Total Hg in Water, Sediment, and Four Species of Aquatic Macrophytes in the St. Lawrence River, near Cornwall, Ontario

Previous research on mercury (Hg) in lakes and rivers has identified pH, alkalinity, and organic carbon levels as the major environmental variables which predict Hg concentrations in fish and invertebrates. However, the factors that regulate accumulation in wetland systems and aquatic plants in particular have yet to be identified. Total Hg in sediments, water, and four species of aquatic macrophytes [yellow pond lily (Nuphar variegatum), eurasian water milfoil (Myriophyllum spicatum), common waterweed (Elodea canadensis), and curly leaf pondweed (Potamogeton crispus)] were measured in twenty-three wetlands along the St. Lawrence River. The total Hg concentration in N. variegatum ranged from 6.4 to 36.5 ng/g and remained fairly constant in relation to measured environmental variables (pH = 7.6 to 8.4; alkalinity = 1,217 to 3,128 eq/L Ca; DOC = 3 to 12 mg/L; organic content of the sediment = 0.9 to 49%). As well, water Hg (3 to 19 ng/L), and sediment Hg concentrations (15 to 882 μg/kg) showed no relationships with Nuphar Hg. Because the plant concentrations were lower than the sediment concentrations, it appears that Nuphar is an excluder of Hg. In contrast, Hg concentrations of the submerged species were significantly higher with the highest concentrations found in Myriophyllum (63 to 240 ng/g) followed by Elodea (58 to 225 ng/g) and Potamogeton (80 to 85 ng/g), and these species were often found to be Hg accumulators in the wetlands. However, as with Nuphar, no correlations were observed with environmental concentrations of Hg and the chemical variables measured. None of these macrophyte species appear to be good biomonitors of sediment Hg contamination.