Ice-Covered Lakes Environment - Freezing of Lakes and the Evolution of their Ice Cover - page 251

In addition to light conditions and temperature, convective mixing strongly affects the
plankton growth under ice. Mixing conditions determine, to a large extent, supply of
nutrients from the deeper lake layers as well as light availability for the plankton, espe-
cially for the non-motile diatom species (Matthews and Heaney 1987). There are also
some reports on the direct effect of mixing on the diatom growth rates, though its
mechanism is still less clear (Marra 1978). Several studies suggest strong correlation
between radiatively-driven convection under ice and growth rate of diatoms (Matthews
and Heaney 1987; Wilson and Wellman 1962; Kelley 1997). According to the reported
observations, existence of convective layer under ice is the only mechanism supporting
non-motile diatom species in the upper euphotic zone provided that vertical mixing
velocities are larger than sinking velocities of the diatoms. However, the role of this early
stage of plankton growth for subsequent diatom bloom after ice breakup is not yet clear.

8.2 Frozen Lakes at Settlements

8.2.1 Monitoring and Remote Sensing

Monitoring of ice conditions in freezing lakes has been an important issue for a long time.
The principal questions have been the lake ice phenology and the thickness of ice, being
related to safety, fishery, shipping and traf c on ice. Systematic monitoring of lake ice

started in the 1800s, first by people interested in nature and then by environmental

authorities. Example in Finland, competitions to predict the break-up date have been
arranged in many lake site communities. Since lakes have limited size, phenology
observations have been easy to perform in inhabited areas. These data are today extremely
valuable for the research of lake ice climatology.
For the society the main question of lake ice is the safety issue, in particular for traf c,

skiing and fishing. Travelling methods on lake ice have ranged from people walking on

ice to trains, where the required ice thickness ranges from 5

125 cm. The additional
questions are storage of material on ice, take lake ice for its latent heat as a cooler, and use
lake ice as a source of fresh water. Lake ice can even be utilized for ice sculpture,
especially when the ice is clear from gas bubbles and looks like glass.
Operational monitoring of ice conditions in lakes is ongoing in several countries based
on ground observations. This is especially required for safety purposes. Ice thickness
determines the bearing capacity in winter, but in spring deterioration of ice changes the
quality and lowers the strength of the ice. A ground data based monitoring system is in use
in the Finnish Environment Institute, which publishes a lake ice thickness map across the
country based on about 30 ice thickness sites (Fig. 8.4 ). Also instructions and warnings
are given for on-ice travelling and activities. Snow and ice thickness are monitored, for
some lakes also the snow-ice and congelation ice fractions (Table 8.1 ).
Satellite remote sensing is based on emission and scattering of electromagnetic radi-
ation from the Earth

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'

s surface. For ice mapping, the main channels are shown in Table 8.1 .