The phase and species distribution of mercury (Hg) in an aquatic system has a controlling influence on its toxicity, transport mechanisms, and ultimate fate. Previous researchers working primarily on lakes, streams, and estuaries have demonstrated significant changes in the aqueous phase and species distribution of mercury on a seasonal or annual basis, while shorter term variations (hours to days) are less well documented. In ecosystems with minimal water depths, such as wetlands, short-term changes in aqueous Hg species concentrations are likely to be more pronounced. Concentration changes in the water column result from atmosphere/water/sediment exchange processes (deposition, evasion, particle settling, resuspension, and advection/diffusion), bio-uptake, methylation and demethylation at the sediment/water interface, and processes operating in the water column such as particle scavenging and photochemical reduction. The Florida Everglades with a shallow water column, year-round warm temperatures, high degree of sun incidence, and a documented mercury accumulation problem is an ideal location to study short-term changes in mercury speciation. Two different types of field efforts have been conducted during 1995 and 1996 to examine short-term variability in mercury speciation in the Everglades: diel sampling, and sunlight exposure/incubation experiments. The studies were conducted in the relatively pristine areas of Water Conservation Area (WCA) 2A, where high methylmercury concentrations in water and biota had been observed previously.

Diurnal studies were designed to determine extent to which total mercury (HgT), methyl mercury (MeHg), elemental mercury (Hg0), and reactive mercury (HgR) varied in concentration in response to environmental factors that vary on a daily basis such as sun intensity, temperature (air and water), redox, pH, and precipitation events. Samples for these Hg species were collected at about 90 minute intervals. All of the Hg species showed significant variations during the diel sampling periods. Elemental mercury showed a 100 percent increase in concentration at maximum sun intensity (near noon) compared to samples collected at night, emphasizing the importance of photochemical reduction in this environment. Night time Hg0 concentrations in the water column were calculated to be in equilibrium with the air mass above the water, suggesting Hg0 production and evasion ceases at night. Seasonally, Hg0 concentrations do not appear to be controlled by the pool of HgT in the water column, but rather appears to be a primarily a function of sunlight intensity. This suggests that photochemical reduction of Hg(II) to Hg0 is the primary process leading to Hg0 production in the Everglades. A simple mass balance shows that the flux of Hg to the atmosphere from diel DGM production and evasion represents about 10 percent of the annual input from atmospheric deposition.

While the diel response of Hg0 was somewhat anticipated, surprisingly HgT, HgR, and MeHg also showed substantial, short-term variability during diel sampling. Variability in HgT and HgR appear to be controlled by two factors: inputs from rainfall and a process that is tied to the solar cycle. Inputs from rainfall result in "spike" increases of water-column HgT concentrations and to a lesser degree HgR. During one diel sampling when a significant storm occurred, concentrations of HgT rose by 50 percent, but had re-equilibrated to pre-storm concentrations within eight hours. Overlain on this incremental signal from rainfall is a nearly sinusoidal trend that appears to be related to solar intensity. The observed trend in HgT (filtered and unfiltered) indicates the total mass of Hg in the water column is changing on a diel basis, with greater concentrations observed during daylight. This observation leads us to conclude that a portion of the total mass of Hg in the water column at this study site is moving into and out of solution on a diel basis. A model that can possibly explain this observation is photochemically driven sorption and desorption, which has been shown for other trace metals. Future research efforts will seek to reveal the details of this process. Diel variability in MeHg concentrations, although substantial, did not appear to be directly linked to photolysis reactions nor rainfall.

The sunlight exposure/incubation experiments were conducted to determine the precise mechanisms of Hg0 production. Each experiment was conducted by uniformly filling pre-cleaned Teflon bottles with filtered surface water from a pristine sampling site in WCA2A. At time = 0, 0.5, 1, 2, 3, and 4 days, bottles were collected for Hg0, HgT, MeHg, and HgR analyses. Results from these experiments show that Hg0 production is a dissolved phase process, and that it is primarily driven by sunlight in the ultraviolet range of the solar spectrum. Methyl mercury shows an initial rapid loss in the first 24 hours, but little loss after that time. Each incubated sample shows a steady loss of HgT from solution, of which only about 50 percent could be accounted for by loss to evasion of Hg0. Future experiments will be performed to determine the fate of this "missing" mercury.

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