RRoss and LQuetin: Palmer Station Antarctica, LTER Report, 1991
RRoss and LQuetin: Palmer Station Antarctica, 1991
in -Term Ecological Research in the United States, A Network of Research
Sites 1991, K.Van Cleve and S. Martin (ed.), LTER Publication No.11,  
Long-Term Ecological Research Network Office, Seattle, Washington.

RESEARCH SETTING

Research at the Antarctic marine LTER site, Palmer Station and surrounding
waters, will focus on the pelagic marine ecosystem and the ecological
processes which link the extent of annual pack ice to the biological dynamics
of different trophic levels.  In polar environments, the annual advance and
retreat of the pack ice is an important physical feature covering vast areas
of the marine environment.  In the Southern Ocean this seasonal cycle of ice
formation and melting affects about 50 percent of the open sea.

Pack ice not only provides marine habitats distinct from open-water habitats,
but also may be the major physical determinant of temporal/spatial changes in
the structure and function of polar biota.  Thus interannual cycles and/or
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.  The amplitude and phase of interannual
variability in the regional extent of pack ice is not the same in all sectors
of the Southern Ocean.  In the region around Palmer Station the maximum extent
of pack ice varies widely, from near zero to halfway across Drake Passage, and
appears to be on a six- to eight-year cycle (Figure 1).

Although the antarctic marine food web is as complex as any other marine food
web, the links between primary producers, grazers and apex predators
(seabirds, seals and whales) are often short and may involve fewer than three
or four species.  Predators tend to concentrate on a core group of species,
especially some extremely abundant euphausiids (krill) and fish close to the
base of the food web. Our general approach capitalizes on this close coupling
between trophic levels, the limited number of species involved, and the fact
that the dominant predators are seabirds that nest on land and are efficient
collectors of their prey items within the foraging area around the rookeries.

Seabirds collect prey samples for us for various physiological and biochemical
analyses, and aspects of seabird foraging and reproductive biology will be
used as indices of prey abundance and availability.  Because these seabirds
are easily accessible during the spring and summer breeding season, these
indices can be evaluated with greater precision than can the biomass and
distribution of their prey by classical oceanographic methods.  Although
aspects of the biology of dominant consumers may be good indices of prey
abundance and availability, we are also examining the mechanisms behind
changes in prey levels within the foraging area--such as changes in water mass
distribution, variability in reproductive and recruitment success of the prey,
and food availability during critical periods of the prey's life history.

Pack Ice / Biological Community Interactions

In the Southern Ocean, pack ice affects primary production in three
communities: open water communities, ice-edge blooms, and ice algae.  The
stable layer created by melting pack ice and increasing incident radiation in
the spring promotes ice-edge blooms that precede blooms in open water in the
surrounding seas.  Ice-edge blooms are a significant component of total
productivity in the Southern Ocean, and variation in the timing and amount of
ice-edge bloom production from variation in the extent of pack ice will affect
both total primary production and the timing and extent of food availability
to the grazers.  Although low levels of primary production are detectable in
winter, because primary production is limited by light, food availability for
hervibores is intensely seasonal and spatially variable.  Although
macronutrients seldom limit primary production, there may be times during
intense ice-edge blooms or within the sea ice when nutrients do limit
production.

Growth and reproduction of the dominant herbivore, the antarctic krill
(Euphausia superba), appear to be food-limited in most areas of the Southern
Ocean.  The population distributions are suggestive of a close coupling
between krill populations, ice dynamics and the associated ice-edge blooms.
In addition, unlike adult krill, larvae and juveniles cannot survive long
periods of starvation.  This inability coupled with a six-month-long winter
suggests that food availability in the winter must be critical for survival of
larvae and juveniles.  Although questions remain about the quantitative
importance of ice algae, larvae and juveniles do feed on ice algae both winter
and spring; and in winters with heavy pack ice cover, larvae and juveniles are
in better physiological condition than in winters with low ice cover.  We
would predict that better physiological condition should lead to better
survival and recruitment of that year class.

Although recruitment and reproductive success vary with environmental
conditions, for animals with lifespans of more than several years like krill,
total biomass in an undisturbed population can be assumed to be constant.
Thus fluctuations in the mesoscale abundance of antarctic krill are usually
attributed to their redistribution of physical forces, not to intrinsic
features of krill biology.  Abundance of adult krill west of the Antarctic
Peninsula varies with year, and is greatest after winters of heavy ice cover
(Figure 1).

In addition to being the dominant herbivore, antarctic krill is the principal
dietary component of both Adelie and chinstrap penguins (Pygoscelis
adeliae and Pygoscelis antarctica).  Adelies are dependent on pack ice
for winter survival (Figure 1) and during critical stages in the
reproductive cycle.  Although Adelie and chinstrap penguins  
depend on krill for
food during summer, in some years krill are scarce within the foraging area
and they must switch to prey such as the antarctic silverfish (Pleuragramma
antarcticum), with a concomitant decrease in reproductive success.

