Geoscience Reference
In-Depth Information
12.2.2 Constituent reactions and interrelationships
Maintaining an adequate dissolved oxygen (DO) concentration is essential to aquatic
ecosystems. As shown in Fig. 12.4, the DO concentration is affected by many fac-
tors, such as atmospheric reaeration, photosynthesis, plant and animal respiration,
biochemical oxygen demand, nitrification, and benthal demand. These factors also
interact with one another. According to their interrelationships, the kinetic processes
in the water column are usually divided into DO cycle, carbon cycle, nitrogen cycle,
and phosphorus cycle. All cycles are more or less related to phytoplankton growth and
respiration in an eutrophication system. In addition, similar cycles exist in the ben-
thic sediment, and flux exchanges occur between the water column and the benthic
sediment.
Figure 12.4
Major kinetic processes in water column.
Dissolved oxygen balance
The dissolved oxygen in aquatic systems is reaerated through exchange with the atmo-
sphere, produced by photosynthesis of aquatic plants, and consumed by respiration
of plants, animals and bacteria, oxidation of carbonaceous matters, oxidation of
nitrogen compounds, and sediment oxygen demand. In the 1-D and depth-averaged
2-D models, the differential equation describing the effects of these processes on DO
concentration is
DC
DO
Dt
K
L
h
(
S
SOD
h
=
C
sDO
−
C
DO
)
+
P
−
R
−
K
d
C
CBOD
−
K
N
C
NBOD
−
(12.53)
m
−
3
, i.e., gram of oxygen per cubic meter);
C
sDO
is the saturation DO concentration (gO
2
where
C
DO
is the DO concentration (gO
2
·
m
−
3
);
K
L
is the liquid-film coefficient
·
day
−
1
);
P
is the rate of DO production due to photosynthesis (gO
2
m
−
3
day
−
1
);
(m
·
·
