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| Enlarge ImageICY RENDEZVOUS—The National Science Foundation’s two research vessels, Nathaniel B. Palmer (left) and Laurence M. Gould, go bow to stern to exchange equipment, supplies, and personnel just west of Marguerite Bay during an unprecedented cruise into the winter pack ice off Antarctica. (Photo by Peter Wiebe, WHOI) |
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| Enlarge ImageA KRILL'S LIFECYCLE—Krill start life as eggs that sink and hatch in spring. They develop through larval stages as they swim back to the surface, reaching the fourth (furcilia) stage by winter. Krill that hatch at the depth of the Antarctic shelf (300-400 meters) can swim back to surface waters before winter and find phytoplankton to eat before they use up their stored supplies. Furcilia that make it survive their first winter by feeding on algae and zooplankton on the undersurface of pack ice. But krill that hatch in water deeper than 500 meters may starve before they can swim back to the surface, and food. (Illustration by Jayne Doucette, WHOI Graphic Services) |
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| Enlarge ImageMARGUERITE BAY AND ENVIRONS, on the Western Antarctic Peninsula, was the research site for four Southern Ocean GLOBEC cruises. Inset: Antarctica and the southernmost tip of South America, where research vessels depart Punta Arenas, Chile, to cross to Antarctica. (Illustration by Jack Cook, WHOI Graphic Services) |
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| Enlarge ImageSCIENTIFIC ICE CAPADES—The GLOBEC Southern Ocean winter cruises offered scientists unprecedented opportunities for a variety of studies of this remote, largely unexplored region. Above, Frank Stewart (left) of Scripps Institution of Oceanography and Jenny Boe of the University of Nevada collect sea ice cores to study the distribution, activity, and dynamics of sea ice microbes in August 2002. (Photo by Peter Wiebe, WHOI) |
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| Enlarge ImageFACE TO BEAK—A curious Emperor penguin approaches Nancy Ford, a technician from Raytheon Corp., as she collected ice samples of Antarctica in August 2002. (Photo by Peter Wiebe, WHOI) |
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| Enlarge ImageGOING FOR THE KRILL—A major focus of the GLOBEC winter cruises to the Southern Ocean was a shrimp-like crustacean called krill, a crucial link in the food chain that supports the thriving community of life around Antarctica. Researchers studied them in a variety of ways. Kendra Daly (right), Kerri Scolardi (middle), and Jason Zimmerman aboard the Palmer deploy a Tucker trawl to catch live krill for experimental studies to measure krill's rates of feeding, growth, and respiration. (Photo by Peter Wiebe, WHOI) |
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| Enlarge ImageBIOMAPER-II is lifted aboard after a tow through water cleared of ice by the icebreaker Palmer. The vehicle has an acoustic system to detect plankton, a video plankton recorder to take pictures of them, and sensors to measure water properties. (Photo by Peter Wiebe, WHOI) |
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| Enlarge ImageDivers Melanie Parker and Kerri Scolardi (University of South Florida) collect juvenile stages of krill under pack ice. (Photo by Stian Alesandrini) |
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| Enlarge ImageSHIP AT REST—The R/V Laurence M. Gould, docked after the 2002 winter cruise, dwarfs the buildings of the U.S. research outpost at Palmer Station, Antarctica. (Photo by Peter Wiebe, Woods Hole Oceanographic Institution) |
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THE ANTARCTIC ECOSYSTEM
Antarctic seas are extremely productive because phytoplankton grows abundantly during the extended daylight of summer and feeds huge populations of krill. Krill are a key animal in this ecosystem, as food for top predators: whales, penguins, and seals. Winters have little light, no phytoplankton growth, and extremely cold temperatures, but a complex food web links a great variety of ocean animals. | » View Flash
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By Peter H. Wiebe, Senior Scientist
Biology Department
Woods Hole Oceanographic Institution At the extreme ends of the Earth, Antarctica is a vast,
rocky continent, mostly ice-covered and barren. Surrounding
Antarctica, the Southern Ocean is equally vast, cold, and
ice-covered. But unlike the land, it teems with life, ranging
from microscopic plankton to top predators: whales, seals, penguins, fish,
and sea birds.
The region’s fecundity is fueled by 24-hour-a-day
sunlight in summer, combined with ocean currents that bring
essential nutrients. These provide the ingredients for rich
blooms of microscopic marine plants and animals at the base
of the food chain—phytoplankton and zooplankton—that are
similar to those in many productive regions of the world's
oceans.
But there is one big difference in the Antarctic ecosystem.
The food moves swiftly to the very top of the chain through
a crucial link: a shrimp-like crustacean called krill, which
swarm in great numbers in pink oceanic patches that range
from tens of square meters to tens of square kilometers.
