Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 21

winter winter

summer summer

0°C 4°C

0°C 4°C

Dimictic lakes

Cold monomictic lakes

Fig. 2.5 Temperature stratification in dimictic and cold monomictic lakes. The dotted rectangle
denotes for autumn and spring overturn of the water mass

(for seawater, see Assur 1958). Therefore the freezing point formula is more clearly lake-
speci

c. The Great Salt Lake of Utah does not freeze although there is snow around in
winter. Also it is well known that Don Juan Pond with the salinity 440 ‰ in Antarctica
does not freeze at C during the austral winter (Meyer et al. 1962).

Unstable density strati

− 50 °

cation does not hold in natural conditions, and therefore
increase of surface water density leads to convective mixing. The temperature of maxi-
mum density of fresh water is close to 4

C, and when the surface temperature reaches this
level, the whole water column is mixed by vertical convection. This is also called the
water mass turnover. The oxygen storage of fresh water lakes is renewed in turnover
events, which are necessary for aquatic life.
In the response of freshwater lakes to thermal forcing, overturn events are crucial. This
event is especially strong in the cooling season, starting from the deepening of the mixed
layer in the autumn. Convective mixing deepens until the surface water temperature
reaches the temperature of maximum density, and then mixing can continue by wind
forcing to lower temperatures. The water temperature lags behind the air temperature
forcing with the

°

depending on the lake depth. For shallow lakes the
memory is less than 1 month, and for deep lakes it can be several months. Water volumes
are fairly small, but if the annual cycle is strong, lakes may store much heat in the
summertime to be released in winter. Summer heat is also stored in the bottom sediments
in shallow lakes.
The annual frequency of the turnover events is the basis of limnological classi

' thermal memory '

cation

of lakes (Fig. 2.5 ). Most seasonally freezing lakes have two turnover events — in spring

cation 3 shows warm upper layer and
cold lower layer with a thermocline layer between, where the temperature changes
sharply. In winter, the strati

and fall — and thus are dimictic. The summer strati

cation is inversed. However, the time between spring and fall
mixing becomes shorter with colder climate, and when cold enough they merge together

3

In limnology the upper layer is called epilimnion and the lower layer is called hypolimnion.