A complex relationship exists between sulfate concentrations and the formation of methyl mercury. Maximum formation of methyl mercury appears to occur around sulfate concentrations of 10-20 mg/L. At sulfate concentrations below this range, increases in sulfate will result in increases in methyl mercury formation. At sulfate concentrations above this value, increases in sulfate will result in decreases in methyl mercury formation. This response results in a “hotspot” of elevated methyl mercury concentrations and fish mercury concentrations in the Everglades (see NRC, 2010). The location of this hotspot would likely shift with variations in water discharge and transport of sulfate.
Monitoring data show clear spatial patterns of fish mercury that are linked to the spatial patterns of sulfate (Figure 4-14) and nutrients (Figure 4-4) in the Everglades. Recent monitoring data (2009-2010) for largemouth bass show low methyl mercury concentrations in the STAs (~0.1 ?g/g), high concentrations in the WCAs (~0.5 ?g/g), and very high concentrations in Shark River Slough in the Everglades National Park (~1.2 ?g/g; much higher than in other portions of the Park) (Gu et al., 2012). For reference the EPA-recommended criterion for fish mercury is 0.3 ?g/g. This spatial pattern reflects variations in the processes controlling fish mercury concentrations. Under high sulfate concentrations in waters adjacent to the EAA, as in the STAs, microbes produce high sulfide concentrations that inhibit the production and bioavailability of methyl mercury (Benoit et al., 2003). As EAA drainage moves south into the WCAs and ultimately into Everglades National Park, sulfate concentrations and the production of sulfide generally decrease, thereby allowing for more formation of methyl mercury by reducing inhibition effects from sulfide. Nutrients potentially also play an important role. High inputs of nutrients from the EAA support high biomass production, which decreases the mercury concentration in biota via a process known as biodilution.7 With decreases in phosphorus concentrations with distance from the EAA, decreases in net aquatic production decrease the biodilution phenomenon and concentrations of mercury in fish and other biota increase.
Long-term observations show that concentrations of mercury in largemouth bass have significantly declined in the WCAs since measurements were initiated in the late 1980s (Figure 4-15). Mercury concentrations in largemouth bass were very high in the early to mid-1990s in the WCAs. Indeed the “hotspot” of fish mercury at that time was located in WCA-3A. However, the decreases in fish mercury concentrations ceased by 1998, and concentrations have remained relatively constant since that time. These decreases in mercury concentrations in largemouth bass are thought to result from declines in sulfate inputs (Kalla et
7Biodilution is a phenomenon through which concentrations of a contaminant (e.g., mercury) in organisms decrease because of increases in nutrient supply and associated increases in biomass (Chen and Folt, 2005; Pickhardt et al., 2002).