Geoscience Reference
In-Depth Information
lake water is referred as the water quality. Salinity, pH, nutrients, chlorophyll, and harmful
substances are particularly interesting. Fully ice-covered lake water bodies are charac-
terized by low level and low range of temperature, strati
cation, weak thermohaline
circulation, weak mixing, and often low level of sunlight. The water velocities are low,
1
10 mm s
−
1
, turbulence—if
is weak, and consequently diffusion is slow. But if the
ice cover is not stable, it has a major impact on circulation dynamics as wind stress can be
transferred to some degree into the water body.
The concentration of dissolved matter (C
d
) is normally estimated from the electric
conductivity (
—
if any
—
-
)asC
d
[mg L
−
1
] = 0.67
S cm
−
1
], where the lower index 25
C
gives the reference temperature for the conductivity. The concentration of dissolved
matter is also expressed as salinity (S), which equals the relative mass of dissolved matter
in a water parcel; in fresh and brackish waters 1 g L
−
1
corresponds to S =1
˃
× ˃
25
°
C
[
μ
°
. The level
of dissolved matter has a weak annual cycle due to consumption of nutrients. The quantity
pH is a measure of the concentration of hydrogen ions. In exact terms, it is negative of the
logarithm of the concentration of hydrogen ions, and therefore the higher the pH is the less
there are free hydrogen ions. The pH of distilled water is 7, for acid water the pH is less
and for basic water it is more.
Lake water contains dissolved gases, with saturation levels of different gases depending
on the temperature (see Fig.
7.1
for oxygen). Solid ice cover is not permeable for gases,
and therefore gas bubbles released from the water body or lake bottom get locked within a
growing ice sheet. Oxygen is a key dissolved gas in water, and its concentration is an
excellent single indicator of the health of a lake. The level of 2 mg L
−
1
is considered for
lower boundary for an ecologically healthy lake environment, while at temperature of 1
‰
°
C
the saturation level is 14.2 mg L
−
1
(Fig.
7.1
). Oxygen is primarily renewed by the
atmospheric
flux, while the output of oxygen from primary production is a secondary
factor. Therefore in the ice season the oxygen renewal is a major issue (Fig.
8.2
).
Suspended matter concentration is changed by sedimentation and resuspension, in
fl
fl
ow
and out
low and chemical processes within the water body. In winter the low level of
water currents means that resuspension is weak that provides an internal cleaning
mechanism. One implication is lower turbidity of the water mass. Major in
fl
fl
ows of
Fig. 8.2
Ice fishing is a
popular hobby in cold region
lakes
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