The Maracaibo lake, which is one of the biggest lakes in the world and the biggest one exposed the sea, has for some months been victim to the spread of the lenna spp, commonly called duckweed or “lenteja de agua” (water lentil) in South America. The latest reports indicate that duckweed covers 2,025 km2 of the Maracaibo lake surface, which corresponds to15% of the total surface. The Maracaibo lake has a surface area of 13.500 km2, and an approximate length of 160 km from North to South and 120 km from East to West. It is located in northwest Venezuelan state of Zulia. The lake basin is one of the most important in the country in terms of cultural heritage, ecological diversity and economical conditions. The economy of the lake includes the location of the most important oil infrastructures of the country; one million barrels per day are extracted in the lakebed by means of hundreds of oil drilling platforms. Fishery is the second largest economical activity on the lake, and the main activity for most of the population living near the lake.

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Zulia state map.

Duckweeds are small free-floating plants that often form dense mats on the surface of still or slowly flowing water. They grow best in eutrophic waters rich in nutrients, such as phosphate nitrate, and with an element of organic enrichment from leaf litter. The leaves are small, often not exceeding 5mm in length. They are very common in tropic lakes, reservoirs, water channels and ponds. The nutrients originate from pollution stemming from excessive usage of fertilizers or possibly from an imbalance of fish populations or waterfowl, which could result in excessive nitrogenous waste products in the water. The re-circulation of nitrogen and phosphorus from the cycle of growth and decomposition of duckweeds may also contribute to the high levels of these elements.

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Photo of a Lemna spp.

The duckweed mats can be composted and used as "green manure." They can also be fed to livestock, rabbits, poultry and fish. It has been estimated that 4 hectares of duckweeds could theoretically supply 60% of the nutritional needs of 100 dairy cows, the manure of which could be recycled to provide fertilizer for the thriving duckweeds. According to Harvey and Fox(1973) one hectare of water area is sufficient to raise 4000-7000 chickens and ducks during a vegetation period. And according to Rejmankova (1981) one hectare of duckweed cover is sufficient to produce protein for 480 ducks during the warm season. The utilization of duckweeds as food for animals is summarized by Landolt and Kandeler(1987).

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Duckweed in the coast of the Maracaibo Lake.

Stopping the inflow of nutrients and the repetitive removal of the duckweed layer will greatly reduce the growth of duckweeds. Methods for controls the Lemna spread included biological, chemical and mechanical controls. Biological control had been proven to be successful. Grass carp, for example, will eat the Lemna species. The use of shade has been successful in reducing the amount of duckweed growth, although very deep shade is often required. Planting trees on the south side of a water body can result in sufficient shading. Lemna does not compete well with other floating leaved plants, such as water lilies, and planting species with floating leaves can substantially reduce the nuisance level of duckweeds. It is necessary to take into account that studies need to be proven before the usage biological controls, in order to avoid unwanted secondary effects in the lake ecosystem.

Chemical controls have also been proved successful; in general, duckweeds are very sensitive to herbicides. In fact, duckweeds are often used to test the toxicity of herbicides and to detect the presence of herbicides in water. Some algaecides are extremely toxic to some species of Lemna. Lemna species are susceptible to herbicides containing diquat, terbutryn and glyphosate. Reglone (diquat)is a liquid concentrate applied directly to the water (Newman, 2001). It is necessary to take into consideration that destroying the duckweed layer with herbicides does not solve the problem of excess nutrients in the water. In addition, the chemical herbicides may be toxic to the animal life, either directly or through biological magnification. Because of the exponential growth rate of duckweeds, herbicides must be used repeatedly (perhaps several times a year). Ideally, in terms of chemical control, it is best to eliminate the influx of concentrated nitrates and phosphates into the water and to avoid the use of concentrated fertilizers Fomin (1987).

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Oil drilling platform surrounding by duckweed in the Maracaibo Lake.

