Geoscience Reference
In-Depth Information
As phytoplankton grows, the dissolved inorganic nitrogen is taken up and incorporated
into biomass. Both ammonia and nitrate are available for uptake but, for physiological
reasons, the preferred form is ammonia nitrogen. The ammonia preference factor
p
NH
3
is given as
C
NH
3
C
NO
3
C
NH
3
k
mN
p
NH
3
=
)
+
(
k
mN
+
C
NH
3
)(
k
mN
+
C
NO
3
(
C
NH
3
+
C
NO
3
)(
k
mN
+
C
NO
3
)
(12.81)
where
k
mN
is the Michaelis-Menten limitation.
Eq. (12.81) shows that when the nitrate concentration is zero, the preference for
ammonia is 1.0; when the ammonia concentration is zero, the preference for ammonia
is zero. When both ammonia and nitrate are abundant, preference is given to ammonia
and the factor approaches 1.0.
The fourth term on the right-hand side of Eq. (12.78) and the first term on the
right-hand side of Eq. (12.79) represent the change of ammonia nitrogen to nitrate
nitrogen due to nitrification. Nitrification is a two-step reaction carried out by aerobic
autotrophs. Nitrosomonas bacteria catalyze the first reaction converting ammonia to
nitrite, and in the second reaction Nitrobactor bacteria convert nitrite to nitrate. The
process of nitrification in natural waters is complex, and depends on temperature,
DO, pH, and flow conditions. The following relation for
K
NH
3
is often used:
C
DO
k
NIT
20
NH
3
T
−
K
NH
3
=
K
NH
3
(
20
)θ
(12.82)
+
C
DO
where
NH
3
is the temperature coefficient for nitrification (about 1.08), and
k
NIT
is
the half-saturation DO concentration for nitrification.
In the nitrification process, the dissolved oxygen is required. The NBOD concen-
tration is related to the ammonia nitrogen concentration by
θ
64
14
C
NH
3
C
NBOD
=
(12.83)
where the stoichiometric constant 64/14 arises because 64/14 g of oxygen is required
to convert1gofammonia to nitrate nitrogen. Eq. (12.83) is inserted into Eq. (12.53),
and
K
N
is set as
K
NO
3
.
The third term on the right-hand side of Eq. (12.79) represents the loss of nitrate due
to denitrification. Denitrification refers to the reduction of nitrate (or nitrite) to N
2
and
other gaseous products. This process is carried out by a large number of heterotrophic,
facultative anaerobes. Denitrification is not a significant loss in the water column,
but can be important in anaerobic benthic conditions. The denitrification process
depends on temperature, DO, etc.; thus, the denitrification rate coefficient
K
NO
3
is
