Duke scientists explain gaps in nutrient availability within North Atlantic

Seasonal wedges of undersea water block upwelling of plant sustaining nitrates

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.

The circulating zone in the North Atlantic Ocean, known as a “subtropical gyre,” swirls in a clockwise direction between the Gulf Stream — the warm current that bisects the Atlantic between the southern U.S. and northern Europe — and the Tropic of Cancer. This gyre is also the location of the Sargasso Sea.

In a paper in the Sept. 29, 2005, issue of the journal Nature, graduate student Jaime Palter and professors Susan Lozier and Richard Barber show that pockets of water that seasonally wedge themselves into the gyre from the Gulf Stream prohibit deep-ocean nutrients from directly upwelling to the “euphotic” zone, the region near the surface where there is enough light to support plant life.

The scientists are in Duke’s Nicholas School of the Environment and Earth Sciences. Their work was sponsored by the National Science Foundation.

Using satellites to detect the presence of chlorophyll in ocean-borne plant life, the investigators noted a striking correlation between where these nutrients were kept wedged off far below the euphotic zone and where the chlorophyll was low.

As seen from space, this satellite-detected chlorophyll was spread out in ring-shaped patterns on the ocean’s surface, with minimum readings corresponding with the low nutrient concentrations, according to 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 brings nutrients to the surface,” said Lozier, a professor of physical oceanography, in an interview. “Do winds cause upwelling? Do surface waters cool and then overturn and sink to drive nutrients up? Do we get a mixing of waters by winds and waves?

“The answer to all those questions is ’yes.’ But none of those processes, even combined, could really explain the patterns of productivity we saw.”

Lozier, Palter and Barber — a professor of biological oceanography — made their deductions by consulting satellite information and years of data on water density, temperature and nutrients from previous ocean studies.

Their study focused on “North Atlantic Subtropical Mode Water.” Lozier described that as “a large volume of water with the same properties” that gets isolated from the Gulf Stream’s edge when cooled by the air above it during winter.

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

“In parts of the ocean where there is no such wedge of low nutrient water beneath the euphotic zone, vertical processes are much more effective at moving nutrients to the surface, and can therefore have a greater biological effect,” added Palter, who is first author of the Nature paper.

How far the mode water moves, and how extensively it blocks the underlying trapped nutrients, depends on the gyre’s power. The power of the gyre is determined by a large scale, cyclical climate pattern called the North Atlantic Oscillation (NAO) — which irregularly swings between “high” and “low” phases over periods of decades, said the researchers.

During the last “low” NAO, occurring in the 1950s and 60s, “really thick” subtropical mode water spread throughout the subtropical gyre, Lozier said. By contrast, the authors determined that mode water layers should be shallower and less extensive during “high” NAOs, potentially making nutrients more accessible to the euphotic zone.

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

These conclusions about NAO effects on nutrient availability are the opposite of what would be expected without accounting for the varying effects of mode water, Lozier said. “We’ve been able to explain that with our ideas.”

When a robust gyre spreads around the blanket of mode water, the nutrient recycling system is disrupted. Lozier said. Floating surface plants “can go to the bank, but there’s no money there.”

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

“The ideas that we lay out here don’t just apply to the subtropical North Atlantic,” she said. “We happened to have a lot of available data there 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 to get a broader view. We may not get the same patterns in other gyres, because the mode waters are different in other basins. But we believe the same mechanics are going to apply.”