The krill connect the microscopic primary producers, which
they eat, to the top predators, which eat them.
This unique and unusually short oceanic food chain is both
strong and vulnerable. It efficiently supports large populations
of big animals. But a small disruption in the chain could
drastically affect the entire ecosystem.
Adding urgency are recent indications of changing conditions
around Antarctica—particularly more frequent calving
of massive icebergs from the continental ice shelf. To manage
and protect this unique environment, we need a more thorough
understanding of the intricacies of the ecosystem and the
potential effects of climate change on it.
Krill are the glue that binds the Antarctic food web, and
20th-century expeditions have learned a great deal about
their life stages, distribution, and abundance—but
only during the warmer, sunlit, ice-free periods of the year.
How do adult and larval krill survive the frigid, sunless
winter—when photosynthesis diminishes essentially to
zero and much of the ocean is covered with pack ice—to
become an abundant food source for large animals the next
spring? To pull back the veil on this critical and previously
shrouded part of the ecosystem, we undertook 11 cruises to
the Southern Ocean, including four unprecedented voyages
into the Antarctic winter ice pack.
Destination: Marguerite Bay
The cruises were part of the Global Ocean Ecosystem
Dynamics Program (GLOBEC), a multiyear, multination, multidisciplinary
series of investigations of several touchstone regions throughout
the world's oceans where marine life and fisheries historically
thrive. Marshalling scientists across several disciplines,
GLOBEC sought to define and measure the myriad factors—oceanic
currents, climatic conditions, seafloor topography, biological
processes, and others—that converge to create and maintain
productive ecosystems. GLOBEC also seeks to provide information
on how vulnerable ocean ecosystems are to climate changes.
Fieldwork for the Southern Ocean GLOBEC program, conducted
between 2001 and early 2003, focused on a broad and relatively
deep (300 to 400 meters) continental shelf region off the
western Antarctic Peninsula, due south of the tip of South
America, from Adelaide Island to Charcot Island. In between
lies Marguerite Bay, which supports a persistent
and large stock of krill and large populations of top predators
that depend on it for food. We suspect that this area may
act as a reservoir for maintaining krill stocks hundreds
of miles away in the Scotia Sea, as far as South Georgia
Island.
Marguerite Bay is surrounded landward by high, snow-covered
mountains and seaward by huge ice shelves. It is dotted by
numerous small islands and persistently covered by sea ice
in winter. Below the sea surface, the bay is gouged by a
trough that cuts diagonally across the continental shelf
and ends in fjord-like features up to 1,600 meters deep in
the interior of the bay. Our principal research goal
was to discern how these features, along with water properties
and currents in the region, conspire to allow krill to flourish
and be retained in the area.
Coping with the chill
Working in the Antarctic fall and winter was challenging,
and the scientists themselves had to learn to adapt. Temperatures
during the fieldwork ranged from 0°C to -28.5°C (32°F
to -19.5°F).
As the late fall turned into winter, bitter cold and near
perpetual night set in. The day was a brief, dim twilight.
Sea ice covering the water made it very difficult to deploy
and tow our instruments to sample ocean waters and marine
life.
Seizing this rare opportunity to conduct research in these
remote locations at these times, a wide range of scientists
had to coordinate a wide range of research spanning the spectrum
of the region’s physics and biology. It was the first
time so many scientists had gathered to study so many aspects
of the Antarctic.
To accommodate the amount and breadth of research, the scientists
had to endure long cruises, 44 to 50 days, in frigid conditions,
and had to use two National Science Foundation research ships
at the same time. One was the 308-foot icebreaker Nathaniel
B. Palmer, which can operate in pack ice and clear a
way. The other was the 230-foot, ice-strengthened research
vessel Laurence M. Gould, which can come in contact
with ice but not force its way through it.
In the fall of 2001, we worked mostly in open water, free
of sea ice. In these conditions, the two ships could
work independently. For instance, scientists studying plankton
and those studying penguins could travel to separate locations
to do sampling needed by each group.
But conditions were much colder when we returned to the
region in the fall of 2002—in fact, the coldest in
20 years of measurements there. Sea ice formed almost instantaneously,
and we were often beset by sea icebergs that made it difficult
or impossible to do our work, or even to get to sampling
locations on a grid we had mapped out to ensure coverage
of the bay. The ships had to remain together in convoy to
get to work sites, with the icebreaker leading the way. Still,
we persisted, huddling in our extreme-weather clothing, with
the ships casting beams of light into the darkness.
