Introduction - Freezing of Lakes and the Evolution of their Ice Cover - page 6

Fig. 1.3 The bearing capacity of lake ice is a major safety question for on-ice traffic. The
photograph shows a rescue operation of a car in Lake P ä ij ä nne, Finland in spring 2005. Published in
newspaper Etel ä -Suomen Sanomat on March 31, 2005, photograph by Mr. Pertti Louhelainen.
Printed by permission

substantial proportional reductions in the volume of liquid water. Also in shallow lakes the
bottom sediments have an important role in the ice season. They store heat in summer and
release the heat back into the water body in winter. The release of heat and gas from the
lake bottom makes a contribution to the circulation in the water body and the structure of
the ice sheet. Impurities may dissolve from the sediments to the near-bottom waters and
create a thin bottom boundary layer due to the salinity strati cation. This layer also has

impact on the near-bottom living conditions.
Lake ice formation and growth have in

uence on the water quality as well as on the
lake ecology (see, e.g., Salonen et al. 2009). The concentration of suspended matter
decreases in the water column during the ice season, since in

fl

ow and turbulence are weak,
and sedimentation is not compensated by resuspension. At the time of ice breakup, the
load of impurities peaks from the snow and ice meltwater. Sunlight is the main limitation
for primary production under ice cover. Photosynthesis is paused for the polar night, and
in the case of a thick snow cover on ice, the dark season can be much longer and extend to
sub-polar latitudes. Another critical factor is that the ice cover reduces the level of dis-
solved oxygen in the water body by cutting the in

fl

ux from the atmosphere (Greenbank
1945; Hargrave 1969; Golosov et al. 2006). This results sometimes in anoxic conditions in
the deep water and

fl

fish kills (Fig. 1.4 ). At the beginning of the ice season, the oxygen