What do we know about
communities of organisms thriving around vents and seeps on the
Arctic sea floor?
Vent and Seep Communities
on the Arctic Seafloor
Peter Vogt
Naval Research Laboratory
Washington, DC
Email: vogt@qur.nrl.navy.mil
Deep, under perpetual ice cover, with reduced photosynthesis and
thus little organic matter coming from above, the sea floor of the
central Arctic Ocean is a marine desert, its life more sparse than
in other ocean basins. But photosynthesis is not the only basis
for life: locally, methane and hydrogen sulfide seep from the ocean
floor, supporting dense oases of organisms that depend on bacteria
able to consume these chemicals. Such "chemosynthetic" life does
not directly depend on sunlight and can thrive even at great ocean
depths.
Two basic kinds of oases are now known from the oceans. Hot vents,
first discovered in 1977, are now known from several places along
the Mid-Oceanic Ridge. These seafloor geysers belch out seawater
heated up to 400°C and laden with nutritious chemicals. Dramatic
colonies of large clams, giant tubeworms, and other strange life-forms
have been discovered at some hot vents.
The
second broad category of chemosynthetic oasis is the "cold
seep,"* which usually involves the upward seepage of methane
dissolved in water or as small bubbles. Mud volcanoes and related
cold-seep features form over great sediment accumulations in which
bacteria digest buried organic matter, producing methane as a waste
product. Specially evolved bacteria oxidize the methane, forming
the foundation of a food chain. Different bacteria have evolved
to oxidize the foul-smelling hydrogen sulfide, itself the waste
product of yet other bacteria living below the ocean floor, which
oxidize sulfate ions of seawater origin. The conspicuous and, by
bacteria standards, large sulfur bacteria (Beggiatoa spp.)
form thin, snow-like mats on the seafloor where seepage takes place.
Bacterial mats form at hot vents also, but Beggiatoa is common
at cool oceanic seeps and some non-seep environments where hydrogen
sulfide rises close to the seafloor and oxygen is present in the
water.
|
|
Neither hot nor cold vents had been found in the Arctic until,
in 1995 and 1996, a team of American, Norwegian, and Russian scientists
discovered an active cold-seep mud volcano on the continental margin
west of the Barents Sea, at 72°N. The Haakon Mosby mud volcano is
about a kilometer in diameter, and stands only a few meters high
in water 1250 meters deep. The mud volcano has a patchy white cover
of sulfur bacteria mats (which few creatures eat), and is home to
a diverse community of creatures whose food web is based is methane-consuming
bacteria. Whereas the mud volcano underlies a presently ice-free
part of the
Arctic, there is no reason not to expect discovery of similar features
in the ice-covered
Arctic Ocean.
In the summer of 1998, scientists inspected the Haakon Mosby mud
volcano first hand, aboard the Russian Mir submersibles that
had been used to explore the Titanic. The submersible sampled bottom
sediments and used a "slurp gun" to suck in unsuspecting bottom
fish in order to study them and augment museum collections. Two
species of small tubeworms were found on the mud volcano, one of
them previously known only from the Antarctic. A number of small,
sand-sized animal species sifted from the seafloor mud of the mud
volcano also depend on chemosynthesis.
|
|
The most common fish found were scalebelly eelpout (Lycoides
squamiventer). Stomach contents of these fish included tubeworms
and other chemosynthesis-dependent creatures, confirming that the
fish are indeed part of the local ecosystem. The eelpout was several
hundred times more abundant on the mud volcano than elsewhere on
the bottom of the surrounding Greenland-Norwegian Sea, and is apparently
attracted to the mud volcano by its abundant food supplies.
A variety of other fish were photographed, but not captured, on
the mud volcano surface. Conspicuous are several species of skate,
which probably form the top of the local food chain by consuming
smaller bottom fish like eelpout. The habit eelpout have of lying
inside small bottom depressions may be their way of escaping detection
and consumption by skates searching for prey. The largest predator
fish in these cold waters is the Greenland shark, but scientists
aboard the Mir submersibles were not lucky enough to glimpse
this rare animal.
The search for hot vents along the Arctic extension of the Mid-Oceanic
Ridge, which separates the North American and Eurasian plates, has
just begun. To date, the northernmost known hot vent is just north
of Iceland, in a shallow-water environment atypical of hot vents
studied elsewhere. There is every reason to expect such vents to
exist farther north, probably with some organisms new to science,
or at least distinct from those found farther south.
Gallery Click the image to see larger image with
taxonomy provided by Dr. M. Eric Anderson, Senior Marine Ichthyologist,
JLB Smith Institute of Ichthyology, Grahamstown, South Africa.
Related Links
Russian research vessel discovers mud volcano
http://www.pao.nrl.navy.mil/rel-96/127-96r.html
Haakon Mosby Mud Volcano (scroll half-way down the page to find
this article)
http://www.mediacen.navy.mil/pubs/allhands/apr98/Aprpg20.htm
Haakon Mosby Mud Volcano: A Warm Methane Seep With Seafloor Hydrates
and Chemosyntheses-bas
http://www.agu.org/
Deep
Sea Vents article from National
Geographic Magazine, October 6, 2000
References
Herman, Y. 1989. The Arctic Seasclimatology, oceanography,
geology, and biology. New York: Van Nostrand Reinhold Co. 888p.
Humphris, S.L., R.A. Zierenberg, and I. Mullineaux 1995. Seafloor
hydrothermal systems. Geophysical Monograph 91. Washington:
American Geophysical Union. 466 p.
Milkov, A., P. Vogt, G. Cherkashev, G. Ginsburg, N. Chernova, and
A. Andriashev. 1999. Sea-floor terrains of Haakon Mosby mud volcano
as surveyed by deep-tow video and still photography. Geomarine
Letters 19: 38-47.
Pimenov, N., A. Savvichev, I. Rusanov, A. Lein, A. Egorov, A. Gebruk,
L. Moskalev, and P. Vogt. 1999. Microbial processes of carbon cycle
as the base of food chain of Haakon Mosby mud volcano benthic community.
Geomarine Letters 19: 89-96.
Vogt, P.R., J. Gardner, and K. Crane. 1999. The Norwegian-Barents-Svalbard
(NBS) continental margin: introducing a natural laboratory of mass
wasting, hydrates, and ascent of sediment, pore water, and methane.
Geomarine Letters 19: 2-21.
*The
term "cold seep" has arisen in the literature to distinguish it
from hot vents on the active (volcanic) spreading plate boundary.
A more accurate term might be "slightly warm" or "lukewarm", because
the seepage consists of porewater, gas and mud which "remember"
the warmth of the earth beneath the seafloor from which they rise.
Temperatures inside the earth increase with depth inside the earth,
even without volcanic activity. Return to article here.
|