Geoscience Reference
In-Depth Information
radiation is continuous or absent, respectively, and the solar altitude is limited by the
zenith distance of the latitude plus inclination of the ecliptic (
ʵ
= 23.5
°
). At latitude
˕
, the
solar altitude at midsummer noon and midwinter noon is 90
° - ˕
+
ʵ
or 90
° - ˕ - ʵ
,
respectively; for
.
There is concern about the increase of UV-B (ultraviolet B-band, 280
˕
=60
°
these altitudes are 53.5
°
and 6.5
°
320 nm) in high
polar latitudes. This is photochemically and photobiologically the most active band of the
solar irradiance on the surface of the Earth, and it disturbs the growth of biological
organisms and causes DNA damage and mutations. UV-A (320
-
380 nm) is less energetic
but can cause skin damage, while the shortest ultraviolet band UV-C is absorbed by the
atmosphere and does not reach the Earth
-
'
s surface.
8.1.4 Primary Production
Usually under ice phytoplankton community is sparse. Due to the poor light conditions,
heterotrophic and mixotrophic species dominate over autotrophic ones. In spring, the
growth of autotrophs starts as soon as the light intensity reaches the level, which is needed
for photosynthesis. Sometimes there are dense algal populations just beneath the ice
cover. All major taxonomical groups of algae can be represented under ice in winter.
Low light availability and low temperatures are the most obvious physical factors
affecting primary productivity in ice-covered lakes. The biological response to these
effects is discussed in detail by Sutter et al. (2012). The physical factors, whose effect on
plankton is undoubtful, are the formation of liquid layers and pockets within the ice sheet
as potential plankton habitats and convective mixing in the upper water column under ice.
Both processes take place in the upper euphotic zone, which is especially thin in ice-
covered lakes and are thereby highly relevant to the primary productivity.
Lake ice sheet may contain liquid water pockets, where primary production is possible
provided that nutrients and light are available. The origin of the liquid water is lake water,
snow and ice meltwater, or liquid precipitation. A common mechanism of formation of a
liquid layer within the ice sheet is locking of the slush layer on ice surface by the snow-ice
on top of it (see Chap. 4 ). The slush layer in lake ice sheet is a documented plankton
habitat, though only few publications are available so far on slush layer algae (e.g., Felip
et al. 1995; Lepp
ranta 2009a). Two other potential algae habitats are the freshwater
pockets forming in the otherwise solid congelation ice during the melting season and the
brine pockets in the ice cover of saline lakes. The latter are analogous to brine pockets in
the sea ice and usually form when the lake water salinity exceeds 1
ä
(e.g., Weeks
1998). Both environments can play an important role in the primary production as pro-
viding optimal light availability for the algae communities of ice-covered lakes. Infor-
mation about the production in brine pockets is available mostly from the marine ecology
(see Thomas and Dieckmann 2010). Also humus pockets in the lake ice sheet have been
found in highly humic lakes (Salonen et al. 2009).
-
2
 
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