Paris, 29 February 2012
French Guiana is experiencing a genuine gold rush: the number of illegal gold-mining sites is estimated at between 500 and 900. According to France's National Forestry Office, over 1,300 kilometers of rivers and 12,000 hectares of forest are directly affected by this activity. It is well known that gold mining has a negative impact on the environment due to the release of mercury into rivers. However, the ecological impact of increased water turbidity caused by gold mining is little known.
To better understand one of its aspects, the researchers decided to study populations of diatoms, single-celled algae measuring between 2 µm and 1 mm. They examined diatom communities at ten currently or recently exploited gold-mining sites and in model rivers never exploited by gold miners. By brushing stones and pieces of wood to which diatoms attach, the researchers were able to compare the populations present in these various locations of the Approuague basin.
The analysis showed that mining activity had a major impact on these communities. At gold-mining sites, the proportion of certain species had increased to the detriment of others. The reasons for this are functional: some diatoms are firmly attached to the substrate, whereas others are able to move around on its surface. The latter are far more resistant to the impact of gold mining.
According to the researchers, the impact of water turbidity can be explained in two ways. On the one hand, in cloudy water, less light gets through to the algae, which leads to a reduction in primary production(2). And on the other hand, the sediment stirred up by the gold miners gradually settles on the algae, asphyxiating them. That is why moving diatoms are more resistant: they can rise above the layer of sediment as it starts covering them. In addition, the researchers did not observe any deformation of the siliceous skeletons of the diatoms. Such deformations only appear when there is high mercury contamination.
In addition to determining the impact of gold mining on communities of algae, the research sought to contribute to the development of new biological indicators to assess and monitor the health of ecosystems in French Guiana's rivers. While diatoms are a widely used indicator in European rivers, using them in the Amazon requires a good grasp of the specific dynamics of local populations and their reaction to human-induced disturbance. The researchers are also looking at other bioindicators such as fish, crustaceans and aquatic insect larvae. Having completed their work in the Approuague basin and, more recently, the Maroni basin, the scientists will continue to study such sentinel species in the Oyapock basin.
© Kamran Khazraie
Confluence of a watercourse showing high turbidity caused by intense gold mining.
© Loïc Tudesque
Diatom of the genus Eunotia (45µm) with low motility
© Loïc Tudesque
Diatom of the genus Surirella (59µm) with high motility
(1)The state of cloudy water. There is a high content (normal or otherwise) of fine or colored material in suspension.
(2)Production of organic plant material by photosynthesis per unit time.
Influence of small-scale gold mining on French Guiana streams: Are diatom assemblages valid disturbance sensors? Loïc Tudesque, Gaël Grenouillet, Muriel Gevrey, Kamran Khazraie & Sébastien Brosse. Ecological Indicators 14 (2012) : 100-106, March 2012.
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