Using automated overwinter sampling devices, we collected preserved phytoplankton samples from multiple depths in Lake Fryxell, a permanently ice-covered lake in southern Victoria Land, Antarctica. Photosynthetic algae were maintained in a stable water column throughout winter darkness. The algal taxa overwintered 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., were observed in winter, but the species was absent in both summers. Two cryptophyte species were more abundant in winter than in summer. We interpret the increase in algal population size as evidence of heterotrophic growth in winter and of mixotrophic behavior throughout the year. For two chlorophyte species, some portion of the population had a distinctive morphology, for example, many Chlamydomonas subcaudata Wille. formed akinetes, whereas many Chlorella sp. contained a large amount of starch or other storage material. During winter, vegetative cells of the most abundant cyanobacterial species, Phormidium angustissimum West et West, occurred at the depth of the summer maximum and at depths below the oxycline. The presence of P. angustissimum below the oxycline may have resulted from the accumulation of settling cells at a depth with a greater density. In contrast to the settling of P. angustissimum, the general absence of the other algal species below the oxycline in winter indicated that these species were not influenced by overwinter settling.
