New! Sign up for our free email newsletter.
Science News
from research organizations

Duke Scientists Explain Gaps In Nutrient Availability Within North Atlantic

Date:
October 3, 2005
Source:
Duke University
Summary:
Duke University oceanographers have developed an explanation for why a vast North Atlantic circulation zone can have a large variability in nutrient supplies needed to sustain ocean plants and, by extension, support the food web of marine life.
Share:
FULL STORY

DURHAM, N.C. -- Duke University oceanographers have developed anexplanation for why a vast North Atlantic circulation zone can have alarge variability in nutrient supplies needed to sustain ocean plantsand, by extension, support the food web of marine life.

The circulating zone in the North Atlantic Ocean, known as a"subtropical gyre," swirls in a clockwise direction between the GulfStream -- the warm current that bisects the Atlantic between thesouthern U.S. and northern Europe -- and the Tropic of Cancer. Thisgyre is also the location of the Sargasso Sea.

In a paper in the Sept. 29, 2005, issue of the journal Nature, graduatestudent Jaime Palter and professors Susan Lozier and Richard Barbershow that pockets of water that seasonally wedge themselves into thegyre from the Gulf Stream prohibit deep-ocean nutrients from directlyupwelling to the "euphotic" zone, the region near the surface wherethere is enough light to support plant life.

The scientists are in Duke's Nicholas School of the Environment andEarth Sciences. Their work was sponsored by the National ScienceFoundation.

Using satellites to detect the presence of chlorophyll in ocean-borneplant life, the investigators noted a striking correlation betweenwhere these nutrients were kept wedged off far below the euphotic zoneand where the chlorophyll was low.

As seen from space, this satellite-detected chlorophyll wasspread out in ring-shaped patterns on the ocean's surface, with minimumreadings corresponding with the low nutrient concentrations, accordingto the Duke team. Depressed nutrient levels ultimately limit the"primary productivity" that supports the food chain.

"Researchers have tried for years to look at what processes bringsnutrients to the surface," said Lozier, a professor of physicaloceanography, in an interview. "Do winds cause upwelling? Do surfacewaters cool and then overturn and sink to drive nutrients up? Do we geta mixing of waters by winds and waves?

"The answer to all those questions is 'yes.' But none of thoseprocesses, even combined, could really explain the patterns ofproductivity we saw."

Lozier, Palter and Barber -- a professor of biological oceanography --made their deductions by consulting satellite information and years ofdata on water density, temperature and nutrients from previous oceanstudies.

Their study focused on "North Atlantic Subtropical Mode Water." Lozierdescribed that as "a large volume of water with the same properties"that gets isolated from the Gulf Stream's edge when cooled by the airabove it during winter.

Because cooler water is denser and heavier, this mode water overturnsand sinks to form a large wedge-shaped mass. Sinking below the surface,it has lower nutrient concentrations than that of the surroundingwaters. Only with time are the nutrients in this mode water massrestored by the sinking and decay of organic matter from the sunlitsurface layers. Since the subtropical gyre has a circulation, the modewater also begins moving around it, potentially blocking nutrients fromupwelling in larger areas, Lozier said.

"In parts of the ocean where there is no such wedge of low nutrientwater beneath the euphotic zone, vertical processes are much moreeffective at moving nutrients to the surface, and can therefore have agreater biological effect," added Palter, who is first author of theNature paper.

How far the mode water moves, and how extensively it blocks theunderlying trapped nutrients, depends on the gyre's power. The power ofthe gyre is determined by a large scale, cyclical climate patterncalled the North Atlantic Oscillation (NAO) -- which irregularly swingsbetween "high" and "low" phases over periods of decades, said theresearchers.

During the last "low" NAO, occurring in the 1950s and 60s, "reallythick" subtropical mode water spread throughout the subtropical gyre,Lozier said. By contrast, the authors determined that mode water layersshould be shallower and less extensive during "high" NAOs, potentiallymaking nutrients more accessible to the euphotic zone.

Indeed, the Duke investigators found that that nitrate concentrationswere 25 percent greater in the mode water during the high NAO thatbegan in the 1980s than in the low-NAO 1950s. And primary productivityrates observed in the 50s and 60s were only half those recorded in thelast two decades, according to their Nature paper.

These conclusions about NAO effects on nutrient availability are theopposite of what would be expected without accounting for the varyingeffects of mode water, Lozier said. "We've been able to explain thatwith our ideas."

When a robust gyre spreads around the blanket of mode water, thenutrient recycling system is disrupted. Lozier said. Floating surfaceplants "can go to the bank, but there's no money there."

The researchers are now preparing to put out instruments that will givescientists their first time series of nutrient level readings in thesesubtropical waters, Lozier said.

"The ideas that we lay out here don't just apply to the subtropicalNorth Atlantic," she said. "We happened to have a lot of available datathere to test our ideas. That's why we have focused there.

"But what we want to do next is start looking at other ocean basins toget a broader view. We may not get the same patterns in other gyres,because the mode waters are different in other basins. But we believethe same mechanics are going to apply."


Story Source:

Materials provided by Duke University. Note: Content may be edited for style and length.


Cite This Page:

Duke University. "Duke Scientists Explain Gaps In Nutrient Availability Within North Atlantic." ScienceDaily. ScienceDaily, 3 October 2005. <www.sciencedaily.com/releases/2005/10/051001101225.htm>.
Duke University. (2005, October 3). Duke Scientists Explain Gaps In Nutrient Availability Within North Atlantic. ScienceDaily. Retrieved March 29, 2024 from www.sciencedaily.com/releases/2005/10/051001101225.htm
Duke University. "Duke Scientists Explain Gaps In Nutrient Availability Within North Atlantic." ScienceDaily. www.sciencedaily.com/releases/2005/10/051001101225.htm (accessed March 29, 2024).

Explore More

from ScienceDaily

RELATED STORIES