| Abstrakt: |
The Pantanal is a 140,000 km2 floodplain wetland stretching across western Brazil and parts of Bolivia and Paraguay. Gold mining with mercury (Hg) amalgamation has thrived since 1980 along its northern rim. We quantified Hg accumulation in sediment cores (N = 5) and food chains in this general region of the northern Pantanal and in a reference region, 200 km deeper into the wetland (Acurizal). Cores were dated with 210Pb and 137Cs using direct gamma-assay. Total Hg was analyzed by cold-vapor atomic fluorescence using a gold-mesh pre-concentration trap. Average pre-1940 Hg accumulation in cores was not significantly different (N = 5, p= 0.14) between both regions and comparable with rates calculated for global reference sites. Post gold-rush Hg (post-1980) deposition averaged 55 ± 11.3 μg m-2 yr-1 in the northern impacted region and was more than 1.5 times higher than the post-1980 rate in Acurizal, implying a regional Hg effect of gold mining. Post-1980 Hg accumulation in Acurizal, in turn, was 2.1 times the rate reported for a global reference during that time period, suggesting an additional basin-wide effect over such reference sites. By combining our core data with assessments of the size of the impacted area and the amount of Hg released to the region since 1980, we estimated that only 2–8% of this Hg was recovered as a sedimentary signal. The remainder of the Hg was lost to the atmosphere, downstream areas, or stored in biota. Hg concentrations in surface sediments in the northern Pantanal (45.5 ± 5.5 ng gdry-1) were significantly higher than those in our reference region (29.1 ± 0.7 ng gdry-1). Hg levels in primary producers were also elevated in the northern Pantanal. Eichhornia crassipes roots contained 2.7–3.0 times more mercury than shoots in both regions and Salvinia auriculata, suggested as a biological monitor for Hg pollution, contained almost four times more mercury in the northern Pantanal (90.7 ± 9.1 ng gdry-1) than in Acurizal (24.5 ± 3.3 ng gdry-1). Plant grazers and scavengers, such as apple snails ( Pomacea sp.) and adult water beetles (Fam. Hydrophilidae), were low in Hg, confirming previous data showing that the channeling of mercury from lower to higher trophic levels along herbivorous links was inefficient compared to transfer along carnivorous links. Collections of 12–16 individuals of four species of Characidae ( Aphyocharax sp., Tetragonopterus sp., Serrasalmus spiropleura and Pygocentris nattereri) in both regions showed elevated Hg body burdens in both piranhas S. spiropleura and P. nattereri from the northern Pantanal (149.9 ± 84.2 and 302.2 ± 159.1 ng gdry-1, respectively). Fish length for each species was not statistically different between regions. P. nattereri length correlated significantly (p<0.001) with Hg content in both regions, but the slope of the regression in the northern Pantanal was 2.6 times the slope for the Acurizal collection, indicating an elevated rate of biomagnification in the Hg-impacted region. Signals of Hg use in mining can be quantified in sediment core chronologies and biological tissues, although species at different trophic levels show dissimilar impacts. Mechanisms involved in Hg magnification along food chains deserve more attention, particularly in tropical regions where the threat of chronic exposure to this neurotoxin may have the greatest implications for biodiversity. [ABSTRACT FROM AUTHOR] |
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