Antarctic silverfish are one of the most abundant fish in high-antarctic
marine environments, and serve as an important prey item for many consumers,
including south polar skuas (Catharacta maccormicki).  Spawning adults and the
first two age classes of antarctic silverfish occur in cold shelf waters, but
older juveniles and subadults move to the warmer East Wind Drift and are often
found within krill swarms.  Silverfish eat copepods, larval krill and other
euphausiids, but rarely adult antarctic krill.  Year class strength appears to
be higher in years when the ice melts earlier, i.e. warmer years, and thus
should be the inverse of that of antarctic krill. Although the mechanisms
behind this variation are not yet clear, we believe that the abundance of
copepods during the early life history may be critical to survival.  The
abundance of these copepods may in turn be a function of the timing of the
ice-edge blooms, as has been found for shelf-dwelling copepods in the Bering
Sea.

Reproductive success of south polar skuas is linked to the extent of pack ice
through the abundance of subadult (Age Class 8+) antarctic silverfish in the
foraging area.  The lag period between high recruitment in antarctic
silverfish and reproductive success in the south polar skuas is thus about
eight years.  Unlike the penguins that arrive at the rookery ready to lay
eggs, south polar skuas depend on local food resources within the foraging
range of the nesting sites to bring the female into breeding condition.  If
prey availability is low the female will not lay eggs, and the skuas will not
attempt to breed. As a result, reproductive success (number of chicks per
pair) is either zero or 1.0 to 1.5 in south polar skuas (Figure 1).  In
contrast, 0.67 to 1.17 chicks are fledged per pair of Adelies, with no total
year class failures.

Predictions suggest that the effects of global change (climate warming, ozone
depletion, and increased human pressure on resources) will be more pronounced
in Antarctica than in mid-latitudes.  Thus detection of these changes above
the ever-present background of high natural variability will be easier in
Antarctica.  Monitoring the ecological effects of changes in sea-ice extent and
thickness, and studying the processes underlying these effects, as recommended
by the International Geosphere-Biosphere Programme (IGBP), will enable us to
predict the impacts of global warming and attendant changes in the annual
cycle of pack ice on antarctica biota.  An additional concern is the seasonal
thinning of the ozone layer over Antarctica which leads to increases in
ultraviolet (UVB) radiation.  Although research on the effects of changes in
the ozone layer is not within the core effort, we are closely connected to a
separate project examining the effects of UVB on primary production.

One of our principal objectives is to separate long-term (decadal) systematic
trends from large interannual variability in populations.  This ability is
vital if we are to measure the effects of increased human pressure on living
resources.  In fact the Antarctic treaty nations signing the Convention for
the Conservation of Antarctic Marine Living Resources (CCAMLR) agreed not to
fish any living resource (including krill) to such an extent that either the
population itself or consumers depending on that food item were affected.
Using the logic that consumers integrate changes over several seasons, CCAMLR
is monitoring the consumers of krill rather than the krill themselves in an
effort to separate interannual variability in local krill biomass from the
effects of commercial fishing. With the Antarctic marine LTER we expect to
understand the processes underlying interannual variability, and thus be in an
excellent position to separate changes due to natural cycles from those due to
commercial fishing pressure.

Site Characteristics

The LTER site region surrounds Palmer Station, located in a protected harbor
on the southwest side of Anvers Island midway down the Antarctic Peninsula (64
degrees 40 minutes S, 64 degrees W).  Within a 9-kilometer (km) radius of
Palmer Station are two exposed, prominent points, and groups of islands that
extend to the edge of the Bismarck Strait to the southeast.  These islands
have a diverse topography which extends into the intertidal and subtidal zones
surrounding and linking the islands.  Palmer Basin, 22 km southwest of Palmer
Station, is the only deep basin in the area.  The maximum depth is 1,280
meters (m), and the basin is connected to the open ocean on the west side of
Anvers Island, and to the southern end of the strait between Anvers Island and
the Antarctic Peninsula to the northeast.  Both channels are about 450 m deep.

The climate is typically maritime Antarctic, with snow and rain common any
time of the year. The temperature at Palmer is relatively mild, averaging
about -10 deg C in July and 2 deg C in January, with temperature extremes
recorded at -31 deg C and 9 deg C.  Annual rainfall averages about 20 inches
and snowfall about 14 inches.

Mesoscale oceanic circulation patterns are reasonably well-known. A southwest
setting flow (East Wind Drift, EWD) beginning around Anvers Island, feeds into
a cyclonic eddy about 300 km south and seaward of the EWD.  The Antarctic
Circumpolar Current (ACC) flows northeast on the outside of this gyre.  The
extent of sea ice is highly variable in the area, with particularly severe ice
conditions occurring roughly every seven to 10 years and lasting two to three
years.

