Introduction - Freezing of Lakes and the Evolution of their Ice Cover - page 5

first samples ever were taken from these lakes in 2013 showing signs of life, mainly
bacteria (Rogers et al. 2013).
A seasonal lake ice cover is normally thin as compared with the lake depth. Its physical
properties show large variability due to the high homologous temperature, 3 the large size
of individual crystals, and the presence of impurities. Lake ice sheets show a simple
strati cation pattern consisting mainly of congelation ice and snow-ice (e.g., Michel and

Ramseier 1971). Snow accumulation on lake ice is a common, and essential feature. Only
in cold and very dry climate regions, the ice cover is snow-free (e.g., Huang et al. 2012).
Even a thin layer of snow signi cantly weakens the heat exchange between the lake water

body and the atmosphere and the transfer of sunlight into the water. Also snow and liquid
water may form slush, which transforms into snow-ice in cold weather (e.g., Palosuo
1965). In freezing lakes, frazil ice seems to be a very rare form of ice, and no observations
have been reported of the occurrence of anchor ice.
The evolution of a lake ice season is primarily a thermodynamic process (see Lepp

ä

ranta

ä

and Wang 2008; Lepp ranta 2009a). When the ice is thick enough, it spans a stable

cover across the lake. Ice breakage and mechanical displacements take place in very large
lakes and also in smaller lakes when the ice is weak. In the melting period, ice loses its
bearing capacity due to internal deterioration (Fig. 1.3 ). Decay of a lake ice cover is a
thermo-mechanical process, starting from the shoreline. Melting of near-shore ice releases
the ice from solid boundaries, and the ice cover may then shift as forced by winds and
currents. The movement of ice causes further breakage of ice that speeds the decay
process.
Land-ice interaction due to thermal expansion and onshore ride-up or piling-up is an
important environmental and practical issue. It has attracted geographers for more than
100 years (Buckley 1900; Helaakoski 1912; Alestalo and H 1979). Due to thermal

and mechanical stresses, lake ice deforms shore areas and loads man-made structures. In
shallow areas, bottom scouring by ice and freezing through the whole water column give
rise to deformation and erosion of the lake bottom. Land-ice interaction has therefore
geological as well as biological consequences.
In the lake water body, physical phenomena and processes are very different under ice
cover from the open water conditions. The ice cover cuts the transfer of momentum from
the wind to the water body that damps turbulence and mixing. The surface water tem-
perature is at the freezing point, and there is very little vertical transfer of heat, apart from
geothermal lakes. In all, the temperature structure and circulation are quite stable.
However, in very large lakes, the ice sheet may experience episodic movements and
disturb the water body. In spring, solar radiation provides a strong downward

ä iki ö

fl flux of heat,

which constitutes the strongest heat flux into fully ice-covered lakes, and the ice melt

water with its impurities is released into the water column.
The volumetric changes in the liquid water body, associated with formation and growth
of ice are of no consequence in deep lakes, but in very shallow lakes, there may be

fl

3 Absolute temperature of a medium relative to its melting point.