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film of water that coats cohesive
sediments, through which molecular diffusion is the dominant transport mechanism for
solutes (Boudreau and J
The diffusive boundary layer is a 0.2
1.2 mm thick
-
rgensen 2001). Bottom currents and associated mixing decrease
the thickness of this layer, thereby increasing the
ø
flux of DO from overlying water to the
sediment (Beutel 2001). Biochemical processes are, in turn, strongly dependent on tem-
perature (Boylen and Brock 1973; Zeikus and Winfrey 1976).
Golosov et al. (2006) have shown that at low temperatures the sediment oxygen demand
is especially sensitive to temperature variations: even several tenths of centigrade increase
of the near-bottom temperature can essentially intensify oxygen consumption in the upper
sediment and produce eventually anoxic conditions in the lower layer. Therefore, the
capacity of bottom sediments to store heat and the heat
fl
flux from water to sediments in
summer and autumn are critical factors for the oxygen conditions during the ice season.
This gives one memory time scale of freezing lakes to be of the order of 6 months. Golosov
et al. (2006) give examples of lakes where the temperature of the bottom water has
increased from 0.5
fl
°
°
C at the end.
The oxygen consumption rate (oxygen depletion rate) is often used as a measure of
combined effect of BOD and SOD, which is easily achieved from time records of DO
concentrations as (C
0
−
C)/t, where C
0
= C
0
(z) and C = C(z; t) are the initial value and the
time evolution of the DO concentration. Typical reported values of the oxygen con-
sumption rate in ice-covered lakes range within 10
−
2
-
1
C in the beginning of the ice season to 4
-
5
1mgO
2
L
−
1
day
−
1
(e.g., Puklakov
et al. 2002), increasing towards the lake bottom. In shallow Lake Vendyurskoe, Russian
Karelia the oxygen consumption rates varied in the beginning of the ice season from
0.4 mg O
2
L
−
1
day
−
1
-
in the upper layer to 1.0 mg O
2
L
−
1
day
−
1
in the vicinity of the
water
sediment boundary (Terzhevik et al. 2010). Along with gradual decrease of oxygen
concentration and development of the anoxic zone in the hypolimnion, the oxygen records
demonstrated short-term
-
fl
fluctuations with periods from several minutes to several days
(Fig.
7.19
).
The evolution of the oxygen content under ice can be approached using a one-
dimensional, vertical model (Golosov et al. 2006). It is based on the diffusion equation
with a sink term. The boundary conditions are given by oxygen
fl
fluxes as absorption of
oxygen at the bottom and no
fl
flux at the surface. The equation is written as
cð
T
Þ
C
@
C
@
t
¼
@
K
@
C
@
z
ð
7
:
33a
Þ
@
z
z ¼ 0
:
@
C
z ¼ H
:
K
@
C
@
z
¼ 0
;
@
z
¼ F
b
ð
7
:
33b
Þ
where K is the diffusion coef
is the rate of oxygen consumption, H is depth, and
F
b
is the bottom absorption. The consumption rate varies from 10
−
8
s
−
1
in the upper water
column to 10
−
7
cient,
ʳ
10
−
6
s
−
1
at the water-sediment boundary (Golosov et al. 2006). The upper
layer has shown similar values in many lakes, but at the bottom the oxygen consumption
-
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