E.E.Hofmann: The Antarctic Marine LTER, BIOMASS Newsletter, 1991
E.E.Hofmann: 
The Antarctic Marine LTER: Studying an Ecosystem Dominated by Ice
BIOMASS Newsletter 13(1):3 (1991)
Palmer LTER Contribution #02


The Antarctic LTER Program will focus on the pelagic marine
ecosystem and the ecological processes that link the extent of annual pack ice
to the biological dynamics of different trophic levels.  Because pack ice is
postulated to be the major physical determinant of temporal/spatial changes
in the structure and function of polar biota, interannual cycles and trends in
the annual extent of pack ice are likely to have significant effects on all
levels of the food web.

Palmer Station and the surrounding waters have been chosen by the National
Science Foundation as the first Antarctic Long Term Ecological Research (LTER)
site.  Palmer Station is the smallest of the three U.S. Antarctic research
stations managed by the NSF.  The Station is located in a protected harbor on
an island midway down the Antarctic Peninsula (64 degs 40 min S, 64 degs 00
min W), and more than 600 miles from the tip of South America.  The climate in
the Palmer region is typically maritime Antarctic, with snow and rain common
anytime of the year.  The temperature at Palmer Station is relatively mild,
averaging about -10 deg C in July and 2 deg C in January. The extent of sea ice
in the region is highly variable, with particularly severe ice conditions
occurring at about 7-to-10-year intervals.  Within the last two decades, each
interval has lasted 2 to 3 years.  The areal extent of ice can vary by 25%
between years

There are several groups of islands with seabird rookeries between Palmer
Station and Palmer Basin, the only deep basin in the area.  Species of
interest for the LTER program include the Adelie penguin (Pygoscelis adeliae),
which dominates the seabird assemblage, and the south polar skua (Catharacta
maccormicki), a gull-like migratory sea-bird.  During the breeding season,
penguins and south polar skuas forage in the upper water column within a
100-km radius of the rookeries, moving southwest into Palmer Basin and west
into the open ocean.  The summer foraging region and wintering grounds of the
seabirds help define the scale of the region to be investigated.

Effects of the ice cycle

The Antarctic LTER program will focus on the pelagic marine ecosystem and the
ecological processes that link the extent of annual pack ice to biological
dynamics of different trophic levels.  In these polar waters the annual cycle
of ice formation and melting affects about 50% of the open sea.  Because pack
ice is postulated to be the major physical determinant of temporal/spatial
changes in the structure and function of polar biota, interannual cycles and
trends in the annual extent of pack ice are likely to have significant effects
on all levels of the food web, from total annual primary production to
breeding success in seabirds.  For example, recent studies suggest a
connection between the extent of winter sea ice and the overwintering or
reproductive success of the seabirds and their prey.  Both Adelie penguins and
Antarctic krill (Euphausia superba), which is the penguins' favored prey, are
positively affected when ice cover is most extensive.  Conversely,
reproductive success in south polar skuas appears to be connected to the
abundance of one age group of the Antarctic silverfish (Pleuragramma
antarcticum), which seems to do better when ice cover is less extensive.

The overall objectives of the Antarctic marine LTER are (1) to document
interannual variability in the development and extent of annual pack ice, and
in life-history parameters of primary producers and populations of key species
from different trophic levels; (2) to quantify the processes that underlie
natural variation in these representative populations; (3) to construct models
that link ecosystem processes to physical environmental variables, and that
simulate the spatial/temporal relationships between representative
populations; and (4) to employ such models to predict and validate the impacts
of altered periodicities in the annual extent of pack ice on ecosystem
dynamics. 

The general approach of the LTER program capitalizes on populations that are
easily accessible near Palmer Station during prolonged breeding seasons, and
that sample the surrounding marine environment.  Beginning in October 1991
(austral spring) a suite of critical biological and environmental variables
will be measured continuously on a small spatial scale representing the
seabird summer foraging area, but on a long and recurrent temporal scale
(every year, for the entire breeding season).  Satellite imagery will provide
a means to continuously monitor certain environmental parameters such as sea
ice extent and thickness, sea surface temperature, and ocean color on larger
spatial scales and throughout the year.  In addition, automatic weather
stations at several selected positions in the regions will continuously
monitor atmospheric pressure, wind speed and direction, and air temperature.
Research at Palmer Station and in the surrounding nearshore waters will focus
on the seabirds, the prey of the seabirds, primary production, and
hydrographic characteristics of the water column.  The LTER program will
provide measurements of processes (reproduction, recruitment) and parameters
(food availability) that are sensitive to environmental change and are
important in the structure and function of the communities.  The inherent
interannual variability in the extent of pack ice allows for natural
experiments on the effects of different pack ice cover on the various trophic
levels.

The spatial scale of sampling prey distribution, abundance, and physiological
condition, water column properties, primary production estimates, and
hydrographic measurements will be extended during two types of research
cruises: (1) time-series cruises in the late spring; and (2) process-oriented
cruises at critical times in biological cycles.  The results obtained from
these cruises are essential for verification of the models of regional
processes, such as primary production, oceanic circulation, and the
biological/physical models of prey abundance.