Mercury accumulation in sediment cores and along food chains in two regions of the Brazilian Pantanal

Abstract

The Pantanal is a 140,000 km2 floodplain wetland stretching acrosswestern Brazil and parts of Bolivia and Paraguay. Gold mining withmercury (Hg) amalgamation has thrived since 1980 along its northern rim. We quantified Hg accumulation in sediment cores (N = 5) and food chainsin this general region of the northern Pantanal and in a reference region,200 km deeper into the wetland (Acurizal). Cores were dated with210Pb and 137Cs using direct gamma-assay. Total Hg wasanalyzed by cold-vapor atomic fluorescence using a gold-meshpre-concentration trap. Average pre-1940 Hg accumulation in cores wasnot significantly different (N = 5, p= 0.14) between both regions andcomparable with rates calculated for global reference sites. Post gold-rushHg (post-1980) deposition averaged 55 ± 11.3 μg m-2yr-1 in the northern impacted region and was more than 1.5 timeshigher than the post-1980 rate in Acurizal, implying a regional Hg effectof gold mining. Post-1980 Hg accumulation in Acurizal, in turn, was 2.1times the rate reported for a global reference during that time period,suggesting an additional basin-wide effect over such reference sites. Bycombining our core data with assessments of the size of the impacted areaand the amount of Hg released to the region since 1980, we estimated thatonly 2–8% of this Hg was recovered as a sedimentary signal. Theremainder of the Hg was lost to the atmosphere, downstream areas, orstored in biota. Hg concentrations in surface sediments in the northernPantanal (45.5 ± 5.5 ng gdry-1) were significantlyhigher than those in our reference region (29.1 ± 0.7ng gdry-1). Hg levels in primary producers were alsoelevated in the northern Pantanal. Eichhornia crassipes rootscontained 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 ± 3.3ng gdry-1). Plant grazers and scavengers, such as apple snails(Pomacea sp.) and adult water beetles (Fam. Hydrophilidae), werelow in Hg, confirming previous data showing that the channeling of mercuryfrom lower to higher trophic levels along herbivorous links was inefficientcompared to transfer along carnivorous links. Collections of 12–16individuals of four species of Characidae (Aphyocharax sp., Tetragonopterus sp., Serrasalmus spiropleura and Pygocentrisnattereri) in both regions showed elevated Hg body burdens in bothpiranhas S. spiropleura and P. nattereri from the northernPantanal (149.9 ± 84.2 and 302.2 ± 159.1ng gdry-1, respectively). Fish length for each species was notstatistically different between regions. P. nattereri length correlatedsignificantly (p<0.001) with Hg content in both regions, but the slopeof the regression in the northern Pantanal was 2.6 times the slope for theAcurizal collection, indicating an elevated rate of biomagnification in theHg-impacted region. Signals of Hg use in mining can be quantified insediment core chronologies and biological tissues, although species atdifferent trophic levels show dissimilar impacts. Mechanisms involved in Hgmagnification along food chains deserve more attention, particularly intropical regions where the threat of chronic exposure to this neurotoxinmay have the greatest implications for biodiversity.