Unprecedented observations
The Southern Ocean GLOBEC cruises resulted in a number of “firsts.” An
important accomplishment was to install arrays of long-term
moorings in strategic locations across the continental shelf
and inside Marguerite Bay. These moorings had sensors that
measured water currents, temperature, salinity, and bio-optical
properties (such as the clarity of the water) continuously
over the two-year period between deployment in 2001 and retrieval
in 2003. Such moorings never before had been deployed on
an Antarctic continental shelf area, and they provided the
first-ever measurements of currents there.
Current surveys based on instruments deployed from the ships
revealed large circular eddies swirling on the continental
shelf, which may help keep krill in the bay, where conditions
favor their survival. The surveys showed that water from
the fast-moving Antarctic Circumpolar Current, circulating
just beyond the continental shelf, rides up onto the shelf,
supplying warmer, more saline, nutrient-rich water into the
Marguerite trough and bay. Such intrusions moderate winter
conditions in the bay and enhance its fertility.
Oceanographers also found a previously unknown southward
coastal current that flowed along Adelaide Island, into Marguerite
Bay, and then south along Alexander Island. Our hypothesis
is that deep and recirculating currents in the bay support
krill reproduction, and the coastal current may move krill
progeny along the coast to other areas.
Scientists and engineers also moored pressure-protected
instruments on the seafloor, both on and off the continental
shelf and in Marguerite Bay, to record marine mammal calls
for a year at a time and open a previously inaccessible window
onto cetacean life in this region.
Two automatic weather stations were installed on Kirkland
and Faure Islands in the middle of Marguerite Bay. They continue
to operate today, providing the first continuous meteorological
observations from this region of the Antarctic.
Tools to catch elusive prey
The aim of the biologists aboard the GLOBEC cruises was
to survey krill and other plankton in the water and map where
their populations are. To accomplish this, we needed a combination
of tools and instruments.
Adult krill swim fast and are notoriously good at avoiding
capture by the relatively small nets traditionally used by
oceanographers. To circumvent this, high-frequency acoustics
has become biologists’ tool of choice for surveying
krill. A transducer emits sound into the water. When
sound waves, propagating at 1,500 meters per second, hit
animals in the water, a portion of the energy is scattered
back to the transducer.
The acoustic signals give an indication of how much animal
life is present at different depths, but they cannot identify
what species are present. So, despite the krill’s agility,
we still use nets to collect samples needed to interpret
the acoustic returns.
To map the distribution of krill and other plankton, we
used a towed vehicle, the BIo-Optical Multifrequency Acoustical
and Physical Environmental Recorder, or BIOMAPER-II. It is
equipped with an acoustic system with five frequencies,
a video plankton recorder system (VPR) to take pictures of
the plankton, and sensors to measure water properties.
Robots and divers under the ice
Also
in our toolbox was a specialized net towed behind the ship
at different depths to collect plankton that are later sorted
and identified aboard ship. This net, the Multiple Opening/Closing
Net and Environmental Sensing System (MOCNESS), has a 1-square-meter
mouth opening that can be signaled to open and close separate
nets to capture plankton at different depths without combining
the samples. We equipped it with a strobe light to temporarily
blind the krill so they could not see the net, thus reducing
their ability to avoid it. We used an even larger MOCNESS
trawl to collect the larger mid-water animals, such as shrimp
and fish.
In winter, krill larvae and other plankton often are found
living in or just under the bottom of pack ice. So we sent
a small remotely operated vehicle (ROV) under the ice. It
was tethered to the ship by a cable that transmitted power
to the ROV and data from it. Operators could directly monitor
and direct the vehicle, which was equipped with a VPR; water
temperature, salinity, and depth sensors; and a tracking
device to signal its location.
Finally, teams of divers also did under-ice surveys of the
krill larvae and collected some of them for experimental
studies back onboard ship to measure the krill’s rates
of feeding, growth, and respiration.
A krill’s life
Antarctic krill, Euphausia superba, is the largest
and often the most abundant of five shrimp-shaped euphausiid species
that inhabit Southern Ocean waters. They grow to lengths
of 6.5 centimeters and can live for perhaps
seven to eight years—although most get eaten early
in life, and few, if any, die of old age.
In most ways, the life history of krill is typical of crustaceans.
Life begins as a fertilized egg that hatches into a larva
called a nauplius. Then, as the larva grows, it goes through
a series of larval stages (called metanauplius,
calyptopis, and furcilia—several stages of each). When the larvae’s
exoskeletons become too small, they molt and grow progressively
larger exoskeletons, until they become adults.
But in some other ways, Antarctic krill have an unusual
life history, facing challenges inextricably linked to their
environment. To survive here, they need not only the long
light conditions of summer, but also the icebound sea of
winter.
