Terrigenous soil and gravel particles that are colonized by attached microbial
communities (phototrophic and heterotrophic prokaryotes and eukaryotes)
are wind-blown onto the lake and accumulate in cracks and depressions on
the uneven ice surface. During the semi-solid ice melting phase in the
summer months, solar heating of the darker particles results in the downward
movement of these aggregates deeper into the lake ice. When the ice re-solidifies,
ice aggregates and associated microbial communities become imbedded in
the ice matrix. The vertical location of the ice aggregate layer represents
a dynamic equilibrium between the downward melting of sediments during
the summer and and about 30 cm per year upward movement of ice from ablation
at the surface and new ice formation at the bottom. A liquid water lens
exists in this layer during the summer (about 150 days) when measurable
solar radiation persists for 24 hours per day. Liquid water, which is a
rare commodity at these lattitudes, supports a viable microbial assemblage
consisting primarily of cyanobacteria (blue-green algae) and bacteria in
Lake Bonney.
The lake ice surface is uneven, with numerous depressions and fissures
that trap soil and gravel blown onto the surface of the lake.
The ice aggregate layer is located 2 m deep in the lake ice. Ice aggregates
were sampled by excavating a large pit (2 x 2 x 3 meters) in the ice and
removing chunks of ice containing the aggregates. At right is a photograph
looking down into the ice pit.
Ice aggregates in situ within the lake ice. This is a photograph
of the ice aggregate layer taken from inside the ice pit. Note the numerous
aggregates, uniformity of the layering, and the vertical light gradient.
Close-up photograph of an ice aggregate imbedded in the lake ice. The bubble
stream emanating from the aggregate is thought to be associated with the
freezing properties of water rather than the biological production of gases.
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