Ice-Covered Lakes Environment - Freezing of Lakes and the Evolution of their Ice Cover - page 267

Fig. 8.16 In the past cold
periods lakes in Central Europe
were covered by ice with
bearing capacity to travel and
enjoy. Painting by Jakob
Grimmer, from Wikimedia

spring conditions directly affecting the radiation input at the lake surface. Apparently,
physical mechanisms of the global warming impact on the ice phenology include time-
delayed effects and feedbacks not resolved to date.
The projected changes in the climate will have a signi cant effect on the qualitative

characteristics of the ice as well as the quantitative changes described above (Fig. 8.16 ). In
geographical terms, the ice season regimes can be grouped into three zones: ephemeral ice
zone, unstable ice zone, and stable ice zone. In the

first zone, ice comes and goes in one
season, in the second the ice breaks at times and results in periods when the lake is
partially open while in the third zone the whole lake has stable ice cover from the date of

first freezing until the ice cover becomes rotten in spring. The boundaries of the zones will
move north when the climate becomes warmer. The extent of the ephemeral zone is
dictated by the mean and variance of air temperature whilst that of the unstable zone is
dictated by the thickness of ice. With decreasing ice thickness, the period of the ice being
breakable increases and finally the ice cover is breakable during the whole ice season.

Today, only very large lakes at around 60

º N in European climate have unstable ice cover

but if the ice thickness is reduced to 20 30 cm or less, medium size lakes will also be

-

unstable.
Consequently the implications of a warming climate to the lake ice season are

rst
shorter season and then thinner ice. However, depending on the snowfall conditions,
snow-ice formation may increase to counter at least part of the effect of rising air tem-
peratures. Thinner ice also means more breakable ice with more areas of open water,
which changes fluxes of heat and moisture between the lake and the atmosphere and may

generate frazil ice. Frazil ice formation may, in turn, lead to accumulation of sediments in
the ice cover via frazil capturing of particles in the water body and via anchor ice
formation. From the point of view of lake ecology, shorter ice season and presence of
openings mean less oxygen de

fl

cit problems. But the openings will also create a new type
of winter environment in lakes, which have hitherto been completely covered with ice.