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“It’s warmer, marine mammals and birds are having massive die-offs,
there are invasive species-in general, it’s changing to a more temperate
ecosystem that’s not going to be as productive.”
“It’s all a good start that people get worried about melting ice and rising
sea levels,” he said. “But we’re now driving a comprehensive change in
the way Earth’s ecosystem works-and some of these changes don’t bode
well for its future.”
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University of Southern California
Public release date: 11-Jan-2008
Greenhouse ocean may downsize fish
By 2100, warmer oceans with more carbon dioxide may no longer sustain
1 of the world’s most productive fisheries, says USC marine ecologist
The last fish you ate probably came from the Bering Sea.
But during this century, the sea’s rich food web-stretching from
Alaska to Russia-could fray as algae adapt to greenhouse conditions.
“All the fish that ends up in McDonald’s, fish sandwiches-that’s all
Bering Sea fish,” said USC marine ecologist Dave Hutchins, whose
former student at the University of Delaware, Clinton Hare, led
research published Dec. 20 in Marine Ecology Progress Series, a
leading journal in the field.
At present, the Bering Sea provides roughly half the fish caught in
U.S. waters each year and nearly a third caught worldwide.
“The experiments we did up there definitely suggest that the changing
ecosystem may support less of what we’re harvesting-things like
pollock and hake,” Hutchins said.
While the study must be interpreted cautiously, its implications are
harrowing, Hutchins said, especially since the Bering Sea is already
warming.
“It’s kind of a canary in a coal mine because it appears to be
showing climate change effects before the rest of the ocean,” he
noted.
“It’s warmer, marine mammals and birds are having massive die-offs,
there are invasive species-in general, it’s changing to a more
temperate ecosystem that’s not going to be as productive.”
Carbon dioxide’s direct effects on the ocean are often overlooked by
the public.
“It’s all a good start that people get worried about melting ice and
rising sea levels,” he said. “But we’re now driving a comprehensive
change in the way Earth’s ecosystem works-and some of these changes
don’t bode well for its future.”
The study examined how climate change affects algal communities of
phytoplankton, the heart of marine food webs.
Phytoplankton use sunlight to convert carbon dioxide into
carbon-based food. As small fish eat the plankton and bigger fish eat
the smaller fish, an entire ecosystem develops.
The Bering Sea is highly productive thanks mainly to diatoms, a large
type of phytoplankton.
“Because they’re large, diatoms are eaten by large zooplankton, which
are then eaten by large fish,” Hutchins explained.
The scientists found that greenhouse conditions favored smaller types
of phytoplankton over diatoms. Such a shift would ripple up the food
chain: as diatoms become scarce, animals that eat diatoms would
become scarce, and so forth.
“The food chain seems to be changing in a way that is not supporting
these top predators, of which, of course, we’re the biggest,”
Hutchins said.
A shift away from diatoms towards smaller phytoplankton could also
undermine a key climate regulator called the “biological pump.”
When diatoms die, their heavier carbon-based remains sink to the
seafloor. This creates a “pump” whereby diatoms transport carbon from
the atmosphere into deep-sea storage, where it remains for at least
1,000 years.
“While smaller species often fix more carbon, they end up
re-releasing CO2 in the surface ocean rather than storing it for long
periods as the diatom-based community can do,” Hutchins explained.
This scenario could make the ocean less able to soak up atmospheric
carbon dioxide.
“Right now, the ocean biology is sort of on our side,” Hutchins said.
“About 50 percent of fossil fuel emissions since the industrial
revolution is in the ocean, so if we didn’t have the ocean,
atmospheric CO2 would be roughly twice what it is now.”
Hutchins and colleagues are doing related experiments in the north
Atlantic Ocean and the Ross Sea, near Antarctica. The basic dynamics
of a greenhouse ocean are not well understood, he noted.
“We’re trying to make a contribution by doing predictive experimental
research that will help us understand where we’re headed,” he said.
“It’s unprecedented the rate at which things are shifting around.”
The researchers collected the algae samples from the Bering Sea’s
central basin and the southeastern continental shelf. They incubated
the phytoplankton onboard, simulating sea surface temperatures and
carbon dioxide concentrations predicted for 2100.
Each of these variables was tested together and independently. Ratios
of diatom to nanophytoplankton in manipulated samples were then
compared with those in plankton grown under present conditions.
The scientists found that photosynthesis in greenhouse samples sped
up two to three times current rates. However, community composition
shifted from diatoms to the smaller nanophytoplankton.
Temperature was the key driver of the shift with secondary impacts
from the increased carbon dioxide concentrations, according to the
study.
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Hutchins and Hare’s collaborators were Karine Leblanc of the Centre
National de la Recherche Scientifique, in France; Giacomo DiTullio,
Peter Lee and Sarah Riseman of the College of Charleston; Raphael
Kudela of the University of California at Santa Cruz; and Yaohong
Zhang of the University of Delaware.
The National Science Foundation supported the research.
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