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Where Fish Fear to Tread

11 February 2004 (All day)
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Dead zone. Sulfide gas (light green areas) seeping from sea-floor sediment off Namibia hints at why fish give the area a wide berth.

The ocean off southwestern Africa supports one of the most plentiful populations of plankton in the world, but for some reason big fish avoid the rich grazing. Now, a team of researchers proposes an explanation: The sea floor appears booby-trapped with poisonous gases.

Anywhere that ocean currents bring nutrients up to the surface, plankton thrive and fish have a field day eating the plankton. But that's not true of the Benguela current, off the coast of Namibia. Fish there are up to 20 times scarcer than in similar regions off Peru. As a result, many plankton fall uneaten to the sea floor. Bacteria decompose them, stripping oxygen out of the water and forming pockets of methane and hydrogen sulfide gas beneath the sediment.

Periodically, these pockets erupt. The air stinks of rotten eggs, sea birds feast on a smorgasbord of dead fish, and oxygen-poor waters send lobsters scurrying onto beaches. Until recently, the explosions were thought be just a local problem. But satellites tell a different story. Images depict huge tracts of the milky, turquoise water, stretching over patches the size of New Jersey and lasting for days. Now a team of oceanographers and fisheries scientists led by Scarla Weeks at the University of Cape Town in South Africa has paired these satellite images with on-the-ground observations of 16 recent eruptions in the Benguela current to look for common triggers and assess the impact on the ecosystem.

Shifts in the weather could be responsible, they report in the February issue of Deep Sea Research. The timing, as well as the size and shape of the eruptions, suggest that a slight lessening of pressure on the sea floor--caused by low-pressure weather systems, warmer water, or stronger upwelling currents--may prompt a few bubbles to escape a large gas pocket. The bubbles would lessen the pressure on the sea floor still more, triggering a bigger eruption, the team speculates. They also suggest that the frequency and extent of the troublesome gas make the region deadly enough to encourage the fish to eat elsewhere.

The fish kills in the region are indeed large, says oceanographer Peter Brewer of the Monterey Bay Aquarium Research Institute in Moss Landing, California. And the big picture of sulfide and other gases creating the patches rings true, although he doesn't believe that the bubble mechanism accurately captures the behavior of gases on the sea floor.

Related sites
An article on the original discovery of the sulfide explosions
The Physical Oceanography of the Benguela Current

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