Jellyfish-Like Creatures May Play Major Role in Fate of Carbon Dioxide in the Ocean


June 30, 2006

Transparent jellyfish-like creatures known as salps, considered by
many a low member in the ocean food web, may be more important to
the fate of the greenhouse gas carbon dioxide in the ocean than
previously thought.

In the May issue of Deep Sea Research,
scientists report that salps, about the size of a human thumb, swarming
by the billions in “hot spots” may be transporting tons of carbon per
day from the ocean surface to the deep sea and keep it from re-entering
the atmosphere. Salps are semi-transparent, barrel-shaped marine
animals that move through the water by drawing water in the front end
and propelling it out the rear in a sort of jet propulsion. The water
passes over a mucus membrane that vacuums it clean of all edible
material.

The oceans absorb excess carbon dioxide from the
atmosphere, some from the burning of fossil fuels. In sunlit surface waters,
tiny marine plants called phytoplankton use the carbon dioxide to
grow. Animals then consume the phytoplankton and incorporate the
carbon, but most of it dissolves back into the oceans when the animals
defecate or die. The carbon can be used again by bacteria and plants, or can return
to the atmosphere as heat-trapping carbon dioxide when it is consumed and respired by animals.

Biologists Laurence Madin of Woods Hole Oceanographic Institution
(WHOI) and Patricia Kremer of the University of Connecticut and
colleagues have conducted four summer expeditions to the Mid-Atlantic
Bight region, between Cape Hatteras and Georges Bank in the North
Atlantic, since 1975. Each time the researchers found that one
particular salp species, Salpa aspera, multiplied into dense swarms
that lasted for months.

One swarm covered 100,000 square kilometers (38,600 square miles) of
the sea surface. The scientists estimated that the swarm consumed up to
74 percent of microscopic carbon-containing plants from the surface
water per day, and their sinking fecal pellets transported up to 4,000
tons of carbon a day to deep water.

“Salps swim, feed, and produce waste continuously,” Madin said. “They
take in small packages of carbon and make them into big packages that
sink fast.”

In previous work, Madin and WHOI biologist Richard Harbison found that
salp fecal pellets sink as much as 1,000 meters (3,280 feet) a day. The
scientists also showed that when salps die, their bodies also sink
fast—up to 475 meters (1,575 feet) a day, faster than most pellets. If salps are really a
dead-end in the food web and remain uneaten on the way down, they could
send even more carbon to the deep.

Salpa aspera swims long distances down in daylight and back up at night
in what is known as vertical migration. Madin, Kremer and colleagues
Peter Wiebe and Erich Horgan of WHOI and Jennifer Purcell and David
Nemazie of the University of Maryland found that the salps stay at
depths of 600 to 800 meters (1,970 to 2,625 feet) during the day,
coming to the surface only at night.

“At the surface,” Madin said, “salps can feed on phytoplankton. They
may swim down in the day to avoid predators or damaging sunlight. And
swimming up at night allows them to aggregate to reproduce and multiply
quickly when food is abundant.”

Because of this behavior, salps release fecal pellets in deep water,
where few animals eat them. This enhances the transport of carbon away
from the atmosphere.

In 2004 and 2006, Madin and Kremer studied salp swarms in a different
ecosystem, the Southern Ocean near Antarctica. Some scientists have
reported larger salp populations there in warmer years with less sea
ice. If this proves true, and if Antarctica’s climate warms, salp swarms
could have a greater effect on phytoplankton and carbon in the Southern
Ocean ecosystem.

Funding for this study was provided by the National Science Foundation,
National Oceanic and Atmospheric Administration, and the Access to
the Sea program at Woods Hole Oceanographic Institution.