Variations in mercury concentration within and across lichen Xanthoparmelia spp. individuals: Implications for evaluating histories of contaminant loading and sampling design

Effects of thallus size and internal zonation on the Hg concentration in the foliose lichen Xanthoparmelia spp. were investigated. Size and zonation effects, if present, provide the potential for temporal records of atmospheric deposition to be recorded in lichens. Our results (n = 49; 0.4-13.8cm in diameter) indicated no significant relationship between Hg and size, although thalli less than 2cm in diameter tended towards lower Hg concentrations; and no zonation of Hg within thalli. Distinct zonation of Hg in thalli has been reported in some studies, but not in others, indicating regulatory mechanisms result by which Hg is released or relocated within the thallus under certain conditions. A secondary objective was to evaluate the variability of Hg in lichen individuals to drive future sampling designs. Within a size range of 2-8cm in diameter, we observed Hg = 154 ± 30ppb (mean ± s.d., n = 38). Bootstrap analysis of this dataset indicated that for a sample size of n = 3 thalli, we can expect a 94% probability that the variability in our sample set will be at least as low as that observed in other studies of Hg in lichen (s.d. ≈50ppb Hg).