Site: McMurdo Dry Valleys LTER
Southern lights over Lake Bonney in April 2008.

The permanently ice-covered lakes studied in the MCM can be considered as an oasis for life in this cold desert because they are some of the few habitats on the Antarctic continent that contain year-round liquid water. Strong microbial linkages have been defined in all of thse lakes and point to the tight coupling of carbon and other nutrients and the reliance of the system on phytoplankton photosynthesis during the short summer period.

Unfortunately, logistical constraints do not allow us to sample these lakes routinely during late summer, autumn, or the long dark winter. The first studies on the summer-winter transition were conducted in 2008 as part of the IPY and showed that phytoplankton primary production ceases in mid-April as sub-ice irradiance reaches a threshold that no longer supports photosynthesis. Despite the cessation of photosynthesis at this time, chlorophyll-a remained high and bacterioplankton productivity continued at high rates.

Results from these extended season studies showed that many of the phytoplankton possesses photochemical systems that resemble those of evergreen trees, in that they maintain their chlorophyll-a content through the winter despite the lack of photosynthesis. Most of the new carbon production during the winter results from carbon from carbon dioxide fixation by chemolithoautotrophic bacteria that obtain their energy from chemical energy rather than sunlight.

Graph for
Primary productivity and chlorophyll-a changes during the transition to polar night in east lobe Lake Bonney, Antarctica.