Very large-
scale fish kills have been attributed to anoxia in
Lake Victoria, Africa. In this case, storms sus-
pend sediments and wash organic material from
surrounding wetlands into the lakes. The re-
sulting anoxia kills large numbers of fish
(Ochumba, 1990). These kills are problematic
because local people rely on the fish for food.
Sewage is released untreated into rivers
and streams throughout the world. Such re-
leases occurred in the United States and west-
ern Europe until the 1970s, when environmental
laws were enacted requiring reductions in the
amount of organic carbon (biochemical oxy-
gen demand) in sewage discharges. When
the load of organic carbon in untreated sewage
stimulates respiration and consumes O
2
at a
rate in excess of that which can be replen-
ished by exchange with the atmosphere, a river
can become anoxic and the fish die. Such
problems have become rare in developed
countries since municipalities have been re-
quired to lower the organic carbon in the
sewage that they release. Consequently, fish
species that are less tolerant of low O
2
are be-
coming reestablished in areas where they have
been absent for many years.
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