The most frequent and recommended method for the removal the duckweed is the manual or mechanical removal of the duckweed cover. This method can also remove a lot of the nitrogen and phosphorus nutrients. This is the current method in use to remove duckweed from the Maracaibo basin. Up until now 90,000 tons have been removed in this manner; however, it has been calculated by Venezuelan Ministry of the Environment that 20 million tons are currently present. 800 workers, consisting of a large number of fisherman, and around 30 special vessels and machines that are normally are used to attend to oil spills, are employed for this purpose.

To combat the growth of the duckweeds the government has invested 300.000 US$, plus they established infrastructures of PDVSA and Municipalities. They are planning to invest 2,000,000 US$ more in the near future. Is necessary to take into account that it is impossible to remove every plant by mechanical means and that re-growth is inevitable. Mechanical methods of control give an instant effect, which can last for a reasonable length of time. A careful watch, however, needs to be kept regarding re-growth of the remaining plants, and remedial action should be put into effect before the problem reaches nuisance proportions again. The removed weed can be composted with chemicals. Continuous removal of this plant would often be necessary.

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Oil drilling platform surrounding by duckweed in the Maracaibo Lake.

Duckweeds prefer still water and increasing the disturbance of the water surface can reduce the amount of duckweed. It has been proven that increasing the amount of boat traffic will reduce the competitive ability of the species and may contribute to their eventual elimination (Newman, 2001).

The accelerating growth of the duckweed in the Maracaibo basin is indicator of the high degree of degradation of the ecosystem of the lake and lake catchments. This implies that environmental controls until now are not enough. Better controls, however, still may not totally imply that the lake is unpolluted. Another strong source of pollution can also come from the lake’s sub-catchments. Rivers and ravines effluents into the lake are known to have high amounts of pollution coming form fertilizer, pesticides, herbicides and organic materials from nearby farms. In addition, some chemicals used in the oil industry to clean the tanks and deposits are sources of pollution, which in some cases are not controlled, causing these chemicals to go directly into the lake.

It is important to mention that these environmental problems stem from the heritage of old practices from the last century, in which only the extraction of the resources was considered, with little consideration for sustainability or environmental risk. Now, this situation is changed, with the new sustainable vision of the current central government, which through the Ministry of the Environment places the preservation of the environment as its priority. Under the Instituto para la Conservación del Lago de Maracaibo, ICLAM (Institute for the conservation of the Maracaibo Lake), adscript to the Ministry of the Environment, the research of the duckweed situation is still in progress. Studies of concentration levels and sources are needed, which includes monitoring of nutrients, organic matter, heavy metal, inorganic and organic pollutions and sedimentation in the lakebed, rivers and ravine effluents.

This first step of information collection will then be coupled with GIS for the final processing of the data, which will assist in the forthcoming decisions concerning subsequent actions.

Efforts to prevent pollution in the lake basin is of interest for all the players and participants that interact in any way with the lake environment, including oil companies, farms, fishermen, nearby communities, and the central and local governments. Towards a joint effort amongst these groups for their mutual benefit, and for the sustainability of the lakes resources, is the way in which we are working.

Bibliography

Fomin, A., Moser, H., Pickl, C., Arndt U. (1987). Ökotoxikologische Untersuchungen saurer Tagebaugewässer der Bergbaufolgelandschaft Lausitz während der Sanierung. (Hrsg.): Abbau von Bodenschätzen und Wiederherstellung der Landschaft. 29. Hohenheimer Umwelttagung, Verlag Günter Heimbach, Ostfildern, 165-178.
Harvey R.M. & Fox, J.L. (1973). Nutrient Removal Using Lemna minor. J. Water Poll. Control Fed. 45: 1928-1938.
Landolt E. & Kandeler, R. (1987). "The Family of Lemnaceae: A Monographic Study" in Veroff. Geobot. Inst. ETH, Stiftung Rubel 95 pp. 382-389.
Newman J. (2001). CAPM Information Sheet 25. Centre for Aquatic Plant Management, Broadmoor Lane, Sonning, Reading, Berkshire.
Rejmankova, E. (1981). On The Production Ecology of Duckweeds. Intern. Workshop on Aquatic Macrophytes, Illmitz, Austria.
http://www.aporrea.org/
http://www.iclam.gov.ve/
http://www.panodi.com/
http://waynesword.palomar.edu/1wayindx.htm