Oceanic Storms Create Oases in the Watery Desert

Topics:

May 17, 2007

For two decades, scientists have puzzled over how vast blooms
of microscopic plants can form in the middle of otherwise barren mid-ocean
regions. Now a research team led by the Woods Hole Oceanographic Institution (WHOI)
has shown that episodic, swirling current systems known as eddies act to pump
nutrients up from the deep ocean to fuel such phytoplankton blooms.

Dennis McGillicuddy, a WHOI oceanographer and leader of the
Eddies Dynamics, Mixing, Export, and Species composition (EDDIES) project,
found that biological activity was surprisingly high when the ocean was stirred by certain
types of eddies. These huge parcels of water were teeming with
diatoms (a large type of phytoplankton) in concentrations 10,000 to 100,000 times the
norm—among the highest ever observed in the mid-Atlantic “Sargasso Sea.”

“Past research has shown that the open ocean is far more
productive than we could explain based on what we knew about nutrients in the
water,” said McGillicuddy. “Scientists have been trying to figure out where the
nutrients come from to make these oases in the oceanic desert, and some of us
hypothesized that eddies were part of the answer. The EDDIES project has
validated that suspicion.”

McGillicuddy and colleagues published their work in the May 18 issue of the journal Science.
The work was primarily funded by the National Science Foundation, while
leveraging a strategic partnership with NASA for satellite measurements
to guide the shipboard sampling.

The Sargasso Sea, like other mid-ocean
regions of the world,
is warmer, saltier, bluer, and clearer than most other parts of the
North Atlantic. The prevailing oceanographic wisdom has
suggested that such open waters were mostly desert-like, unproductive
regions
populated by a few smaller plant species. Yet observations showed
oxygen and
other biologically important elements being consumed at a higher rate
than the
theories and models could account for. There had to be some natural
nutrient
source.

Now, McGillicuddy and colleagues have found that eddy-driven
nutrient transport actually primes the ocean’s “biological pump,”
fertilizing
the waters with nutrients from the deep. Fed by this unusual upwelling,
the phytoplankton
population explodes and, in turn, attracts more zooplankton and other
animals
higher up the food chain. The fate of all of that biomass is also
important, as
plankton blooms can remove substantial amounts of carbon dioxide from
surface
waters and sink it to the deep ocean.

The EDDIES project team included
chemists, biologists, and physical oceanographers from WHOI, the
Bermuda Institute of Ocean Sciences (BIOS), Rutgers University, the
University of Southampton (UK), the University of California-Santa
Barbara, the Virginia Institute of Marine Science, Humboldt State
University, and the University of Miami. The report by McGillicuddy et
al. was published in conjunction with another article by Benitez-Nelson
et al. about the roles that eddies play in marine production.

“Eddies are the internal weather of the sea,” said
McGillicuddy, “the oceanic equivalent of storms in the atmosphere.” The largest
eddies can contain up to 1,200 cubic miles (5,000 cubic kilometers) of water
and can last for months to a year.

These distinct parcels of water are formed by differences in
ocean temperature and salinity that give water different densities. On a
rotating planet, these different water masses tend to dance around
one another rather than mix. The density inside an eddy can be higher or lower than the
surrounding water, like high and low-pressure systems in the atmosphere. The
balance between density and pressure differences, along with earth’s rotation (the
Coriolis force) gives eddy currents their distinctive clockwise or
counterclockwise spin. The direction of the spin depends on whether the eddy contains
cooler “mode water” or a warmer core.

Working from a long-debated but mostly untested hypothesis, EDDIES
investigators measured how these swirling currents can perturb the layers of
the ocean and cause an upwelling of nutrient-rich water into the sunlit “euphotic”
zone—the top 330 feet (100 meters) that light penetrates.

In nearly six months of ship-based work in the summers of
2004 and 2005, the researchers employed a combination of remote sensing, video
plankton recorders, electronic plankton nets, ocean drifters, tracers, and traditional measurements of water
properties and current speeds.

They started with NASA satellite measurements of sea surface
height to locate eddies in the Sargasso Sea, south and east of the Gulf Stream
in the North Atlantic. The 18-member research
team then sailed into those eddies with the research vessels Oceanus (operated by WHOI) and Weatherbird II (operated by BIOS).

In addition to proving the connection between eddies and
mid-ocean plankton blooms, the researchers discovered that winds can act to
dampen or amplify the effects. Biological activity was most prodigious in
mode-water eddies, in which wind-driven currents interact with the clockwise
spin of the eddy to enhance upwelling of nutrients. Cyclonic eddies also stir
up plankton growth, but the process is tempered by the eddy-wind interaction
that tends to cause downwelling in the centers of counter-clockwise spinning
eddies.

The Woods Hole Oceanographic Institution is a private,
independent organization in Falmouth,
Mass., dedicated to marine
research, engineering, and higher education. Established in 1930 on a
recommendation from the National Academy of Sciences, its primary mission is to
understand the oceans and their interaction with the Earth as a whole, and to
communicate a basic understanding of the ocean’s role in the changing global
environment.