Geoscience Reference
In-Depth Information
—
the lakes are then cold monomictic. Lake with perennial ice can be polymictic or
amictic, but if they are completely ice-covered the oxygen storage is not renewed.
In shallow brackish, saline or hypersaline lakes, wind-driven or wave-driven mixing
can drive the turnover. But in deep lakes, vertical mechanical mixing may be limited, and
permanent salinity strati
cation develops. The layer with a sharp change in salinity is
called a halocline, analogous to the thermocline. If the halocline is very weak, the bottom
water may be renewed locally by increase of salinity due to evaporation or salt rejection in
freezing. If this is not the case, the lake is classi
ed as meromictic, referring to the
existence of a lower layer of water, which does not intermix and hence is anoxic. In
principle, ventilation of such layer could take place by in
ow of more saline water, but
that is possible only in particular environmental conditions. Example in Finland there are
small meromictic lakes, where the strati
fl
cation has developed after the Last Ice Age.
Example 2.4.
Consider a cold lake with temperature T
b
and salinity S
b
in the lower layer
and salinity S
0
< S
b
in the upper layer. Vertical ventilation of the water body is possible, if
ˁ
(T
m
, S
0
)>
ˁ
(T
b
, S
b
). This can be exactly solved from the equation of state (Eq.
2.4
), but a
first-order approximation can be made using Eq.
(
2.5
) with a salinity term
ʲ·
S
b
,
0.8 kg m
−
3
added. Then we can obtain the ventilation condition (salinity in
ʲ
*
‰
):
a
b
½T
b
T
m
ð
S
b
Þ
2
S
b
S
0
\
10
−
2
C
−
2
,
Since
ʱ
/
ʲ
*
°
it
is seen that for T
b
T
m
(S
b
)=3
°
C, we must have
-
S
b
-
.
The temperature pair of the maximum density and the freezing point Tf)
m
, T
f
) is the
basis of the classi
S
0
< 0.1
‰
cation of the cooling process in lakes. In fresh and brackish lakes, as
soon as T
0
< T
m
, the lake is potentially capable to freeze, because then the density
decreases in cooling, and in calm conditions only a short, cold period may result in a thin
ice cover. Kirillin et al. (2012) called
the period, when the surface temper-
ature is below T
m
and the surface is still ice-free. The condition T
0
≤
'
pre-winter
'
max(T
m
, T
f
) means
always a risk of ice formation. However, the strength of the inverse stratification becomes
weaker with increasing salinity (Fig.
2.4
b). In saline and hypersaline lakes, the potential
freezing condition comes only when the surface temperature has reached the freezing
point. Lakes can be classi
ed for the ice season (see also Fig.
2.2
) into (Table
2.4
).
Proglacial lakes form a special group of ice-covered lakes. They form in front (epi-
glacial lakes), on top (supraglacial lakes) or underneath (subglacial lakes) of glaciers and
ice sheets, and glacial melt water is their main water source (Menzies 1995). Epiglacial
lakes have normally a seasonal ice cover, and they are much as normal cold region lakes.
Supraglacial lakes have seasonal or perennial ice cover and the body is in liquid state
usually in summer only (e.g., Lepp
ranta et al. 2013). Subglacial lakes have a perennial,
glacial ice cover, and they are usually under a heavy pressure caused by the thick ice.
ä
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