Sick oceans

Freshwater woes

El Nino

The SeaStar satellite carries the SeaWiFS instrument which monitors the color and condition of the world's oceans.
Courtesy NASA.

Sulfur (milky turquoise color) in the ocean off Namibia. a) March 18; 2001, b) March 29; c) April 3.
Courtesy Scarla Weeks

The aroma of rotten eggs is familiar to neighbors of industrial plants, but it can also arise naturally when organic stuff rots in the absence of oxygen, as at volcanic vents.

Along Namibia's coast, eruptions of hydrogen sulfide are a regular feature, although they're not featured in scratch-and-sniff tourist brochures. The eruptions are not entirely miserable from some perspectives.

Although millions of fish may die, scavenging birds feast on their carcasses.

Namibians also feast on lobsters escaping to shore from the oxygen-deprived water, where they are easy game for beachcombers.

Got my eye on you!
Now, for the first time, the disturbance has been tracked from space -- and it turns out to be inordinately large. "What shocked the Namibians and a lot of the scientific community was the frequency, spatial extent, and how long these events could last," says Scarla Weeks, a research associate in oceanography at the University of Capetown (see "Massive Emissions..." in the bibliography).

For hundreds of years, the Namibian Coast has been known for its hydrogen sulfide perfume. The fish kills, combined with the satellite data, have made the giant belches a pressing subject of scientific study.

The Namibian Coast has the most intense upwelling of fertile deep-ocean water in the world, says Weeks, who collaborated on the research with colleague Andrew Bakun and Bronwen Currie, a biogeochemist with Namibia's National Marine Research and Information Center.

Mixed blessing
Weeks explains that the shallow sea bottom along the coast is covered with a thick ooze of decomposing diatoms -- free-floating plants nourished by the strong upwelling. Upwelling generally makes great fishing, and fishing is Namibia's third-largest industry, and second-largest source of foreign exchange.

But there may be too much of a good thing. "As a result of almost continual phytoplankton production," Weeks says, "there is massive diatom fallout." Dead diatoms fall through the water, where they are decayed by aerobic bacteria.

Anaerobic bacteria take over the decomposition task in the thick, muddy sediment, producing hydrogen sulfide. When the gas enters the water column during an eruption, it separates into hydrogen and sulfur, and the hydrogen atoms combine with oxygen to form water. That reaction removes oxygen from the water, creating deadly low-oxygen conditions.

Double whammy
Although hydrogen sulfide is itself a respiratory toxin to fish and marine invertebrates, the reduced oxygen may have greater ecological consequences in the long run, Weeks says.

Hydrogen sulfide that enters the atmosphere makes "this awful, noxious smell of corroding metal," says Weeks. "But other than the noxious smell and headaches, there are no known effects on the human population."

Weeks and her colleagues tracked the eruptions with the OrbView-2 SeaWiFS satellite. "The surface is milky green due to the elemental sulfur," she says. When the hydrogen sulfide oxidizes, it leaves sulfur at the surface water that reflects visible light.

The low-oxygen conditions are not simply a transient, local affair. Weeks followed one "event" out to sea, where it covered 20,000 square kilometers and lasted at least three weeks.

These hydrogen sulfide eruptions are the largest in the world, Weeks adds. "Certainly you get hydrogen sulfide outgassing in places like the Baltic Sea and the Black Sea, but the important thing is that nowhere else does this actually happen remotely to the intensity of Namibian waters, and yet it's completely natural."

Natural, perhaps, but given the stakes to the critical fishing industry, and the suspicion that the eruptions may be getting more frequent and intense, an international collaboration is starting to examine possible links to global warming or other large ecological disturbances.

-- David Tenenbaum

BIBLIOGRAPHY
Massive Emissions of Toxic Gas in the Atlantic, Scarla Weeks et al, Nature, Jan. 31, 2002 (Brief Communications).

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