Winter Phytoplankton Densities during the 1990-1991 season

Using automated overwinter sampling devices, preserved phytoplankton samples
were collected from multiple depths in Lake Fryxell, a permanently ice-covered lake in
southern Victoria Land, Antarctica. Photosynthetic algae (i.e. algae possessing chloroplasts)
are maintained in a stable water column throughout winter darkness. The algal taxa
"overwinter" in different ways, in a species specific manner. Typical vegetative cells were
the most abundant form for all species found in the water column. Populations of one
chlorophyte, Stichococcus sp., and two cryptophyte species increased during winter. We
interpret the increase in algal population size as evidence of wintertime heterotrophic
growth, and mixotrophic behavior in the context of the entire year. For two chlorophyte
species some portion of the population had distinct morphology, e.g. akinetes for
Chlamydomonas subcaudata and cells containing a large amount of starch or other storage
material for Chlorella sp.. During winter, vegetative cells of the most abundant species of
cyanobacteria, Phormidium angustissimum, occurred at the depth of the summertime maximum and
at depths below the oxycline, which may represent a "false bottom". Other than this false
bottom and the absence of diatoms, settling did not appear to influence the overwintering
algal community.