The median wetland water MeHg concentration reported here was slightly higher relative to those reported from the Florida Everglades (range of reported medians 1995–2005: 0.19–0.28 ng/L; Scheidt and Kalla, 2007) and on the lower end of concentrations reported in a recent study from agricultural and managed wetlands in California (0.04–7.3 ng/L filtered; Alpers et al., 2014). Methylmercury concentrations in the open water portions of Lake Zapotlán were similar to reported values from natural lakes in temperate and northern oligotrophic lakes in North America (< 0.3 ng/L; Bodaly et al., 1998), despite the relatively high inputs of THg in natural and wastewater runoff, and high MeHg in wastewater. The explanations for these lower MeHg concentrations are potentially more complex than those for reduced THg in this PYZD-4409 system. Although particle setting is an important mechanism that affects all particle-bound Hg species, there are a number of species-specific mechanisms that can result in reduced MeHg concentrations. Potentially high MeHg photodecomposition rates in this shallow and warm subtropical freshwater system may account for low MeHg concentrations measured in surface waters (Seller et al., 1996, Zhang and Hsu-Kim, 2010 and Black et al., 2012). The biogeochemical conditions of this environment may not favor net MeHg production, which seems to be reflected in the low %MeHg in lake sediments. This could be a function of the high pH (Kelly et al., 2003 and Golding et al., 2008) or high concentrations of divalent cations (inferred from pH and conductivities of ~ 1000 μS/cm) limiting the uptake of bioavailable Hg(II) by methylating bacteria (Daguene et al., 2012). In eutrophic systems, such as Lake Zapotlán, so-called bloom dilution may be a factor, where during periods of high primary production of phytoplankton the amount of bioavailable Hg in the water column is sorbed to, and distributed among, a greater number of algal cells. Previous work has shown that this can result in lower inorganic Hg and lower MeHg concentrations throughout the food web (Pickhardt et al., 2002).