The Aquatic Cycling of Mercury in the Everglades (ACME) project was initiated in 1995 to provide a better understanding of the factors resulting in high mercury (Hg) levels in resident wildlife of the Florida Everglades. The overall objective of the ACME project was to examine in detail the biogeochemical processes that control mercury cycling in the Everglades, with particular emphasis on the mercury methylation and bioaccumulation. Ultimately, our intent is to integrate all our findings through the use of a variety of models so that potential effects of various ecosystem restoration alternatives on mercury toxicity can be tested.

To date, the ACME project has provided many new insights into the processes controlling mercury cycling in the Everglades, as well as a better general understanding of biogeochemical processes operating in wetlands. Surface water Hg concentrations are low (total Hg concentrations generally less than 5 nanograms per liter), and are variable both spatially and temporally. At the ecosystem scale, seasonal variations in total Hg concentrations in surface water appear to be primarily controlled by rainfall, the dominant source of Hg to the Everglades, although there is significant variability in these seasonal trends. There are no apparent spatial trends in total Hg in surface water, suggesting that atmospheric-Hg deposition of is randomly distributed.

Methylmercury (MeHg) in surface water shows a general trend toward increasing concentrations from north to south, with maximal concentrations observed in southern Water Conservation Area (WCA) 2 and central WCA 3. Seasonally, MeHg concentrations are generally highest in the summer, when deeper water levels and warmer conditions exist. These trends in MeHg concentrations are in agreement with the MeHg concentrations observed in sediments, and measured rates of microbial methylation. At any particular location, Hg methylation rates were found to be greatest in southern Water Conservation Area (WCA) 2 and central WCA 3, and the site of greatest methylation activity was the surficial sediments. Microbial mercury demethylation is also occurring in Everglades peat, but unlike methylation estimates, demethylation rates showed no strong spatial trends. The primary Hg methylating agents were determined to be sulfate reducing bacteria, which emphasizes the need to examine sulfur cycling in parallel with mercury cycling studies. Excess sulfate is currently being delivered to the Everglades, and probably originates from agricultural runoff in the northern part of the system. Studies of dated cores from WCA 2A show that the influx of excess sulfur began in the early part of this century, concomitant with an influx of excess phosphorus. The excess sulfate stimulates bacterial sulfate reduction over large areas of the northern Everglades, and has a significant but complex relation to the extent and distribution of MeHg in the ecosystem.

Fourteen diel studies conducted from 1995 to 1998 demonstrated the importance of short-term changes in redox gradients that control the speciation and fluxes of Hg from the sediments to the water column, and from the water column to the atmosphere. In addition, close-interval sampling (centimeter scale) conducted during the diel studies documented the existence of strong chemical gradients originating from the near-surface sediments, and are the primary transport mechanism of MeHg to the water column. Photochemical reduction and demethylation reactions both occur on a diel basis in the Everglades, and appear to be regulated by dissolved organic carbon, which limits light penetration into the water column. Mass-balance calculations show that most of the seasonal variability in MeHg concentrations in the water column can be accounted for by the estimated efflux rates from sediments and the photochemical demethylation rates in the water column.

Accumulation of MeHg in biota follows the same general trends observed for water-column MeHg concentrations and methylation rates, however, seasonal variability can be significant. Examination of gut contents of Gambusia (the dominant forage fish in the Everglades) showed that zooplankton are an important dietary component. This observation was in contrast to observations made early in the project, however, when very few zooplankton were found in the water column. Biota sampling on a diel basis showed that zooplankton are only present in the water column at night, and hide from predators during the day.

The ACME project has demonstrated the dynamic nature of the mercury cycle in the Everglades, and similar processes are likely operating in wetlands elsewhere. The existence of transient chemical gradients appears to be the key to understanding the complex relations between this atmospheric derived pollutant, rapid biogeochemical cycling and efficient bioaccumulation.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)