Pond ecosysiem
249
III — _ŒË BO‘ITOM INTERFACE
A —Oxygen demand and detrital decay
Reported
oxygen
uptake
by submerged,
undisturbed
sedinents
in
a
variety
of
aquatio
environments all peak at 3 to 4
gm
oxygen/sq m/day
(Hopkinson
et
al
1978;
Schroeder
1975;
Teal
and
Kanwisher
1961).
Interstitial
water at
measure
a
decrease in the redox potential of pond water from +250 mv,
1 cm above the
sediment
surface
to <—150 mv, 1/2 mm into the sedinents. Assume
that the
DO concentration decreases to zero through a 100 micron thick laminar water
layer
(Kanwisher
1963) just above the sedinents plus a 100
micron
depth
into
the sediments (Geesey 1982).
Substituting into Eqn.
2 the
reported
sediment
oxygen
uptake rate as J,
and a diffusion depth of
200
microns
across
which
the
DO
decreases from a pond value of 4 ppm
to
zero
100
microns
into
the
sediment
surface
gives
a
value
for
the
diffusion
coefficient
of
x
10'5 cm sq/sec.
Here again,
the typical
value
for
diffusion
of oxygen in water at temperatures of 20 to 30° C
is
observed.
Varying, within reason, the values assumed for the parameters of Eqn. 2 can
change the value of D that we obtained by a factor of Ï3. However, the fact
that
the
reported values of sediment oxygen uptake all peak at
the
sane
value
implies a limiting physical constraint.
A logical choice
for
this
constraint
is
laminar
diffusion.
The high BDD of
precipitated
organic
matter
(algae,
feed and manure residues) exceeds the rate at which
oxygen
can diffuse into the sediments. Detritus becomes anoxic within
Anoxic
decay implies 2 to 10 tines more carbon being fixed in extra—
cellular
products
than
is fixed within
microbial
cells.
Extracellular
products,
the
familiar microbial sline,
may be key in the food value
of
detritus.
Had
fermentation
been oxic,
most of
the
formed
particulate
organic carbon would be fixed in the growth of microbial cells.
B — Ammonium cycling
Anoxic
decay of precipitated algae and other organic matter
releases
the
N of protein as ammonia.
Counter to the flow of precipitating organic
matter adding N to the sediments is the diffusion of ammonia from
sediment
interstices
to
pond water.
Sediment interstitial water is usually
acid,
often
with a pH 56.5.
This means that essentially all ammonia is
in
the
ionized ammonium form.
Krom and Berner (1980) reported a diffusion coeffi—
cient
for
ammonium ions in marine sedinents as "5 x 10"6 cm
sq/sec
when
accounting
for adsorption of ammonium on sediment particles and twice this
value in open water. Assume a sediment porosity of 30%; diffusion from 1 cm
-
sediment
depth across which the ammonium concentration decreases
from
50
ppm.to 1 ppm; and, a diffusion coefficient of 5 x 10'€ cm sq/sec. Substitu—
ting these values into Eqn.
2, the predicted flux of ammonia from sedinent
into
the overlying pond water is 0.l gm/sq m/day.
Assuming that there are
at
least several lO's of cm of sediments below the pond and that the
pond
bottom.
was
dried prior to filling,
there will also be a similar
rate
of
diffusion of ammonium down into the sediments and away from the pond.
Based
on
the
above
discussions
of
ammonia
transfer
across
the
air/water interface and cycling within the water column, an ammonia balance
can now be made (Figure 1).
249
III — _ŒË BO‘ITOM INTERFACE
A —Oxygen demand and detrital decay
Reported
oxygen
uptake
by submerged,
undisturbed
sedinents
in
a
variety
of
aquatio
environments all peak at 3 to 4
gm
oxygen/sq m/day
(Hopkinson
et
al
1978;
Schroeder
1975;
Teal
and
Kanwisher
1961).
Interstitial
water at
measure
a
decrease in the redox potential of pond water from +250 mv,
1 cm above the
sediment
surface
to <—150 mv, 1/2 mm into the sedinents. Assume
that the
DO concentration decreases to zero through a 100 micron thick laminar water
layer
(Kanwisher
1963) just above the sedinents plus a 100
micron
depth
into
the sediments (Geesey 1982).
Substituting into Eqn.
2 the
reported
sediment
oxygen
uptake rate as J,
and a diffusion depth of
200
microns
across
which
the
DO
decreases from a pond value of 4 ppm
to
zero
100
microns
into
the
sediment
surface
gives
a
value
for
the
diffusion
coefficient
of
x
10'5 cm sq/sec.
Here again,
the typical
value
for
diffusion
of oxygen in water at temperatures of 20 to 30° C
is
observed.
Varying, within reason, the values assumed for the parameters of Eqn. 2 can
change the value of D that we obtained by a factor of Ï3. However, the fact
that
the
reported values of sediment oxygen uptake all peak at
the
sane
value
implies a limiting physical constraint.
A logical choice
for
this
constraint
is
laminar
diffusion.
The high BDD of
precipitated
organic
matter
(algae,
feed and manure residues) exceeds the rate at which
oxygen
can diffuse into the sediments. Detritus becomes anoxic within
Anoxic
decay implies 2 to 10 tines more carbon being fixed in extra—
cellular
products
than
is fixed within
microbial
cells.
Extracellular
products,
the
familiar microbial sline,
may be key in the food value
of
detritus.
Had
fermentation
been oxic,
most of
the
formed
particulate
organic carbon would be fixed in the growth of microbial cells.
B — Ammonium cycling
Anoxic
decay of precipitated algae and other organic matter
releases
the
N of protein as ammonia.
Counter to the flow of precipitating organic
matter adding N to the sediments is the diffusion of ammonia from
sediment
interstices
to
pond water.
Sediment interstitial water is usually
acid,
often
with a pH 56.5.
This means that essentially all ammonia is
in
the
ionized ammonium form.
Krom and Berner (1980) reported a diffusion coeffi—
cient
for
ammonium ions in marine sedinents as "5 x 10"6 cm
sq/sec
when
accounting
for adsorption of ammonium on sediment particles and twice this
value in open water. Assume a sediment porosity of 30%; diffusion from 1 cm
-
sediment
depth across which the ammonium concentration decreases
from
50
ppm.to 1 ppm; and, a diffusion coefficient of 5 x 10'€ cm sq/sec. Substitu—
ting these values into Eqn.
2, the predicted flux of ammonia from sedinent
into
the overlying pond water is 0.l gm/sq m/day.
Assuming that there are
at
least several lO's of cm of sediments below the pond and that the
pond
bottom.
was
dried prior to filling,
there will also be a similar
rate
of
diffusion of ammonium down into the sediments and away from the pond.
Based
on
the
above
discussions
of
ammonia
transfer
across
the
air/water interface and cycling within the water column, an ammonia balance
can now be made (Figure 1).