Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 219

The meltwater has normally a very low salinity. The layer of low-saline surface water
increases the vertical stability. But solar heating and in

ow of low-salinity water may also
produce a thermally unstable but non-convective layer with temperatures exceeding T m in
the very vicinity of the ice (Ko

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ski and Wisznewski 1934; Jonas et al. 2003). This
layer is not necessarily supported by stabilizing salinity gradient but can be also produced
by vertical differences in the absorption rate of the solar radiation (Kirillin and Terzhevik
2011).
While dilution due to ice melting stabilizes the surface waters in spring, an opposite
process takes place in the early winter, when salt release from newly formed ice desta-
bilizes the upper water column and produces convection. In saline water, ice formation
can drive deep convection, but in freshwater lakes the in

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uence is soon compensated by
the temperature increase beneath the ice. Convection driven by salt

fl

flux can play an
important role in the survival of plankton under ice in winter (Jewson et al. 2009). In
shallow lakes, the salt extruded from ice can be mixed across the water column and
increase the average water density in the lake. Then in spring, the melt water from ice and
snow forms a layer of lower density on the lake surface.
The salinity distribution may also affect the convection at the lake bottom. Near-bottom
waters are often distinguished by increased content of dissolved salts released from the
sediment or brought by groundwater in

fl

cation is strong
enough, it can prevent ventilation of the near-bottom waters by the spring overturn and
contribute to the deep anoxia development in the following summer (Mironov et al. 2002;
Pieters and Lawrence 2009).
In large lakes, the area of shallow pelagial sediments is usually small compared to the
entire surface of the lake bottom, and the heat seasonally stored in the sediment plays a
minor role in driving the lake circulation. In addition, lakes of large surface area, espe-
cially those located in the dry continental climatic zones, remain snow-free for the entire
ice-covered period (Lake Baikal is the most prominent example). There, convective
mixing penetrates down to several tens or even hundreds meters depth (Farmer 1975;
Shimaraev and Granin 1991; Weiss et al. 1991).
Another in

fl ow. If the resulting salinity strati

ow-related effect, appearing in freshwater lakes only at temperatures close
to T m ,iscabbeling

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an increase of water density due to mixing of two water masses with
differing temperatures and salinities (Kirillin et al. 2012b). The mixture of two such water
masses is more dense than either of them, as seen in the convex shape of density isolines
in TS space, and therefore diffusion induces convection. The effect is known to produce
deep mixing in the ocean (called also as double diffusive convection in oceanography), but
is usually negligible in freshwater lakes. At low temperatures, however, small differences
in the salinity between near-shore waters and the core water masses are the source of the
contraction by mixing.

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