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| Enlarge ImageIn October of 1997, pastel Arctic sky illuminates the tiny, isolated town composed of the Canadian icebreaker Des Groseilliers, frozen into the ice for the year, and outlying laboratory structures. Inset: Drifting within the ice, the ship traveled over 1,739 miles (2,800 kilometers) during the year. (Courtesy of SHEBA Project Office) |
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| Enlarge ImageDrifting within the ice, the ship traveled over 1,739 miles (2,800 kilometers) during the year.
(Illustration by Jayne Doucette, WHOI Graphic Services) |
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| Enlarge ImageThe four major species of copepods in the Beaufort Sea all have different sizes, different life cycles, and different prey. L to R: Metridia longa (~2.5 millimeters), Calanus glacialis (~4mm), Calanus hyperboreus (~7mm). The smallest, Oithona similis (0.5mm) is below the center. The largest species, Calanus hyperboreus, is a critical link in the Arctic food web, eating phytoplankton and microzooplankton when the returning spring light triggers their growth. They are eaten in turn by many larger animals. (Photo by Carin Ashjian, WHOI) |
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| Enlarge ImageA view from the ship Des Grosielliers during Arctic spring, showing the blue biology lab ("Blue Bio"). The long straight line is a newly opened crack, or lead, in the melting ice, coated with a skin of freshly frozen ice. (Photo by Carin Ashjian, WHOI) |
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| Enlarge ImageTwin plankton nets, called 'bongo nets', hanging over the side of the ship. The nets are towed through the water to capture copepods, which are counted to track their abundance over a yearly cycle. (Courtesy of Carin Ashjian, WHOI)
(Courtesy of Carin Ashjian, Woods Hole Oceanographic Institution) |
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CARBON ALSO FLOWS THROUGH IT
The Arctic ecosystem has a unique, complex food web that is fashioned by its distinctive plankton, animal species, and environmental factors. Carbon also cycles through the web from atmosphere to seawater and back. Phytoplankton and algae take up carbon dioxide from seawater and transform it into the organic carbon of their tissue. Then it flows through successive levels of eating animals that convert their prey’s carbon into their own tissues or into sinking fecal pellets. Along the way, some carbon dioxide escapes back to the atmosphere through the organisms' respiration. (Illustration by Jayne Doucette, WHOI) | » View Flash
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By Carin Ashjian
Associate Scientist
Biology Department
Woods Hole Oceanographic Institution Capped with a formidable ice and snow cover, plunged into total
darkness during the winter, buffeted by blizzard winds, and bitterly
cold, the Arctic Ocean is one of the most inaccessible and yet
beautiful environments on Earth. Life here endures some of the greatest
extremes in light and temperature known to our planet. Yet despite
these inhospitable conditions, the Arctic Ocean is teeming with life.
Great polar bears roam the Arctic ice and swim the Arctic seas.
Supporting these top predators is a complex ecosystem that includes
plankton, fish, birds, seals, walruses, and even whales. At the center
of this food web, supporting all of this life, are phytoplankton and
algae that produce organic material using energy from the sun.
The Arctic’s extreme environmental conditions have limited our
opportunities to study this complex food web. Expeditions to the remote
Arctic are difficult and expensive. When we can get there at all, it is
usually only in summer. Such gaps in our observations have compromised
our ability to understand the food web’s intricacies and
vulnerabilities—at a time when the ecosystem appears to be increasingly
vulnerable.
Scientists now know that warming temperatures are
affecting the Arctic Ocean, producing changes that may have cascading
effects on the Arctic’s interlinked and delicately balanced food web.
Changes in the food web not only threaten life in the Arctic region,
they also could have impacts on Earth’s climate. Populations of Arctic
plankton for example, not only provide food at the base of the food
web, they also convert carbon dioxide from the atmosphere into organic
matter that eventually sinks to the ocean bottom—effectively extracting
a heat-trapping greenhouse gas from the atmosphere.
Life and light springs eternal
Every spring, after the long dark night of Arctic winter, the sun
reappears over the horizon. Then, a cumulative sequence of events
begins, and life in the Arctic springs into action.
With
each day longer than the previous one, light begins to penetrate
through the thick cover of snow and ice to the undersurface of the ice,
where ice algae begin to grow, like mold on a damp ceiling. One green,
string-like form, Melosira, grows long, hanging under the ice
like Spanish moss. It eventually detaches from the under-ice surface
and sinks to the seafloor where it is consumed by the animals living
there.
As days lengthen, light and warmth increase, and the
winter snow cover that accumulated over the ice begins to melt. Once
the snow melts, enough light can penetrate through the ice to spur the
growth of phytoplankton— very small, drifting, plantlike organisms that
live in the water. They become available as food for higher organisms
in the food web, the zooplankton—tiny marine animals that, in turn, are
eaten by larger animals, from fish to jellyfish to whales.
A rich and vulnerable ecosystem
Nowhere is the plankton ecosystem less well-understood than in the
Arctic Ocean. Without more detailed knowledge about the workings of
these ecosystems and the life histories of the individual life forms in
them, we cannot predict how they will be affected by climate changes.
But those changes already appear to be happening.
Scientists
have documented dramatic shifts in Arctic ice cover, water temperature
in the Arctic Ocean, and the atmosphere above it—all potentially due to
the effects of a warming climate. Such changes are likely to affect,
and may alter, the Arctic food web and ecosystem. They may change the
amounts of water, nutrients, and plankton coming into the Arctic Basin,
or change the timing of spring growth.
