Acceptance and utilization of LTER data requires that:
FILE NAME: PRINCIPAL INVESTIGATOR: OTHERS:
KEYWORDS: lake, limnology, phytoplankton, density, phylum
ABSTRACT: 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.
VARIABLES: location, date, depth (m), phylum, species density (cells/mL),
phylum density (cells/mL), phytoplankton density (cells/mL)
RESEARCH LOCATION: Lake Fryxell is located in Taylor Valley in the
McMurdo Dry Valleys of southern Victoria Land, Antarctica. The McMurdo Dry
Valleys is the largest of the desert oases on the Antarctic coast and is located
in the Transantarctic Mountains along the western edge of McMurdo Sound. The valleys
run perpendicular to the coast and contain numerous perennially ice-covered lakes.
Lake Fryxell is fed by nine major inflow streams which drain Canada and
Commonwealth Glaciers and glaciers in the Kukri Hills. The streams are neutral
pH and range in conductivity from 23 uS to 200 uS, with solutes accrued by
dissolution of marine aerosols, calcite and primary weathering of silicate
rocks. Nutrients are gained through weathering of apatite and dissolution of
nitrate from atmospheric deposition. Lower nutrient concentrations occur in
streams with abundant algal mats. Streamwater DOC concentrations are as high
as 9 mg C/L during the initial flow and then decrease to 1 mg C/L or less.
Fulvic acid accounts for only 7-10% of streamwater DOC, suggesting that
streamflow may contribute labile organic substrates to the lake.
The annual cycle of dry valley ecosystems is controlled by extreme changes
between summer and winter. During summer, beginning at the end of
October, there is continuous sunlight and temperatures above freezing occur for
periods of several days. These summer conditions support growth of
photosynthetic algae in the pelagic zone and benthos of the lake, and in many
meltwater streams. The winter period of total darkness begins in mid-April and
ends in mid-August. The ice-cover protects the lake ecosystem from the extreme
winds and cold temperatures of winter. Stream algal mats survive winter in a
dormant "freeze-dried" state. In the "spring" (mid-August to late October) and
"fall" (mid-February to mid-April) the valleys are in twilight during portions
of the day, and air temperatures are warmer than in winter, but always below
freezing.
In Lake Fryxell, streamflow mixes with a 1-2 m layer just below the ice-cover.
Below this layer, the water column is very stable and chemical profiles are
consistent with molecular diffusion. The major ions and DOC increase with depth.
Bottom waters at 18 m have a salinity of 25% of seawater and a DOC concentration
of 30 mg C/L. The DOC profile and high DOC concentration in sediment is a
significant DOC source. Fulvic acid accounts for 20-24% of the DOC and the
fulvic acid chemistry is uniform except for higher sulfur content at depth.
Photosynthesis by phytoplankton begins in September and continues into fall.
During summer, about 1% of the surface light penetrates the ice cover and light
intensity decreases exponentially to values about 0.3uM/m^2/s at 9 m. In this
euphotic zone, O2 concentrations are supersaturated due to freeze out of O2
carried by streamwater and to algal production of O2. A sharp oxycline occurs
at about 9.5 m, and below the oxycline H2S concentrations increase with depth.
METHODS: 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.
TIMING: Samples were obtained once in 1990 (April 15) and on six dates in
1991. The sampling device for the 1990 samples was deployed on December 15,
1989 and recovered on December 24, 1990, with sampling depths of 5.03, 5.0,
6.35, 6.95, 7.54, 8.15, 8.9, 9.38, 9.97, and 10.59 m. On December 25, 1990, two
sampling devices were deployed. One collected samples on April 16, June 24, and
September 6, 1991 at depths of 7, 8, 9, 10.25, 12, and 15 m. The other device
collected samples on August 1, 15, and 30, 1991 at depths of 5.1, 5.71, 6.32,
6.93, 7.54, 8.15, 8.90, 9.37, 9.98, and 10.59 m. Data are presented at 0.5 m
depth intervals centered on the sampling depth.
CITATIONS: McKnight, D.M, Howes, B.L., Spaulding, S.A., Taylor, C.D. and D.D. Goehronger.
199?. Phytoplankton dynamics in a stably stratified Antarctic lake during
winter darkness. In press.
COMMENTS:
STATUS: Public Access (Type 1).
VARIABLE DESCRIPTION: LOG:
NOTE: Data contained in these files has been subjected to quality
control standards imposed by the investigator. The user of this data
should be aware that, while efforts have been taken to ensure that these
data are of the highest quality, there is no guarantee of perfection
for the data contained herein and the possibility of errors exists. If
you encounter questionable data, please contact the MCM LTER data manager
(; (303)492-4639) so that the data can be
corrected or qualified. Thus, these data may be modified and future
data will be appended.
VARIABLE TYPE UNITS MISSING VALUE INDICATOR MINIMUM MAXIMUM PRECISION
Location String None Required entry n/a n/a n/a
Latitude String DD-MM-SS South Required entry n/a n/a n/a
Longitude String DDD-MM-SS East Required entry n/a n/a n/a
Description String None Null n/a n/a n/a