Mesoscale eddies can ignite large-scale plankton blooms, such as this one spotted from space off southwestern Florida (in red).

Don't Bet on the Bloomin' Plankton

Ocean currents that stimulate marine organisms by sucking up carbon and nutrients from the sea bottom don't seem to mitigate the buildup of carbon dioxide in Earth's atmosphere--and they might even constitute a net source of the greenhouse gas--new research suggests.

The tiny plants and animals collectively called plankton compose the foundation of the marine food chain. Their populations tend to wax and wane, sometimes creating blooms that can cover thousands of square kilometers of ocean--big enough to be visible from space. One powerful influence on their prosperity seems to be ubiquitous but intermittent currents called mesoscale eddies. Generally ranging from 100 to 200 kilometers wide and strongly influenced by winds and thermal convection, the eddies dredge up detritus from the ocean bottom and spread it across the surface where plankton feed or grow.

Unlike persistent ocean currents such as the Gulf Stream, mesoscale eddies tend to appear and disappear almost unpredictably, making them difficult to study. Nevertheless, scientists have been attempting to gauge their influence on plankton, as well as their role in the planet's carbon cycle, by seeking eddies that recur at specific locations. Two teams sampled the water at many points within eddies in the Pacific Ocean near Hawaii and in the Sargasso Sea region of the Atlantic. The researchers were able to track what happens to carbon that comes up from the deep and how it supports the local plankton populations. They found that a lot of the carbon fails to sink back down to the depths. Instead, it is recycled as organic matter, where it remains in the surface waters and can even be ejected into the atmosphere as carbon dioxide, says chemical oceanographer Claudia Benitez-Nelson of the University of South Carolina in Columbia and lead author of the Hawaii paper. The bottom line, the teams report tomorrow in Science, is that in many cases the eddies can carry quite a bit of carbon up from the bottom and deposit less of it back down below.

The research suggests that natural systems such as mesoscale eddies might not automatically trap and sequester atmospheric CO2, says ocean scientist Donald Rice of the National Science Foundation in Arlington, Virginia, which funded the research. Eddies are complex, he says, so it's foolish to count on them to brake global warming before they're better understood.

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