Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 235

Fig. 7.16 Reflectance (left) and transmittance (right) in lakes 1 — Lovoj ä rvi, 2 — P ää j ä rvi, 3 — Iso
Valkj ä rvi, 4 — Vanajavesi, 5 — Peipsi, and 6 — Vesij ä rvi in the melting season. Lakes 1 - 3 are highly
humic, and lakes 5 - 6 are snow-free

7.5 Wintertime Budgets of Gases and Nutrients

7.5.1 Oxygen

Ice-covered lakes behave as closed systems with regard to dissolved oxygen (DO).
Without intense photosynthesis and water aeration, there is no oxygen supply, and the
consumption of oxygen by bacterial plankton in decomposition of organic matter and the
absorption of oxygen by bottom sediments control the DO content in a lake (Hargrave
1972). In general, the oxygen level is an excellent index of the health of the lake. The
dimension of DO concentration is mass per volume, the unit is normally mg L − 1 (milli-
grams per litre). Often the oxygen content is expressed as the percentage of its saturation
level, which depends on the temperature (see Fig. 7.1 ).
In ice-covered lakes, oxygen renewal is marginal, and therefore the oxygen content
decreases during the ice season (Fig. 7.17 ). The initial conditions at the instant of freezing
are important, since the colder the water is after autumn mixing the more there is oxygen.
Oxygen depletion is strongest in the lower layer, where anoxic conditions are reached
particularly in long winters. The consumption of oxygen is higher in eutrophic lakes and
therefore these are more likely to suffer in the ice season. Lakes, where such conditions are