Palmer is especially well-situated for studies of birds, seals, and other
components of the marine ecosystem, and it is equipped with a large,
well-outfitted laboratory and seawater aquaria. The station consists of two
major and several smaller buildings.  Peak population is 43, including support
personnel.   Many Palmer-based research projects are undertaken in conjunction
with the research ship Polar Duke.  Small boats support research near the
station.

Adelie penguins dominate the seabird assemblage, but the islands and points of
land in the area also support chinstrap penguins and south polar skuas.  About
12,000 pairs of Adelie penguins are distributed among five main island
rookeries within two miles of the station. The Adelies from Palmer Station are
believed to winter in the pack ice of the Bellingshausen Sea near Palmer.
About 600 pairs of south polar skuas reside on about a dozen islands in the
vicinity.  During the summer breeding season, the seabirds depend on resources
in the adjacent deep-water foraging area: 50-km radius for the Adelies, 160 km
for the south polar skuas.  Both antarctic krill and silverfish are found
within Palmer Basin, within the EWD and on the eastern edge of the ACC.

RESEARCH PROGRAM STATUS

Our central hypothesis states that many significant biological processes in
the antarctic marine environment are strongly affected by physical factors,  
particularly the annual advance and retreat of pack ice and variations in
ocean currents.  The overall objectives of the Antarctic marine LTER are:  

-- 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;

-- to quantify the processes that underlie natural variation in these
representative populations;

-- to construct models that link ecosystem processes to physical environmental
variables, and that simulate the spatial/temporal relationships between
representative populations; and

-- to employ such models to preduct and validate the impacts of altered
periodicities in the annual extent of pack ice on ecosystem dynamics.

Each component (seabirds, prey, phytoplankton) has a series of testable
working hypotheses.  Our general approach capitalizes on populations of
seabirds that are easily accessible near Palmer Station during a prolonged
breeding season, and that sample the marine environment.  We are monitoring a
suite of critical biological and environmental variables continuously on a
small spatial scale (Palmer Station and environs)  representing the seabird
summer foraging area, but a long and recurrent temporal scale (every year, the
entire breeding season).  We will use satellite imagery to continuously
monitor certain environmental parameters such as sea ice extent and thickness,
sea surface temperature, and potentially color (fluorescence) 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 marine environment
focuses on the seabirds, the prey of the seabirds, primary production and
hydrographic characteristics of the water column throughout the austral
spring/summer.  Aspects of the foraging behavior of the penguins and south
polar skuas serve as indices of the abundance of the two prey items (krill and
silverfish).  We are monitoring processes (reproduction, recruitment) and
parameters (food availability) that are sensitive to environmental change and
are important in the structure and function of the communities.

To verify that this area is representative of the entire region we are
extending the spatial scale of sampling of prey distribution, abundance, and
physiological condition, water column properties, primary production
estimates, and hydrographic measurements during two types of research cruise.
(1) Time-series cruises (two weeks) are scheduled for the same time frame
every year (late spring) to survey the foraging area of the seabirds and
determine how well the continuous measurements of the nearshore marine
environment represent the basin as a whole.  (2) Process-oriented cruises (six
to eight weeks) at critical times in biological cycles will confirm that local
monitoring of critical environmental parameters will allow modeling of
regional processes, particularly of primary production and oceanic
circulation.

The first of these pairs of cruises are planned for the fall and spring of
years representing the extremes of pack ice cover.  The inherent interannual
variability in the extent of pack ice allows us to "conduct" natural
experiments of the effect of pack ice on the various tropic levels by
monitoring parameters and processes during and after seasons of different pack
ice cover (Table 1).

FUTURE DIRECTIONS

We anticipate that the core program will serve as a nucleus for
satellite or collaborative proposals intended to address questions of
interest not presently perceived as intrinsic to the testing of our
hypotheses.  For example, the initial research plan does not encompass
either the antarctic terrestrial or the marine benthic ecosystems
because exchanges of organic matter/nutrients appear to flow one
way--from the marine to the terrestrial, and from the pelagic to the
benthic realm.  However, resarch on terrestrial and benthic ecosystems
could easily be conducted from Palmer Station.  Thus this LTER site
may be useful for terrestrial and benthic studies in the future.

We expect that cross-site comparisons of control mechanisms of
different populations (biological versus physical) and patterns of
interannual spatial and/or temporal variability in populations or
parameters like nutrients or dissolved organic carbon (DOC) will prove
enlightening.  For example, primary production in the Southern Ocean
is generally believed to be light-limited, although some nutrient
limitation may occur in ice algae and at the end of ice-edge blooms.
Cross-site comparisons of the pattern and control of primary
production and the resulting community structure and function of
nutrient-limited and light-limited ecosystems would be valuable.
.