Mercury Speciation and Distribution in Coastal Wetlands and Tidal Mudflats: Relationships with Sulphur Speciation and Organic Carbon

Sediment cores were analysed from four coastal wetland sites within the Minas Basin, Bay of Fundy to compare mercury speciation and sediment characteristics. The coastal wetland sediments were low in total mercury (mean = 17.4 ± 9.9 ng g−1); however, MeHg concentration was 92 times higher (mean of 249 pg g−1) than intertidal mudflat sediment (mean of 2.7 pg g−1). Total mercury concentrations in intertidal mudflat cores were also low (0.5–23.7 ng g−1) and correlated (Pearson correlation = 0.98; p < 0.01) with % organic carbon; with low concentrations of MeHg present only below depths of 6 cm (mean = 2.7 ± 1.0 pg g−1). Total mercury concentrations were negatively correlated (correlation = 0.56, p < 0.05) with inorganic sulphur (acid volatile sulphides (AVS) and pyrite) while MeHg concentrations were inversely correlated (Pearson correlation = −0.68; p < 0.05) with the pyrite content but not with AVS. Methyl mercury concentrations were not significantly correlated with organic carbon content in the wetland sediments, and mercury-in-biomass enrichment factors were lower (total mercury mean 1.5 ± 1.9 and MeHg mean = 3.6 ± 4.8) than published measurements from mercury polluted sites. Modelling estimates found on average 4.4 times more total mercury mass in the intertidal mudflat sediments relative to vegetated wetlands. A negative relationship was observed between MeHg concentrations (below 20 cm depth) and modelled tidal inundation. The mineral fraction within wetland sediments contained 96.2% of the total mercury mass; however, the highest concentrations of mercury species were in root biomass. This research confirms that vegetated coastal wetlands are key areas for formation of bioavailable methyl mercury, and mercury distribution is tied to organic carbon and sulphur speciation.