Green and mean: a thick, floating weed is expanding fast across Venezuela's Lake Maracaibo.

Lake Maracaibo in northwestern Venezuela, home to one of the country's main ports, has been under an environmental alert for several months now, thanks to a 20-million-ton bed of an aquatic weed that covers more than a quarter of the lake. Aside from the obvious environmental problems surrounding its mysterious growth, the weed is blocking a 13,280-square-kilometer outlet to the sea. Considering that the lake borders one of the main petroleum exploration and transport centers of this oil-rich nation, the economic risks are high.

No one really knows why this green giant, as the lake's fishermen call the phenomenon, has grown so. But battling the beast is like swimming against a swift current. Armadas of 2,000 men in boats of all sizes pull 50,000 tons a day of the plant matter out of the water, an amount the weed manages to reproduce in less than 48 hours. "In June, the largest area covered by the plant was 30% of the basin," Marcelo Monot of the Fundacion Ecologica Maranata told reporters recently. "The plant is capable of covering 254 kilometers in 12 to 14 days."

The outlook for the lake is worrying. Although the weed, known as lemna, or duckweed, is less than five millimeters in diameter, its tendrils form intricate webs that create a water-resistant bed on top of the water. "lf nothing is done in time, Lake Maracaibo could become a swamp," says Nola Fernandez, head of the sanitary engineering department of the Universidad del Zulia in Maracaibo, a city not far from the lake that bears its name.

The growing green goo does not yet threaten fish in the lake, yet environmentalists warn that the habitat could die since the weed is cutting oxygen levels by blocking photosynthesis in plant life below the surface. As if that's not enough, the rainy season at the end of last year has helped feed the weed, according to the Venezuelan government's weather agency

Through this waterway moves close to 1.5 million barrels of oil a day--the entirety of oil production in Venezuela's central-west region--to be exported or distributed to refineries around the country. For a country that largely finances itself on oil revenues, the green giant could become a threat. State-owned oil company Petroleos de Venezuela (PDVSA) reports no problems so far navigating tankers in and out of the lake, nor with its network of oil pipes and towers. Nevertheless, the company could lose more than US$33 million daily if it cannot get its oil out, says Carlos Rivero, chief of security and sanitation for PDVSA. The oil company has $2 million invested in a clean-up strategy for the duckweed problem. That's wise, since PDVSA ships much of its oil to the Caribbean and the United States through Lake Maracaibo.

Other industries depend on the lake as well. Coal miner Carbozulia and concrete maker Cemetos Mara, together representing 20% of the state of Zulia's economy, also use the lake to move their materials. The duckweed invasion is threatening Energia Electrica del Zulia, a power company The thermoelectric power plant uses the lake's waters to produce electricity, and it has fought to block the weed from nearing its operations. Should it disrupt the flow of water to the generator, there could be problems getting electricity to Maracaibo.

Freddy Rodriguez, president of the Maracaibo Lake Basin Conservation Institute, says that the greatest concentration of the weed lies in the northern part of the lake, near the Strait of Maracaibo, while the southern and central areas, where boat traffic is heavier, are clear. Rodriguez says there have been some complaints from tourism companies, chambers of commerce and small fishermen, which have seen a decline in their businesses, but he says that there are no hard numbers on economic losses.

Rare bloom. Venezuela's duckweed explosion is not a common problem, ecologically speaking. It rarely blossoms thanks to natural predators--fish and other creatures--which won't allow it to grow out of control.

The plant itself, from the family Lemnaceae, is non-toxic and grows best in water with low salinity, which makes its presence in salty Lake Maracaibo even more of a mystery In the last few decades salt levels have risen in the lake with increasing oil activity. Researchers are left scratching their heads. Australian biochemist Ronald Leng has said that, in theory, the plant has taken off in Maracaibo because of high levels of copper and phosphorus in the waters there.

Lenin Herrera, the coordinator of a commission to probe the phenomenon, says it will spend $340,000 to conduct 17 research projects to understand the floating menace. In the commission's first report, it recommended eliminating the plant by using manual equipment or by mechanical or hydraulic means; it rejected the idea of using chemical or herbicidal agents or burning the plant.

Duckweed has proven to be useful in the past. A study by the University of Zulia reports that duckweed collected in some of the less-polluted regions of the lake could be used as fertilizer or as an animal feed.