The effect of agricultural run-off on mercury biogeochemistry in the Everglade

High methylmercury levels in Everglades fauna were first noted in 1990, but the factors that have contributed to the accumulation of mercury in the system are not yet fully understood due in part to the complex cycling of mercury between the air, soils, and water. Surface waters entering the Everglades from the north are high in nutrients and organic matter due to agricultural activities upstream, and this run-off may be enhancing the production of methylmercury within the wetland by increasing mercury loading and creating conditions that favor mercury-methylating microbes. The objectives of this study are: (1) develop analytical approaches for analysis of total mercury and methylmercury in Everglades soils, (2) to assess mercury accumulation rates in soils from the northern Everglades that have been impacted to different degrees by agricultural run-off, thereby revealing any trends resulting from the transport of mercury into the system with surface waters, and (3) to determine the specific effects of phosphorus on soil methylmercury concentrations by examining soils subjected to a range of a phosphorus loading rates in mesocosm-scale study where only phosphorus levels have been manipulated.;The procedures established for sample collection, processing, and analysis provided for accurate measurement of total mercury and methylmercury in certified reference materials. Detection limits for both methods were low enough to permit accurate measurement of methylmercury and total mercury in Everglades soils (1.34 ng Hg/25.6 ml digest for total Hg, and 1.96 pg (as Hg)/25 ml distillate for McHg). Total mercury concentrations range from 89--420 ng Hg/g dry soil, and mercury accumulation rates are significantly greater at sites highly and moderately impacted by agricultural run-off (75 and 84 ug/m2/yr, respectively) than at relatively unimpacted sites (58 ug/m2/yr) (statistical significance determined using Pearson Product Moment Coefficient, p#0.05). These accumulation rates are greater than can be accounted for by published atmospheric deposition rates, and suggest that mercury enters the Everglades from sources other than wet and dry deposition from the atmosphere.;Methylmercury concentrations in samples collected in the Phosphorus Dosing Study range from 28--299 pg Hg/g dry wt., while total mercury concentrations range from 89--391 ng Hg/g dry wt. There are no statistically significant correlations between soil phosphorus concentrations and methylmercury levels, nor are there significant correlations between soil methylmercury and water column concentrations of orthophosphate or total phosphorus. Even though eutrophication may ultimately contribute to conditions that favor mercury methylation, elevation of phosphorus levels alone does not appear to increase methylmercury concentrations in Everglades soils.;The results of these studies suggest that agricultural run-off is a source of mercury to the Everglades, but that the phosphorus component of the run-off has no specific effect on methylmercury concentrations in soils.