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Photo: Satellite image of duckweed in Lake MaracaiboScientists puzzled by plant invasion on Venezuelan lake

Carol Marzuola
6 August 2004
Source: SciDev.Net
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[CARACAS] Venezuela's Lake Maracaibo is facing the world's biggest known invasion of duckweed, an aquatic plant. And while scientists agree over potential causes, there is widespread disagreement, fuelled in part by a lack of basic research, about if and when the weed will die out.

The lake is the largest body of freshwater in South America and officials say duckweed (Lemna obscura) now covers between eight and ten per cent of its 13,280 sq km surface. But others, citing satellite photos obtained from NASA in July, claim that the duckweed coverage has reached 20 per cent.

The persistent floating plant has been causing problems for fishermen, small vessel operators and coastal residents around the lake since May, as well as emitting an unpleasant smell as it rots. Although there is little data on what ecological problems the plant is causing, most scientists agree that duckweed is adding to an existing lack of oxygen in parts of Lake Maracaibo. Duckweed blocks out sunlight and prevents algae from producing oxygen in the water column. As the duckweed invasion continues, natural aquatic flora and fauna are likely to be replaced with those that can survive without oxygen.

A duckweed invasion of this magnitude is unprecedented, according to agronomist William Haller at the University of Florida's Centre for Aquatic and Invasive Plants.

"I cannot understand how this has happened," says Haller. "This is the first time I have ever known Lemna species to cause such a huge problem. I’ve never seen much of a duckweed problem outside of a five-hectare pond. You never get duckweed in these huge lakes."

The source of the duckweed is probably local, says Elias Landolt of the Geobotanical Institute in Zurich, Switzerland, who confirmed the plant's identity from a live sample. But Landolt explains that it is difficult to determine whether or not the species is native to South America because there are few botanical collections from the affected region.

Landolt says that Lemna obscura only grows in waters with high nutrient contents, and can tolerate some salinity. Lake Maracaibo is slightly saline as a strait connects it to the Gulf of Venezuela. And though scientists have yet to identify what changes in water quality caused duckweed to appear in the lake this year, they agree that favourable conditions are now sustaining its growth. The lake, heavily polluted from years of raw sewage dumping and agricultural run-off, as well as petroleum and chemical industrial activities, is excessively rich in nutrients.

According to Jose Rincon, an aquatic ecologist at the University of Zulia in Maracaibo, levels of nitrogen and phosphorous in the lake have increased by at least 1,000 per cent during the past three decades.

Federico Troncone, head of water quality and hydrobiology at the Institute for the Conservation of Lake Maracaibo (ICLAM), a government agency, claims that heavy rainfall has triggered the outbreak. Levels of rain during the December to April dry season were nearly double normal. According to ICLAM's theory, an increased flow of freshwater might have disrupted the so-called 'hypolimnetic cone', an area of dense, nutrient-rich and contaminated water in the centre of the lake, and caused nutrients to spread.

Troncone admits, however, that since the problem began, ICLAM has only completed two expeditions, in May and June, to collect samples from ten stations in the lake. Another collection is planned for early August.

Other large-scale research programmes have been proposed, but are pending approval and funding from the Ministry of Environment and Natural Resources. The lack of current data has led to disagreement among scientists about when, how and even if the weed will disperse.

Optimists such as Troncone think that natural causes — wind, and shifting nutrient and salinity levels — will eventually cause the duckweed to die out.

Venezuela's environment minister Ana Elisa Osorio recently announced that the duckweed would be gone by mid-September, citing natural causes and a duckweed removal program. The government has reportedly allocated US$2 million for the task of removing the weed from the shoreline, but sceptics say the work is slow and ineffective.

Some researchers also warn that there is no existing technology capable of harvesting such massive amounts of Lemna, and are doubtful that manual removal can keep up with the pace of duckweed growth. "I think there is more of it dying every day that they will ever remove," says Haller. "And there’s more of it growing every day than they’ll ever catch up with."

Other scientists say that the only way to stop the duckweed invasion to is to reduce the amount of raw sewage pouring into the lake. According to ICLAM officials, an average of 10,000 litres of municipal wastewater is produced every second in the Lake Maracaibo basin, yet only 20 per cent of it is currently treated.

Nola Fernandez, an environmental engineer at the University of Zulia, says the duckweed, which under ideal conditions can double in size in two days, could invade up to 50 per cent of the surface of the lake by early next year if left unchecked.

Others warn that it is impossible to predict what will become of the duckweed without having identified the factors that caused it to spread in the first place. "We have to understand the source of the problem," says Rincon. "Why did Lemna come to the lake this year in particular? I believe that a bigger effort is needed to see the complete picture."