A population is a group of organisms of the same species. Like canaries in the coalmine, changes in populations of organisms can be important indicators of environmental changes.

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 overwinteringalgal community.

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 Samples were obtained and preserved in situ through the winter season using automated time series samplers. The samplers were composed of an array of syringes suspended at a series of depths on an in situ mooring. Each syringe was hydraulically connected to a manifold and linked to a gear pump. A floating piston in each syringe was hydraulically driven to draw water at prescribed time intervals and introduce Lugol's solution (KI, I and acetic acid) as a preservative.
 Algae were identified and enumerated with the use of a Nikon Diaphot inverted microscope under oil immersion at 1000x magnification. Subsamples were allowed to settle in chambers following the method of Utermohl. Strip counts were made with a minimum of 100 individuals of the most common taxa enumerated. Identifications were based primarily upon keys by Prescott, Tikkanen, and Seaburg et al. Cells with intact chloroplasts were counted as "live". Visibly-degraded cells were not counted. We noted qualitatively that few visibly degraded cells were found in the winter samples, which is comparable to the summer samples. Subsamples were also examined at 100x to enumerate protozoans. 
In order to  evaluate changes in cell volume between winter and summer, cell volumes were determined by measuring cell dimensions and approximating cell shape to geometric shape. Dimensions of 10 cells of each taxa were measured in four samples from different depths collected in April 1990. In a previous study, 100 cells of each taxa were measured to determine variance in size.


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