The great bowhead
whale, for example, depends on plankton patches found along the
northern coast of Alaska for food during its migrations between the
summer feeding grounds off of Arctic Canada and its overwintering
grounds in the Pacific. Climate-induced changes in the availability of
these plankton patches may have dramatic impacts on the whales, with
either more or less food available along their migration route.
Populations of Arctic plankton are a conduit for the uptake,
processing, and transformation of carbon dioxide. Warming related
changes in the Arctic environment, such as ice cover, may have impacts
on this planktonic conduit. Changes in the amount of carbon that flows
and cycles through this food web will change the amount of carbon
retained in the ocean or respired back into the atmosphere. These
changes may fundamentally alter the structure of Arctic ecosystems.
The SHEBA ice camp
To begin to shed light on the dimly understood Arctic ecosystems and
food webs, I lived during parts of 1997 and 1998 at the SHEBA (Surface
HEat Budget of the Arctic) ice camp, a major science encampment both on
and in the ice in the part of the Arctic Ocean known as the Beaufort
Sea, where I studied seasonal and life cycles of the Arctic plankton
ecosystem. The heart of the camp was the Canadian Coast Guard
icebreaker Des Groseilliers—a big, bright red ship
conspicuously and intentionally frozen into the ice for the entire
year. It served as a comfortable and opulent (for ice camps!) hotel and
laboratory base. Separate labs were scattered on the ice surrounding
the ship, housed in structures ranging from small tents to a 20-foot
blue metal, bear-proof container, affectionately named the “Blue Bio,”
because it supported biological studies.
Leaving the ship
for the labs held risks for the unwary. During summer months, we
observed firsthand the effects of a warmer climate, as our stable ice
platform melted into an impressive resemblance of Swiss cheese. Life
jackets were required equipment when venturing off the ship onto the
ice—in case we made a false step between the holes on our way to the
labs. In addition, the threat of polar bears was very real, and all
eyes were on watch at all times. Often, the tracks of visiting polar
bears were visible in the snow in the morning. We had several
opportunities to watch these fascinating creatures from the safety of
our warm, red ship.
My studies focused on copepods—small (1
to 7 millimeters-long) crustaceans that are a critically important link
between phytoplankton and larger animals. I concentrated on four
species of copepods that dominate the zooplankton community both in
terms of numbers and biomass (weight). Each has a different size,
different life cycle strategies, and different roles in the food web
marked by the quantity and type of prey that they consume.
Two of the species are fairly large and are members of the genus Calanus, which
are believed to be omnivores (eating both plant-like phytoplankton and
tiny animal-like microzooplankton). One medium-sized species, Metridia, is an omnivore and a voracious consumer of Calanus copepod eggs and juveniles. The fourth is the extremely abundant and very small Oithona, also an omnivore.
An abundance of diverse life
At the SHEBA ice camp in 1997 and 1998, I studied the abundance,
reproduction, growth and development rates, and sizes of these four
copepod species over their seasonal cycles with my colleague Bob
Campbell from the University of Rhode Island. We verified that previous
studies dating back to the 1950s through 1970s underestimated the
biomass of zooplankton in the Arctic Basin. It may be as much as 10
times greater than we previously believed.
We confirmed that Calanus copepods live to be three years old, reproducing during the summer when food is plentiful. Two species (Oithona and Metridia) have more prolonged reproduction that extends over much of the year. The fourth species (C. glacialis) cannot successfully reproduce in the central Arctic, we think, so C. glacialis populations there must be brought in by currents from the surrounding Arctic shelves.
These different life histories may have important consequences for the
species, and they give scientists new insights into understanding the
species’ chances to survive changing seasonal cycles or food
availability that might occur with climate change.
Tracking who eats whom
In an ongoing project, Campbell, Eveleyn and Barry Sherr of Oregon
State University, and I are exploring planktonic food webs in western
Arctic shelves and basins. We are measuring the rates at which the
tiniest animal plankton, called microzooplankton, consume Arctic
phytoplankton. We then measure the rates at which the copepods (which
are middle-sized plankton, or mesozooplankton) consume phytoplankton
and microzooplankton. We also are finding out the food preferences of
the different groups. Do copepods prefer a phytoplankton or a
microzooplankton diet, for example? Do they eat ice algae?
To
do this, we conducted feeding experiments during two six-week-long
cruises to the Arctic in the summer of 2002, on the U.S.C.G. Healy,
the United States’ newest icebreaker—a ship not frozen into the ice!
First, we collected plankton by towing plankton nets to catch the
copepods and determine their abundance in the water. In feeding
experiments, we collect animals and seawater, select a known number of
copepods, and incubate them with their prey to measure how much they
eat in 24 hours.
Then we couple the grazing (feeding) rates
of individuals with their abundances in the water to calculate the flow
of carbon through the food web. This fundamental information is
critical to our basic understanding of Arctic food webs, and it gives
biological modelers the data they need to predict more accurately the
potential impact of environmental and climate change on the fate of
carbon in this ecosystem.
Summering in the ice
In the summer of 2004, we will embark on two more cruises on Healy to
continue this work. We plan to concentrate on investigating the
potential of ice algae as a food source for copepods. We also want to
document whether one of the Calanus copepods (C. glacialis) can reproduce using its stored fat alone, or whether it requires available food.
Every trip to the largely unexplored Arctic seas brings new challenges,
surprises, and insights for researchers. Every trip also contributes to
our understanding of this remote, severe, but very active ocean and its
role in sustaining life on Earth.
Posted: September 15, 2004 [top] |
