Proglacial Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 186

usually these zooplankton are present at low densities. Benthic microbial mats are less
common in saline lakes, but even the most saline ones contain some biota. Other bio-
logical habitats include pockets within the lake ice, the under surface of the lake ice, and
the micro-strati ed chemoclines of meromictic lakes where gradients of conductivity,

oxygen, pH, and sulfur provide numerous niches for photosynthetic sulfur bacteria, other
anoxic bacteria, and methanogenic Archaea. With many higher organisms unable to
survive there, the Antarctic continent could be seen as a microbial world with the lakes
being its principal bastion.

6.3 Supraglacial Lakes

6.3.1 Occurrence of Supraglacial Lakes

Supraglacial lakes form and grow from the penetration of solar radiation into an ice massif
(Figs. 2.17 and 6.3 ). Within the ice, the heating comes from the balance between solar
radiation and conduction, while at the surface there are also strong terrestrial radiation
losses. Therefore, the water body may grow in the presence of an ice cover. Figure 6.3
shows a situation when the ice thickness must have been more than 30 cm to carry the
5-ton bandwagon in a water pumping operation.
Supraglacial lakes are found on the Antarctic and Greenland ice sheets and in many
mountain glaciers. In Greenland they are much more developed than in Antarctica and
have a major impact on the ice mass balance (Hoffman et al. 2011; Liang et al. 2012).
Lake water masses may also transport latent heat deeper into the ice sheet by convection
across the lake water body and penetration of lake water into fractures (Chikita et al. 1999;
Joughin et al. 2008; Hoffman et al. 2011). Lake out

ows may form moulins into the ice
advecting heat across the ice sheet and reducing the sliding friction at the base of the ice
sheet. Supraglacial lakes in mountain glaciers possess potential to create

fl

floods in the
nearby environment (Bajracharya and Mool 2009). They are extremely low in biota
(Keskitalo et al. 2013).
Supraglacial lakes form in blue ice regions in the surface layer of ice sheets and
glaciers. The condition for the existence of the bare, blue ice surface is negative mass
balance. The albedo is low due to the bare ice or open water surface (around 0.4

fl

0.6 for
ice cover and 0.1 for open water). Therefore these lakes absorb solar energy about 5

-

10
times more effectively than the snow-covered ice (albedo around 0.9). Solar heat is
distributed across the optical thickness of the ice, which for clean blue ice is 1

-

2m.On
the other hand, conduction of heat in the ice is slow, and as a result the solar heat is
trapped, blue ice surface layer warms, and melting of ice may start beneath the surface and
grow into water bodies (Figs. 1.2 and 6.3 ). In summer the surface radiation balance is
close to zero but

-

the radiation transmitted through the surface (QT) T ) is typically

100 W m − 2

ä

50 - (Lepp ranta et al. 2013b).