Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 41

time for each substance can be de ned in a manner similar to the fl flushing time of the water

mass. Freezing has no in uence on the total budgets of impurities, but the concentrations of

the impurities are different in ice and water. Therefore, freezing and melting in

fl

fl uence on the

concentration of impurities in the surface water layer beneath the ice.

2.4 Ice-Covered Lakes

2.4.1 Zonation of Freezing Lakes

In regards with ice, lakes can be divided into three main zones: ice-free zone, seasonal
lake ice zone, and perennial lake ice zone. Years in these zones are qualitatively different.
Between the perennial and seasonal zone there is a quasi-perennial zone, where lakes
sometimes have ice-free summer, and between seasonal and ice-free zones there is an
ephemeral zone, where ice occurs but some years are ice-free. The boundary between the
seasonally and perennially ice-covered lakes is quite thin, since there are not many lakes,
which are seasonal but occasionally possess multi-year ice. Proglacial lakes can be taken
as a sub-category in the perennial lake ice zone, as there the water body is always in
contact with land ice.

2.4.2 Seasonal Lake Ice Zone

The zone of seasonally freezing lakes covers large areas of the continents (Fig. 2.13 ). This
zone is close to the diagram of Hutchinson and L er (1956), showing that lakes located

between 40 ° and 80 ° latitudes may freeze in the seasonal cycle. Bates and Bilello (1966)
estimated the southern boundary of seasonal ice cover in the Northern Hemisphere to
follow approximately the latitude 45

ö f fl

N, higher in the Western Europe and lower in Asia
and North America. In this latitude band there are also permanently ice-covered lakes in
very high altitudes. Basically, where the mean air temperature in the coldest month is
below the freezing point, shallow freshwater lakes are potential to freeze over (Fig. 2.2 ).
The seasonal ice zone can be further sub-classi

°

ed into lakes with a stable ice or
unstable ice cover. Lakes of the former class freeze over annually (Fig. 2.14 ), while in the
latter class the ice cover breaks and full ice coverage is not necessarily achieved every
year. This feature is lake-speci c, since ice formation is strongly dependent on the

regional climate conditions, as well as on the depth, size and morphology of the lake
basin. For example, in the present climate in Europe, the whole Finland belongs to the
stable ice zone, but two large lakes nearby, Ladoga (Prokacheva and Borodulin 1985) and
Peipsi (Reinart and P

rn 2006), have unstable ice cover.
Apart from the ephemeral zone the mean annual maximum ice thickness in seasonally
freezing lakes ranges within 20

ä

200 cm. No reliable data exist on the maximum possible
thickness of seasonal lake ice; based on the sea ice data from the Siberian shelf, it can be

-