Ice-Covered Lakes Environment - Freezing of Lakes and the Evolution of their Ice Cover - page 244

may freeze in winter. Normally, however, cold region lakes have liquid water and at least
microbial activity throughout the year. Carbon dioxide dissolves easily in water and
therefore the storage is renewed via the atmosphere in lakes, which possess an open water
season.
The ice and snow layers also act as habitats of biota. As soon as there are liquid water
layers or pockets in ice and snow cover, primary production takes place with its selective
light absorption spectrum. This is typical in spring or summer, but also takes place in slush
layers at the snow-ice interface and in the brine pockets on saline and brackish lakes. But
the ice and ice margin ecosystems seem to be a minor factor in lakes not reaching the
richness observed in polar sea ice environment.
Light can become a limiting factor to primary production. In the polar night there is no
sunlight, and in the case of thick snow on ice the radiation can be attenuated too much
before reaching the water body. Primary production uses light quanta, which is expressed
in moles. 2 The rate of quanta 10 3 mol m − 2 s − 1 at the Earth

μ '

s
surface. The level of quantum irradiance needed to maintain primary production is of the
order of 25

fl flow in solar radiation is *

mol m − 2 s − 1 (see Arst 2003), which corresponds to irradiance power of
around 5 W m − 2 . If there is a dry, clean snow cover, irradiance just beneath the snow
surface can be 50 W m − 2 at noon in clear days, and that has attenuated to below 5 W m − 2
at the depth of z =

μ

ʺ − 1

log(10) 25 cm, where ʺ is the light attenuation coef cient of

*

snow.
Under snow-free ice, there is enough light in the surface layer for photosynthesis, and
an increase of the primary production under ice is often observed when the snow melts
(Kelley 1997; Salonen et al. 2009). Hence, the physical-biological interactions are more
manifold than in the preceding snow-covered period.
Primary production needs also nutrients, which are classi ed into macro- and micro-

nutrients. The former category contains sodium, magnesium, phosphorus, sulphur, chlo-
rine, potassium, calcium, iodine and silicon, while the latter contains e.g. iron, copper and
zinc. Nutrient supply is a limiting factor. In winter, low temperature and freezing con-
straint the activity of soil microbes and slows geochemical processes that reduces the
in ux of nutrients via surface and groundwater runoff. In fully ice covered lakes, circu-

lation and mixing are weak, and the water body is strati

fl

ed. This factor further limits the
cycling of nutrients within the water body. Also nutrients are lost in sedimentation, and
due to the absence or weakness of turbulence at the bottom, less is gained by resuspen-
sion. In all, nutrient supply can bring a strongly limiting factor to primary production in
ice-covered lakes. But ice-covered lakes, which receive nutrient enrichment from natural
or human sources, can show high production rates.
Benthic photosynthetic communities may become dominant in ice-covered lakes, since
conditions are more stable at the bottom. Sedimentation brings nutrients down from the
water body, nutrients are released from the bottom sediments, and there are more active
bacterial decomposition and nutrient recycling processes than in the overlying water body.

2 The number of quanta in one mole equals the Avogadro ' s number N A = 6.022142 × 10 23 .