Relationships of eutrophication to the distribution of mercury and to the potential for methylmercury production in the peat soils of the everglades
Elevated mercury concentrations have been reported in fish and wildlife from the Everglades in recent years. The hypothesis that eutrophication caused by the impact of phosphorous- (P) rich agricultural runoff stimulated methylmercury accumulation was put forward because eutrophication had been shown to be a cause for methylmercury accumulation in other ecosystems. We tested this hypothesis by obtaining total mercury (Hg(T)) concentrations and accumulation rates and estimating the potential for microbial methylation and methylmercury degradation in peat soils collected along a P gradient in water conservation area 2A (WCA-2A). A negative correlation observed between Hg(T) and P concentrations in soils (r2 = 0.64) was explained by increased peat accretion rates in a nutrient-enriched area (7.1-7.5 mm yr-1) as compared to an unenriched area (1.922.50 mm yr-1), estimated using lead-210 and cesium-137 dating. Total Hg accumulation rates (post-1964) were comparable for the enriched and unenriched sites (29-30 and 29-37 μg m-2 yr-1, respectively). Thus, calculations of Hg(T) accumulation rates are confounded by differences in peat accretion rates in the Everglades. Potential rates for both methylation (2.3-48.6 ng g-1 day-1) and demethylation (6.5-113.2 ng g-1 day-1) were higher in samples from WCA-2A than in samples collected in an area of the Everglades that had never been exposed to nutrients. However, trends suggesting the relationships of these activities to the P gradient in WCA-2A were not detected, and methylation to demethylation ratios did not correlate with soil P concentrations. The results suggest that (i) nutrient-enriched agricultural runoff originating upstream of WCA-2A did not contribute significantly to Hg(T) built up in the northern Everglades and (ii) eutrophication did not affect the potential for net methylmercury formation in peat soils.
Vaithiyanathan, P; Richardson, CJ; Kavanaugh, RG; Craft, CB; Barkay, T
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