Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 17

Table 2.3 Classification of lakes based on the salinity (S) of the water
Freshwater lakes S<0.5 ‰ Salinity of concern only in ice season

Brackish lakes 0.5 ‰ ≤ S < 24.6 ‰ Freezing point < temperature of max density

Saline lakes 24.6 ‰ ≤ S <35 ‰ Freezing point ≥ temperature of max density,

salinity at most at the oceanic level

Hypersaline lakes S ≥ 35 ‰ Salinity greater than the oceanic level

l / Dð 1 D , where D>1 is the fractal dimension or
the Hausdorff dimension of the shoreline; thus l

shoreline increases as a power law

→ ∞

as ʔ s 0. Therefore the length of

→

the shoreline is not a well-de ned geometrical property of a lake. Similarly, the number of

lakes in an area depends on the resolution of the map and is a questionable quantity.
Instead, the surface area and the lengths of the major and minor axes of a lake are well
de ned. Apart from the descriptive geometry, however, the fractal concept has not

brought much new applicable information to lake research. It is clear that the coastline
geometry is an important factor during the ice season but the length scale of this factor is

finite and process dependent.
The geochemical classi

firstly based on the salinity 1 (S), cation of the water quality is

which is, by de nition, the relative mass of dissolved salts and expressed in parts per

thousand or per mille (

‰ ). Salinity is a state variable of natural waters and in fl uences on

the mixing, circulation, cooling, and freezing. The classi cation reads (Table 2.3 ).

Brackish water salinity is much less than the salinity of normal seawater (35 ‰ ); the

‰

transition at 24.6 is often chosen, because this is the salinity where the temperature of

maximum density equals the freezing point (see Sect. 2.1.2 ). The chemical composition of
the lake water plays an especially significant role in hypersaline lakes, for which lake-
specific equations of state would be desirable to describe the growth and properties of the ice.
Additional geochemical classi

ers are pH and concentrations of speci c substances; e.g.,

oxygen, nutrients, organic matter. A speci c class of lakes are tidal lakes, such as lagoons,

which exchange water with the ocean (Shirasawa et al. 2005). In the northern part of the
Baltic Sea, due to glacial land uplift lakes become slowly isolated from the sea and the
salinity consequently decreases (e.g., Lindholm et al. 1989). These lakes are called fl adas.

2.1.2 Physical Properties of Lake Waters

A fundamental property of lake water is its density, denoted by ˁ . This is provided by the

equation of state, which is written in general form as

q ¼ qð T ; S ; p Þ ð 2 : 3 Þ

1 In limnology, the amount of dissolved salts (c d ) is normally expressed in mass/volume (grams/
litre). Clearly c d = ˁ S, where ˁ is the density of the solution.