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The OAS of liquid lake water can be also found in lake ice captured by growing ice
(see Sect. 3.3 ; Lepp
ranta et al. 2003a, b). In freshwater congelation ice, the capture is not
very effective, and the OAS levels are much lower in the ice than in the parent water
(Fig. 3.8 ). Therefore bubble-free congelation ice is normally more transparent than the
liquid water of the same lake. In the melting season, in particular, the presence of liquid
water has major impact on the light transfer. Liquid solution in ice or snow in
ä
fl
uences on
the light transfer: when it
fills gas inclusions, scattering is reduced, and vice versa. In very
humic lakes, liquid humus inclusions have been found from congelation ice resulting in
stronger absorption of short wavelengths (Arst et al. 2006; Lei et al. 2011). Apart from the
melting season, liquid water content of congelation ice is very small, and, consequently,
chlorophyll is then practically absent.
Snow-ice formed by
flooding has a contribution of about 50 % from the lake water
with all its impurities (Lepp
fl
ranta and Kosloff 2000). However, snow-ice has also a large
gas content, which dominates the light transfer. Chlorophyll can be found in slush layers
inside snow-ice when there is light available (Lepp
ä
ä
ranta 2009a). Frazil ice formation
would be effective in capturing particles from the water body and would therefore
in
uence the ice properties as in rivers. Atmospheric deposition adds to the impurities in
the near-surface layer, whether congelation ice or snow-ice.
For light transfer in brackish and saline lakes, the presence of brine pockets makes a
signi
fl
cant difference to freshwater lake ice (Weeks 1998). Brackish ice has been observed
to be less transparent than freshwater ice in the same region (Arst et al. 2006). Primary
production can become large in the brine pockets adding the chlorophyll a absorption
spectrum to the light transfer (Arrigo 2003).
Because of the strong scattering, ice and snow surface have very high re
ectances
compared with open water surface. The variability of lake ice albedo is large depending
primarily on the thickness of snow and presence of liquid water (e.g., Mullen and Warren
1988; Perovich 1998; Henneman and Stefan 1999; Arst et al. 2006). Table 3.9 shows
references and ranges of the albedo for different surface types. These albedo values are
similar to those observed in polar seas (Perovich 1998).
The albedo of snow depends on the liquid water content and grain size, impurities and
cloudiness. Dry,
fl
fine-grained snow has an albedo as high as 0.9. With increasing grain
size and especially increasing liquid water content, the albedo becomes lower, and for wet
Table 3.9
Typical level and range of albedo over lake surfaces
Surface
Albedo (%)
Sensitivity to surface material
Reference
Range
Dry snow
85
80 - 95
Grain size
Wet snow
50
40 - 70
Wetness
Dry ice
50
30
60
Gas content
-
Wet ice
30
20 - 40
Wetness, gas content
Open water
7
5 - 10
Water quality
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