Scientists on the British Antarctic Expeditions discovered
80 years ago that krill eggs sink to depths of 500 meters
or more before hatching, perhaps to avoid predation near
the surface. (That requirement fits nicely with the depths
of the western Antarctic continental shelf.) But the larvae
eventually have to swim back up to sunlit surface waters
to find enough food (phytoplankton and zooplankton) to grow
through their larval stages to adulthood.
Antarctic water is very cold, only 1°C to -1.8°C,
and the cold temperature slows the krills' larval development. Krill
eggs hatched in the austral spring only make it to the fourth,
or furcilia stage, before winter sets in. By that
time, pack ice covers the water, and no phytoplankton grow.
Neither krill larvae nor adults have stored enough lipids
(fat) to provide energy to see them through until spring. So
how do they make it?
Survival tactics
Two field seasons of the Southern Ocean GLOBEC program in
the Antarctic fall and winter have significantly improved
our understanding of how krill survive the winter. Part of
the answer is that krill larvae that reach the surface congregate
in, or just under the bottom of, pack ice.
In the open ocean, anything that can be used as surface
will be used as one—to grow on, huddle on, feed on,
or get caught on. In the pitted underside of the ice are
phytoplankton, ice algae, microzooplankton, and organic detritus.
We found that larval krill have flexible feeding habits and
can eat this diverse, albeit scarce, buffet.
We found from shipboard studies that at least some of the
larval krill are able to obtain enough food within and under
the ice to meet their nutritional needs during the austral
winter, though they could not find enough food to grow.
But they can delay their growth, molting, and development,
or even suspend them for a time. They can even survive some
period of starvation by digesting some of the carbon and
nitrogen in their own exoskeletons and muscles.
Hot spots and cold spots
As for the adult
krill, we discovered two “hot spots” where
large populations of krill accumulated: Labeauf Fjord in
Marguerite Bay and Crystal Sound just north of Adelaide Island.
The krill in these areas occurred in a dense layer between
50 and 120 meters below the sea surface. We found another
hot spot off the northern end of Alexander Island, a region
of rough bottom topography. We are currently analyzing our
data to explain these hot spots.
Not surprisingly, large numbers of seals, penguins, and
whales also frequented these areas. While scientists on Palmer surveyed and counted krill, sea birds, penguins, seals, and
whales, other investigators aboard Gould focused on experimental
studies of seals and penguins. They temporarily captured
a number of animals to measure their physiological properties
and later released them. At the same time, they attached
tags carrying temperature and pressure sensors and a transmitter
that could send data via satellite back to a computer logging
system.
The data recovered from tagged Crabeater seals and Adelie
penguins revealed their diving and feeding behavior, and
researchers discovered some movements that they hadn't suspected.
For instance, Adelie penguins can dive to much greater depths
and can travel farther and faster than scientists previously
believed. Because the tags recorded top predators' activity
for some time, we were able to see that hot spots of krill
identified during the cruises continued to be focal points
for the predators long after our ships had left the area.
Elsewhere in the region, to our surprise, krill did not
make up the majority of the zooplankton population. Instead,
animals more typical of other ocean ecosystems, such as copepods
(small crustaceans) and pteropods (small planktonic snails)
predominated in the water. We still believe krill are the
most important part in the chain linking primary phytoplankton
producers to the top predators, but in some areas, other
zooplankton play important roles in the ecosystem.
The Antarctic frontier
There is still more
to learn about the ecosystem. What about the adult krill?
Large adults were abundant in 2001, when the weather was
milder. They were largely absent—as
were the larval krill—during the second year, when
conditions were colder. Where did they go? Was this
related to the early onset of pack ice formation in 2002?
Even with the icebreaker, we could not reach several places,
because the ice pack was impenetrable. In these areas, we
suspected the adult krill would be found.
Newer technologies, though, will soon help us meet the challenge
of the Antarctic. For example, autonomous vehicles (AUVs),
robotic vehicles that don't need tethers, and moorings equipped
with biological sensors could gather data under the ice when
ships cannot take us there. Some of these vehicles and moorings
are now being developed.
undefined A more powerful icebreaker now
being developed specifically for Antarctic research will
provide better access to ice-covered seas.
The Antarctic region is a formidable and sometimes forbidding
place in which to work, but it is also a region of great
beauty. Even more, it is susceptible to climate change and
is a linchpin in the forces that cause global climate variability—since
melting polar ice will create cascading effects through the
world. It will be important for us to be able to anticipate
the impacts of climate change on the Southern Ocean ecosystem.
To do that, we anticipate that future research programs will
build on GLOBEC’s legacy of an integrated, multidisciplinary
ecosystem approach, and we will do more work in the harsh,
dark Antarctic winter.
Posted: April 25, 2005 [top] |
