Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 240

depends strongly on the sediment composition. The molecular diffusion coef cient of

10 − 9 m 2 s − 1 . In the bottom boundary layer, the time-scale of oxygen
consumption is much shorter than the time-scale of molecular diffusion.
Golosov et al. (2006) examined the evolution of vertical oxygen pro

oxygen is 2 ×

le under lake ice
using a self-similarity model, which also contained the surface layer of the bottom sed-
iments. They used the Dirichlet boundary conditions: C 0 = constant at the surface, sup-
ported by their data, and C = 0 at the bottom. Initially C = 0 was reached at the depth

ʴ

beneath the sediment surface. The pro les of oxygen content and oxygen consumption

were given as second or third order polynomials in the water and in the sediment, and the
oxygen consumption depended on the water temperature. In the beginning, oxygen was
fed from the water to the sediment, then the bottom became anoxic, and the anoxic layer
grew upward from the bottom.

7.5.2 Methane

Lakes have been recognized as important sources of methane, which accounts for about
20 % of the greenhouse effect (see Kirillin et al. 2012b). There are four emission path-
ways: ebullition flux through aquatic vegetation.

Isolation of lakes from the atmosphere by the ice cover can result in accumulation of
methane bubbles beneath and inside the ice with subsequent increase of the emission after
the ice break-up. Prokopenko and Williams (2005) suggested this effect to produce out-
bursts in Lake Baikal of several teragrams of methane for the whole lake area. Later,
Schmid et al. (2007) demonstrated that most of the methane produced under ice is dis-
solved and oxidized in the deep water column of Lake Baikal. Though a slight increase of
the methane concentration under ice was observed, the level did not exceed signi

fl flux, diffusive fl ux, storage fl ux, and fl

cantly
the atmospheric concentrations. With certain care, the suggestion about the negligible
methane emission on annual scales can be extrapolated to the majority of deep oligo-
trophic lakes.
Yet, depending on the trophic regime and the size of lakes, estimations of methane

fl fluxes vary in a wide range, from micrograms to several grams carbon per square meter

per year with the highest methane

fl fluxes found in hypereutrophic lakes. Strayer and Tiedje

(1978) reported of the methane fluxes in hypereutrophic Lake Wintergreen as high as 92

and 44 g cm − 2 year − 1 , due to ebullition and diffusion, respectively. The present evidence
suggests that the majority of methane production occurs in anoxic sediment (Rudd and
Hamilton 1978). Thus, an appreciable effect of the ice cover on methane production can
be expected in shallow eutrophic lakes with signi

fl

cant duration of the ice-covered period.
The winter anoxia, typical for such lakes, can signi cantly affect the methane production.

Quantitative estimations of this effect and possible consequences of climate-driven
changes in ice phenology on the methane emission are not yet available.