Geoscience Reference
In-Depth Information
model. Because the nitrite nitrogen usually changes to nitrate very quickly, a direct
process from ammonia nitrogen to nitrate nitrogen is assumed and only three states
(organic nitrogen, ammonia nitrogen, and nitrate nitrogen) are modeled. Their kinetic
processes are described by
−
ω
ON
DC
ON
Dt
S
ON
h
(12.77)
=
K
M
a
NC
f
Phy
,
ON
C
Phy
−
K
ON
C
ON
h
(
1
−
f
ON
,
d
)
C
ON
−
DC
NH
3
Dt
=
K
M
a
NC
(
1
−
f
Phy
,
ON
)
C
Phy
+
K
ON
C
ON
−
K
Phy
a
NC
p
NH
3
C
Phy
S
NH
3
h
−
K
NH
3
C
NH
3
−
(12.78)
DC
NO
3
Dt
S
NO
3
h
=
K
NH
3
C
NH
3
−
K
Phy
a
NC
(
1
−
p
NH
3
)
C
Phy
−
K
NO
3
C
NO
3
−
(12.79)
m
−
3
);
K
ON
is the min-
eralization rate coefficient of organic nitrogen (day
−
1
);
K
NH
3
is the nitrification
rate coefficient of ammonia (day
−
1
);
f
Phy
,
ON
and 1
where
C
ON
is the concentration of organic nitrogen (gN
·
f
Phy
,
ON
are the fractions of
respired phytoplankton recycled to the organic and ammonia nitrogen pools, respec-
tively;
f
ON
,
d
is the fraction of the dissolved organic nitrogen; and
S
ON
,
S
NH
3
, and
S
NO
3
are the organic nitrogen, ammonia, and nitrate fluxes from the sediment bed,
respectively.
The first terms on the right-hand sides of Eqs. (12.77) and (12.78) represent the
production of nitrogen due to phytoplankton death and respiration.
f
Phy
,
ON
of the
produced nitrogen is organic, while 1
−
f
Phy
,
ON
is in the inorganic form of ammonia.
DiToro and Matystik (1980) assigned 0.5 for
f
Phy
,
ON
in the Great Lakes model.
The second terms on the right-hand sides of Eqs. (12.77) and (12.78) represent
the change of organic nitrogen to ammonia nitrogen due to mineralization. Nonliv-
ing organic nitrogen must undergo mineralization or bacterial decomposition into
ammonia nitrogen before utilization by phytoplankton. This process is affected by
temperature and phytoplankton population, and thus the following relation for
K
ON
is often used
−
C
Phy
k
mPc
T
−
20
K
ON
=
K
ON
(
20
)θ
(12.80)
ON
+
C
Phy
where
ON
is the temperature coefficient for mineralization (about 1.08), and
k
mPc
is
the half-saturation phytoplankton concentration for mineralization.
The third term on the right-hand side of Eq. (12.77) represents the settling of the
particulate organic nitrogen in the 1-D and depth-averaged 2-D models. Note that it
has a different formulation in the 3-D and width-averaged 2-D models, as shown in
Eq. (12.63).
The third term on the right-hand side of Eq. (12.78) and the second term on the
right-hand side of Eq. (12.79) represent the nitrogen uptake for phytoplankton growth.
θ
