Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 203

Fig. 7.1 Oxygen saturation as a function of temperature

the freezing point (Hom nne in Finland, maxi-

mum depth 95 m, in windy autumn the turnover continues down to 1

é in 1903). Example in the large lake P ä ij ä

C (Pulkkanen
and Salonen 2013). Eventually, a persistent inverse winter thermocline develops, and then
freezing state can be reached fast. At freeze-over of freshwater lakes, initial winter
strati

2 °

-

cation is set up.
Also in pre-winter conditions, factors having usually secondary effects on the stratifi- -

cation dominate the stability of the water column, in particular hydrostatic pressure vari-
ations in very deep lakes. The temperature of maximum density decreases with increasing
pressure, T m = T m (p), at the rate of 0.020

C bar − 1 (or about 0.2 C/100 m water depth).

Therefore, with the isothermal water column held at the temperature T m = T m (p 0 ), a
transition depth exists where the local temperature equals T = T m (p). Deeper down, a
temperature increase produces gravitational instability, called thermobaric instability.
Hence, downward movement of initially stable water may turn into free convection, if the
wind mixing is strong enough to penetrate below the transition depth (Farmer and Carmack
1981; Carmack and Weiss 1991). Once created, thermobaric convection penetrates deep
into a neutrally strati

° °

cantly to the ventilation of
deep lakes such as Lake Baikal (Shimaraev and Granin 1991; Shimaraev et al. 2011).
Lake waters cool together with the ambient atmosphere (Fig. 7.2 ). Convection takes
place deeper and deeper as the temperature of maximum density is approached from
above. Therefore in deeper lakes the timing of potential ice formation is later than in
shallow lakes. Freshwater lakes mix through at their temperature of maximum density
T m = 3.98

ed water column and contributes signi

cation is set up. Surface
water experiences a minor supercooling, depending on the particular weather conditions
(usually

° C, and after some further cooling stable winter strati

C).
Brackish, saline and hypersaline lakes, unless very shallow, are strati

0.1 ° C but may be up to 1 °

*

ed in salinity,
and mixing does not need to reach the bottom but only the halocline, which thus repre-
sents the maximum mixing depth in fall. In limnology such lakes are called meromictic. In