Environmental Engineering Reference
In-Depth Information
Organic
matter
High energy
O
2
Production
autotrophs
(plants)
Decomposition
heterotrophs
(bacteria/animals)
Solar
energy
Atmosphere
Chemical
energy
CO
2
Inorganic
nutrients
Low energy
Photosynthesis
6CO
2
+
6H
2
O
C
6
H
12
O
6
+
6O
2
Respiration
Carbon
dioxide
Water
SugarOxygen
FIGURE 5.18
Production/decomposition cycle. (From Chapra, S.C.,
Surface Water-Quality Modeling
,
WCB McGraw-Hill, New York, 1997.)
During biomass production by heterotrophs (secondary production), energy is also required, gen-
erally produced by chemical oxidation. Oxygen is most eficient in terms of energy production, and
organisms that use oxygen as an energy source (such as ourselves) are referred to as aerobic organ-
isms, and an aquatic environment with oxygen is an aerobic environment. During aerobic processes,
oxygen is consumed and carbon dioxide is released.
Anaerobic organisms, on the other hand, obtain chemical energy by reducing oxidized com-
pounds such as NO
3
, MnO
2
, FOOH(s), SO
4
, and CO
2
, releasing soluble compounds such as ammo-
nia (NH
4
), hydrogen sulide (H
2
S), soluble Mn, Fe, and methane (CH
4
).
Organisms have variously adapted to aerobic or anaerobic environments (Figure 5.19). Fish gills
promote the diffusion of DO from water to the blood, similar to the lungs. However, there is con-
siderably less oxygen in water than in air and oxygen diffuses more slowly in water than in air,
making the process more dificult. Fish may increase the low of oxygen by swimming or opening
and closing their gill laps (the
operculum
) to pump water across the gills. Some aquatic insects will
surface and use atmospheric oxygen, using tubes or siphons to obtain atmospheric oxygen, or they
will surface and bring air bubbles with them to the subsurface. Other aquatic insects have gills or
hairs to facilitate oxygen transfer, or diffuse oxygen across the body wall. Chironomid midge larvae
(bloodworms) have hemoglobin (rare in insects), allowing them to live in environments with low
DO concentrations.
FIGURE 5.19
Oxygen-related adaptations. (From EFISH: The Virtual Aquarium. Available at http://cnre.
vt.edu/eish/; and from State Hygienic Laboratory at the University of Iowa, Available at http://www.uhl.
uiowa.edu/services/limnology. Photographs by Bob Jenkins and Noel Burkhead. With permission.)
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