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Rotten-Egg Gas Suffocating Fish Off African Coast

Bijal P. Trivedi
National Geographic Today
January 31, 2002
 
Satellite photographs have revealed that the Namibian coast off
southwestern Africa is belching up massive amounts of hydrogen sulfide
gas—commonly known as "rotten egg" gas.

As the gas is
forced up from the seabed, it robs the water of oxygen and basically
suffocates the marine organisms in the area. "During these episodes,
the fish wash up on the beach en masse either dead or in a stupefied
state from lack of oxygen," said Scarla Weeks of the University of Cape
Town in South Africa.



Weeks said that "lobsters walk out of the ocean gasping for breath," much to the delight of the local residents and the seabirds who snatch them up for food.

When the Namibian government contacted Weeks and her colleagues to investigate the noxious smells coming from the sea, the team thought the situation was a local phenomenon that extended along the Namibian coast for perhaps a few kilometers.

But the satellite photos revealed that the gas eruptions cover an area about 7,722 square miles (20,000 square kilometers). "What was most surprising was the extent of the emissions," said Weeks, who reported on the situation in the January 31 issue of the journal Nature.

The situation has generated much concern because of fears that the gas emissions could devastate local fisheries, which are the third largest component of the Namibian economy.

The highest level of gas emissions have been observed in areas that are known to have nursery grounds for larvae and juvenile fish, which are particularly susceptible to low oxygen levels. Weeks and her colleagues now suspect that the gas emissions were responsible for the catastrophic loss of two billion young Cape Hake fishes in 1992 to 1993.

The gas originates from a belt of sea sediment or mud that that lies parallel to the Namibian coastline. As the gas reacts with the seawater, the sulfur granules turn the water a murky shade of green that can be seen from satellites.

The Namibian coastline is affected by the Benguela current, which originates in the Southern Ocean and is rich in organic material and nutrients.

When strong trade winds sweep across the Namibian coastline, they cause a tremendous upwelling of these cold Antarctic waters, creating a fertile environment for phytoplantkton—microscopic unicellular plants—to bloom. The abundant phytoplankton are a major food source for high levels of marine life that make the area one of the richest fishing grounds in the world, said Weeks.

The phytoplankton include heavy concentrations of organisms called diatoms. As these diatoms die and sink to the ocean floor, they form a thick layer of oozing mud.

Within this mud belt, which ranges up to 20 meters deep, there are areas in which the oxygen levels are zero—"azoic zones" or "dead zones," Weeks explained.

As bacteria break down the diatoms in these dead zones, hydrogen sulfide gas is generated as a byproduct. The gas is trapped in the mud and builds up, until it is eventually released when the region is disturbed by surrounding forces.

The mud belt off the Namibian coast is at least 460 miles long (740 kilometers) and 47 miles (76 kilometers) at its widest point, off Walvis Bay. But until last year, no one had determined how widely the hydrogen sulfide gas emissions extended or how long they lasted.

"We still don't know much about the frequency of these events or what triggers the release of these gases from the mud belt, although there have been written accounts of this phenomenon dating back to the beginning of the 20th century," said Weeks, who noted that a massive release of the gas is occurring now. She and her colleagues are continuing their use of satellite images to study the gas emissions.

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