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.

Summer Phytoplankton Densities 1992-2001


  Phytoplankton were collected over four austral summers (1991-92 through1994-95) to examine seasonal and annual fluctuation in species composition and biovolume in Lakes Fryxell, Hoare, West Lake Bonney, and East Lake Bonney. All of these lakes are perennially ice-covered lakes located in the Dry Valleys of South Victoria Land, Antarctica.
 The phytoplankton consisted primarily of cryptophyte and chlorophyte flagellates, and filamentous cyanobacteria. Some common taxa were vertically stratified (Oscillatoria limnetica, Phormidium angustissimum, Pyramimonas sp., Oscillatoria sp.), while others showed no distinct vertical stratification (Chlamydomonas subcaudata, Cryptomonas sp.). The stratification of the phytoplankton reflects the gradients of nutrients and light, and the stability of the water column.

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 Discrete samples for phytoplankton enumeration were collected from the oxygenated portion of the water column (below the bottom of the ice to a depth of 10 m) at 0.5 m intervals. Sampling was done primarily between the hours of 14:00 and 20:00 (during the austral summer, the illuminated period is 24 h/day) by either peristaltic pump or Kemmerer bottle. Samples were collected in 1 l bottles and preserved immediately with Lugon's solution (American Public Health Association, 1985). Identification and counts were made with an inverted microscope by the method of Utermohl (1958). At least 100 individuals of the most numerous algae were counted per sample at 100x magnification. The total number of individuals counted was dependent on the number of taxa, but ranged between 200 and 500. Counting error ranged between 13 and 26%, depending on species. Algal species identifications were made using Geitler (1932), Seaburg et al. (1979) and Prescott (1962). Cell volumes were estimated for dominant taxa by measuring cell dimensions of 50-100 individuals and using closest geometric formulas of additional dates and depths to determine changes in cell volume over time. For rare taxa, volume estimates were made from fewer cell measurements.
Primary productivity was measured using the method of Strickland and Parsons (1972). In situ 24 h incubations were made in triplicate 300 ml light and duplicate 300 ml dark bottles with Na14CO3 (3 uCi per 300 ml, New England Nuclear). Following 24 h incubation, samples were well shaken and filtered through Whatman GF/F filters in the dark. The filters were placed in scintillation vials and acidified with 1 ml of 5% acetic acid in methanol to remove [C14 carbonates. The [C14] fixed by biological activity was determined in Aquasol (New England Nuclear) by liquid scintillation.
 Samples for nitrate and orthophosphate were frozen within several hours of collection, later filtered through 0.45 um Nucleopore filters and analyzed by air-segmented continuous-flow absorption spectrophotometry (Alpkem RFA-300) (Antweiler et al., 1993). Chlorophyll extractions were made in 95% ethanol (Jesperson and Christoffersen, 1987) and measured in a Turner Designs Model 10 Fluorometer.


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