Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 230

Lateral heterogeneity in the solar heating of the upper layer can produce a secondary
circulation pattern, whose character has been poorly investigated. The snow cover is
unevenly distributed over the ice surface. In large lakes, the deep central parts remain
often snow-free, whereas shore areas are snow-covered (Kouraev et al. 2008). In thin
snow patches the albedo decreases faster, the ice surface becomes exposed, and increased
irradiance beneath the ice results. Also in spring, the positive albedo feedback favours the
development of a heterogeneous ice cover. In autumn the opposite is true, i.e. negative
feedbacks favour homogenization of ice cover as seen in the small variability of ice
thickness.
The differential solar heating of the water body yields horizontal temperature gradients
and produces density currents (Zhdanov et al. 2001). Horizontal temperature transects
were mapped by Forrest et al. (2008) in the convectively mixed layer of Pavilion Lake,
and the result revealed remarkable differences in the temperature of the convective layer,
ascribed by the authors to variations of the ice cover thickness. The patchy surface may
create small-scale circulation patterns, too. Apart from variations in the ice and snow
thickness, the depth differences between shallow and deep parts of lakes are able to
produce lateral temperature gradients and initiate a density-driven circulation (Farmer
1975; Bengtsson 1986).
In freshwater lakes, it is possible that convective mixing raises the temperature of the
whole water body to 4 C, apart from the thin boundary layer beneath the ice, before ice

breakup. This implies a fast

°

final melting of ice after the porous ice sheet has broken.
When the ice has disappeared, the temperature pro

cation
sets up leaving a very short time for convection in open water conditions. This way spring
overturn mostly takes place under ice cover, and the renewal of oxygen in deep water
remains limited.

le soon passes 4 ° C, and strati

7.4 Light Conditions

7.4.1 Optically Active Substances

Pure water is most transparent to blue light. In shorter and longer wavelengths, light
absorption increases strongly (Fig. 7.14 ). Absorption by solid and liquid phases of pure
water is similar but scattering is in

uenced by the size, shape and orientation of ice
crystals (Warren 1982; Mullen and Warren 1988). Liquid water inside snow or ice traps
radiation due to absorption and lower scattering, and the more there is liquid water, the
higher is the absorption.
The optically active substances (OAS) in lake waters are, in addition to the pure water,
coloured dissolved organic matter (CDOM), 5 suspended matter, and chlorophyll
a (Fig. 7.13 ) (Arst 2003). CDOM absorbs strongly blue light with absorption decreasing

fl

5 Also known as the yellow substance.