Life in a cold environment - Antarctica: Global Science from a Frozen Continent - page 201

Figure 6.25 Lake Fryxell, Taylor Valley, Victoria Land. This is one of several Dry Valley lakes

that melts suf ciently during summer to have open water around the edge (Credit: Brien
Barnett, NSF).

in Antarctic freshwaters that is not generally seen at lower latitudes. ' Top down '

' '

grazing control is very limited, and the microbial loop predominates. This consists

of energy and nutrients flowing through a food web composed of microscopic

phytoplankton, bacteria and protozoans.
As with the nearshore and littoral marine environments, ice plays a very large role
in the biology of Antarctic lakes. In the maritime and continental zones, and the
colder sub-Antarctic islands, lakes are seasonally or permanently covered by ice.
This has diverse consequences, affecting, for instance, light transmission, water
column stability, thermal and chemical strati cation and oxygen availability, as well

as the direct physical damage from ice nucleation in contact with biological material,
and damage from moving ice. While many lakes are obviously recent in origin, there
is evidence that some have persisted through at least one full glacial cycle, while
some in the Dry Valleys are thought to be tens or even hundreds of thousands of
years old. In recent years the existence of many subglacial lakes deep under the
Antarctic continental ice sheets has come to prominence. The largest of these is Lake
Vostok, 200 km long and 500m deep, lying below at least 3.6 km thickness of the
continental icecap. Initially thought to have been completely isolated from the
modern world since the formation of their overlying ice sheets, it is now known that
many subglacial lakes are linked by under-ice water flows, but they still harbour the