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Transparent Animal May Play Overlooked Role in the OceanSwarming by the billions, gelatinous salps transport tons of carbon to the depths |
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| Enlarge ImageSalps are transparent, tubular, jelly-like animals that live in all oceans but are seldom seen. This species, Salpa aspera, can multiply into extensive swarms when conditions are right. Feeding on microscopic plants, they remove significant quantities of organic (carbon-containing) material from upper ocean waters. The brown spot is the salp's stomach. (Photo by Laurence Madin, Wood Hole Oceanographic Institution) |
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| Enlarge ImageSalps reproduce in two ways and have two adult forms: a solitary form, as in the photo above, and a linked form, seen here. In this species, Salpa aspera, the linked form is a chain that can contain nearly 100 individuals. The brown spots are the individual salps' stomachs. (Photo by Laurence Madin, Woods Hole Oceanographic Institution) |
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| Enlarge ImageSalpa aspera forms large, periodic summer swarms in the northwest Atlantic Ocean between Massachusetts and North Carolina. Differently shaded areas on this map show the locations and extent of salp swarms in each of two summers. Numbers 1-3, 4-5, and letters A-B indicate locations having dense salps in three additional summers. Some swarms covered more than 38,000 square miles of the ocean. Contour lines show bottom depths. (Map courtesy of Laurence Madin, Woods Hole Oceanographic Institution) |
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» Dive and Discover's Expedition 10: Studying Salps off Antarctica
Dive and Discover's Expedition 10 explored one of the coldest, most remote places on our planet—the Southern Ocean surrounding Antarctica. Using scuba diving and other sampling techniques, scientists studied the mysteries of salps—transparent jelly-like creatures that are important to the entire Antarctic food chain.
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Salps don’t get much respect. They’ve been around for millions of years, but hardly anyone even knows they exist.
Even many who have heard about these transparent, jelly-like creatures
consider them a dead-end in the ocean food web: They cruise around,
vacuuming up microscopic plants, but don’t get eaten by other
animals, making them a marine equivalent of inedible cows.
But in the May issue of Deep Sea Research,
scientists report that salps may play an important and overlooked role
in determining the fate of the greenhouse gas carbon dioxide in the
ocean. Swarming by the billions in salp “hot spots,” they transport
tons of carbon per day from the ocean surface to the deep sea and keep
it from re-entering the atmosphere, the scientists say.
Biologists Laurence Madin of Woods Hole Oceanographic Institution and
Patricia Kremer of the University of Connecticut led researchers to the
Mid-Atlantic Bight region (between Cape Hatteras and Georges Bank) in
four summers since 1975, and each time found that one species, Salpa aspera, multiplied into dense swarms that lasted for months.
One swarm covered 38,600 square miles (100,000 square kilometers) of
the sea surface, containing perhaps trillions of thumb-sized salps. 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.
Sending carbon to the depths
The oceans absorb carbon dioxide from the
atmosphere, including some of the carbon dioxide emitted by fossil-fuel burning. In sunlit surface waters, tiny
marine plants⎯phytoplankton⎯use it to grow. Animals then consume
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 plants, or it can return to the air as heat-trapping carbon
dioxide.
Salps send carbon down a third path by making heavy, fast-sinking fecal
pellets that efficiently ship carbon to the deep sea, where it is
sequestered from the atmosphere.
“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 3,280 feet (1,000 meters) a day, far faster than most pellets. The
scientists also showed that when salps die, their bodies also sink
fast—up to 1,575 feet (475 meters) a day. If salps are really a
dead-end in the food web and remain uneaten on the way down, they send even more carbon to the deep.
Night migrations to the surface
Salpa aspera swims long
distances down in daylight and back up at night. Madin and 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 1,970 to 2,625 feet (600 to 800 meters) 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 continues to warm, salp swarms
could have a greater effect on phytoplankton and carbon in the Southern
Ocean ecosystem.
— Kate Madin
[The author, who has a Ph.D. in invertebrate zoology, is
married to Madin and occasionally studies salps with him.]
Funding for this study was provided
by the National Science Foundation, National Oceanic and Atmospheric
Administration, and the Rinehart Initiative for Access to
the Sea at Woods Hole Oceanographic Institution.
Posted: June 30, 2006 [top] |
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