Agriculture Reference
In-Depth Information
in anoxic decomposition of organic matter in the sediment at a rate decreasing
exponentially with depth,
R
=
R
0
exp
(
−
αz)
+
R
1
(
2
.
39
)
and it is removed by nitrification in the overlying water and in worm burrows.
The rate of NH
4
+
formation and the density of the worms vary with seasonal
temperature changes. Figure 2.12 shows concentration profiles of NH
4
+
in the
sediments measured over 2 years and the corresponding profiles predicted by the
model using independently measured parameter values. It shows that the main
features of the profiles and their seasonal dynamics are satisfactorily predicted.
By comparison, a model using the same parameter values but only allowing for
diffusion in the vertical direction over-predicted the concentrations several fold.
Aller found similar good agreement between observed and predicted profiles and
fluxes of SO
4
2
−
and Si in the sediments. He concluded that the model accounted
satisfactorily for the important processes operating.
The application of this approach is illustrated in Figures 2.13 and 2.14, which
show the effects of tubificid worms on the movement of P between a submerged
Concentration of NH
4
+
in solution (
µ
M)
0
100
200
300
400
0
100
200
300
400
0
100
200
300
400
0
0
0
5
5
5
10
10
10
15
15
15
July 1974
November 1974
March 1975
0
100
200
300
400
0
100
200
300
400
0
100
200
300
400
0
0
0
5
5
5
10
10
10
15
15
15
July 1975
October 1975
March 1976
Figure 2.12
Concentration profiles of NH
4
+
at different times in sediments in Long
Island Sound. Points are measured data; lines are predicted with Equations (2.37) and
(2.39) using independently measured parameter values (after Aller, 1980a). Reprinted
with permission from Elsevier
