Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 231

Fig. 7.13 Diffuse attenuation coefficients of light in selected lakes in Estonia and Finland: 1 Ä ntu
Sinij ä rv, 2 Paukj ä rv, 3 Koork ü la Valgj ä rv, 4 Vesij ä rvi, 5 P ä ij ä nne, 6 Lammi P ää j ä rvi, 7 V ô rtsj ä rv, 8
Tuusulanj ä rvi, 9 Valkea-kotinen (Arst et al. 2008)

exponentially with wavelength, and, consequently, CDOM moves the colour of the water
from blue toward yellow wavelengths. The in

uence of suspended matter depends on the
optical characteristics of particles, but normally radiation is absorbed and scattered with
strength weakly decreasing with wavelength. Chlorophyll a absorbs strongly blue and red
light (wavelength bands 430 - 440 and 660 - 690 nm); therefore plants have usually green
colour. The absorption and scattering of the OAS can all be assumed additive, since the
OAS concentrations are low enough.
Gas bubbles are the dominant optically active impurity in snow and ice. Due to a large
number of air inclusions in snow, scattering and therefore light attenuation within the
snow cover and snow-ice layer is strong, causing a drastic reduction of irradiance with
depth. Also the OAS present in lake water are found in snow and ice. In highly humic
lakes, a fraction of CDOM is captured in congelation ice growth and by

fl

flooding that adds
to the absorption at short wavelengths. Suspended matter would be effectively captured
into the ice sheet in frazil ice formation but this occurs rarely in frozen lakes. Since the
size of gas bubbles is much larger than the wavelength of light, the attenuation of light
when passing through ice is now strongly wavelength dependent.
Solar radiation penetrating the surface of a lake ice cover equals QT T =(1

fl

Q s and
constitutes mostly photosynthetically active radiation (PAR) or visible light (see Sect. 4.1.2 ) .
The PAR-band attenuation coef

− ʱ ) ʳ

10 m − 1 for snow and 1m − 1 for con- cient is ʺ s * ʺ i *

· ʺ − 1 , or 0.3 m in snow and 3 m
in ice (Fig. 7.14 ), 5 % of the light is left. Light attenuation in clear, bubble-free congelation
ice is close to that in liquid lake water with similar attenuation spectra. Since the concen-
tration of impurities is lower in congelation ice than in lake water, the ice may be even more
transparent than the water especially in turbid and humic lakes. In contrast, gas bubbles in the
ice scatter light, which lowers the light transmittance and

gelation ice (Jakkila et al. 2009; Lepp ä ranta et al. 2010). At 3

fl flattens the attenuation spectrum.