Mercury in canopy leaves of French Guiana in remote areas
Introduction
As in the Amazonian basin of Brazil, large areas of French Guiana (South America) have been subjected to gold mining since the 19th century. This activity uses large quantities of mercury (Hg) (1–4 kg Hg for 1 kg of gold produced), which are mainly lost in soil, river sediment and the atmosphere. In order to estimate, understand, and finally prevent the resultant Hg pollution, studying the Hg cycle has been a major goal of the last 20 years (see for example Schroeder W., 1998). To estimate the anthropogenic impact we must first evaluate the contribution of the two natural origins of the Hg present in soil: geological soil content and atmospheric deposition. Atmospheric uptake is an important process by which Hg accumulates in plants (see for example Lindberg et al., 1992, Lindberg et al., 1994, Munthe et al., 1995, Benesh et al., 2001). We focus here on the contribution of the background atmospheric deposit, measuring specifically the contribution from the leaves of the canopy through litterfall deposit.
Total Hg concentration in leaves has been measured by Roulet et al. (1999) in different parts of Brazil and in French Guiana; results have shown a higher content, close to double, at the French Guiana site (mean values in French Guiana ranging from 130 to 150 ng g−1, compared to 40–120 ng g−1 in Brazil). Is this high content representative of the entire French Guiana canopy in remote areas? Our goal here is to establish a representative value for this total Hg deposit in litterfall for the tropical rainforest of French Guiana. This requires first sampling representative species in the canopy, and second, performing this sampling in remote areas free from gold mining Hg pollution and from human settlement, to detect the contribution from the background atmospheric level. Previous estimates of Hg content in the tropical forest canopy have been made but not systematically.
Section snippets
Study area
Two sampling sites were chosen. The first is the field station of ‘Les Nouragues’, the French CNRS observatory in a remote area (4°05′ N, 52°41′ W) situated 100 km away from the oceanic coast at an altitude of 50 m above sea level (asl). The site is under regular trade wind influence throughout the year with a prevailing northeast direction. This atmospheric circulation involves the first 1000 m of the atmosphere (the low troposphere), an air layer which carries most of the atmospheric load.
Analyses and results
After each campaign, the collected leaves were dried following the same procedure as used for sediments (2 days in an oven at 60 °C). This procedure does not appear to affect the total Hg content in sediments. Dried leaves were then sealed in plastic bags and sent to the Centre d'Analyse du CNRS (Vernaison, France). The total Hg concentration was measured using the cold vapor technique with atomic absorption spectrometry detection (Ama 254, Spectra France). The dried leaves were directly placed
Mean leaf concentration
At Les Nouragues, the total Hg concentration ranges from 32.4 to 114 ng g−1, whereas at Pic Matecho concentration ranges from 52.4 to 103.0 ng g−1. For a given species, dispersion appears to be small whereas leaves can be of different ages. Both measurements in the central nerve indicate that Hg concentration is systematically lower (5–10 times lower) than in the blade. The mean values obtained at Les Nouragues (59±28 ngHg g−1) and at Pic Matecho (70±16 ngHg g−1) cannot be considered to be
Conclusion
The purpose of this study was to estimate the canopy contribution to the Hg present in the soils of the French Guiana tropical rainforest through the litterfall deposit only. This canopy contribution originates mainly from the atmospheric background load. A study of Hg concentrations in the foliage of the canopy was carried out in two remote areas where gold mining has not occurred and which are free from any other anthropogenic influences. The sampling (11 species) of the canopy is
Acknowledgements
We are very grateful to S. Mori for helpful comments. This project was supported by The Mission du Parc de la Guyane, the Programme ‘Mercure en Guyane’ (INSU-CNRS, Ministère de l'Environnement, Europe), and the University of Grenoble.
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