Limnol. Oceanogr., 44(7), 1999, 1815–1825

Trophic accumulation of mercury (Hg) in aquatic ecosystems is of global concern due to health effects associated with eating fish with elevated Hg levels. The methylated form of Hg bioaccumulates so it is important to understand how inorganic Hg is transformed to methylmercury in the environment. Here, a new site for Hg methylation, the periphyton communities that are prevalent in the Florida Everglades, is described. It is hypothesized that periphyton communities that support an active microbial sulfur cycle support Hg methylation. This new methylation site has implications for trophic transfer of methylmercury since periphyton can be the base of the food web in aquatic ecosystems. Mercury accumulation in food webs is a global problem (Clarkson 1990; U.S. Environmental Protection Agency 1997) that is exacerbated in wetlands (St. Louis et al. 1994), including the Florida Everglades (Ware et al. 1990; Cleckner et al. 1998). Increased rates of atmospheric mercury (Hg) deposition relative to historical times worldwide (Mason et al. 1994) may contribute to increased production of methylmercury (MeHg) in aquatic ecosystems. Mercury is methylated in situ through the action of microorganisms (Winfrey and Rudd 1990; Gilmour and Henry 1991), and it is the methylated form that bioaccumulates in food webs (Bloom 1992). Because anthropogenically derived Hg must undergo methylation before it bioaccumulates in food webs, MeHg production is a key process in the complex biogeochemical cycle of Hg. Wetlands in particular support elevated Hg methylation rates and MeHg concentrations (St. Louis et al. 1994; Hurley et al. 1995; Krabbenhoft et al. 1995; Gilmour et al. 1998). Specifically, aquatic sediments and wetland soils have been identified as key locations for MeHg production (Korthals and Winfrey 1987; Ramlal et al. 1993; Gilmour et al. 1998; Krabbenhoft et al. 1998a), with sulfate-reducing bacteria (SRB) being the important mediators (Compeau and Bartha 1985; Gilmour et al. 1992). Here, a novel site for MeHg production, the periphyton communities that are an abundant component of biomass in many wetlands, including the Everglades, are discussed. The data presented support the following hypothesis: periphyton communities that support an active microbial sulfur cycle, including dissimilatory sulfate reduction, also support Hg methylation. This finding is particularly important because, more so than sediments, periphyton is a direct food source to higher organisms such as invertebrates and small fish (Browder et al. 1994; Hill et al. 1996). We have been examining factors that affect MeHg bioaccumulation in the Florida Everglades as part of the Aquatic Cycling of Mercury in the Everglades (ACME) study. Our initial studies on Hg and MeHg distribution in water (Hurley et al. 1998), MeHg distribution and production in sediment (Gilmour et al. 1998), diel cycling of Hg (Krabbenhoft et al. 1998b), and trophic transfer of MeHg (Cleckner et al. 1998) in the northern Everglades provide detailed site descriptions and background information for the locations discussed here. Briefly, the main study area is Water Conservation Areas (WCA) 2A and 3A—two large, diked marshes in the northern portion of the remnant Everglades (Hurley et al. 1998). About 60% of the inflow water to WCA 2A originates from the Everglades Agricultural Area and is discharged through a distribution canal on the northern edge of the marsh. As a result, a strong nutrient gradient exists across WCA 2A, with high nutrient levels in the north and lower levels in the south. Sites F1 and U3 were chosen as end members of this gradient in WCA 2A (Fig. 1). WCA 3A is somewhat less affected by agricultural runoff, with precipitation accounting for about 60% of the input water. Our site numbers within WCA 3A begin with a 3A designation and run from 3A33 in the north to 3ATT in the south, with 3A15, our main study site, in central WCA 3A. Cattail (Typha spp.) is the dominant vegetation in the eutrophic area of WCA 2A (site F1), while sawgrass (Cladium jamaicense), spike rush (Eleocharis spp.), and bladderwort (Utricularia spp.) are codominant at the other sites. Everglades periphyton communities, consisting of living algae, bacteria, detrital particulate organic matter, and, in some cases, particulate calcium carbonate, are complex structures that range from filamentous green mats (main algal types are Spirogyra spp. and Mougeotia spp.) in eutrophic areas to calcareous mats (main algal types are diatoms and blue-green algae) in less impacted areas (Browder et al. 1994; McCormick et al. 1996). The term ‘‘periphyton’’ in this paper refers to both ‘‘true periphyton,’’ immobile organisms attached to substrates, and ‘‘pseudoperiphyton,’’ organisms that are ‘‘free-living, mobile, or creeping within the true periphyton’’ (Vymazal et al. 1994). Calcareous periphyton mats are usually attached to submerged macrophytes, particularly Utricularia spp. and Eleocharis spp., and/or to sediments. However, many mats break away from their substrates and become free-floating because of trapped oxygen formed during photosynthesis (Browder et al. 1994). Periphyton samples and filtered surface-water samples were collected using clean techniques from ACME study sites (Olson et al. 1997; Cleckner et al. 1998; Hurley et al. 1998). To maintain the integrity of periphyton and associated redox microstructure, there was no effort to separate algae from macrophytes or to remove invertebrates from the periphyton. Water and periphyton samples were analyzed for MeHg using distillation, ethylation and cold vapor atomic fluorescence spectroscopy (Horvat et al. 1993; Olson et al. 1997). All periphyton samples were analyzed in triplicate, with an average relative standard deviation (RSD) of 19%, while approximately 15% of the water samples were analyzed in duplicate, with a mean relative percentage difference (RPD) of 12%. Additionally, to characterize the types of algae comprising the periphyton communities of the Everglades and to