Mercury speciation in the Lower St. Lawrence Estuary

Mercury speciation was determined in the water column and sediment pore waters of the Lower St. Lawrence Estuary, and total Hg (HgT) was measured in seven biological species, including pelagic and demersal fish and crustaceans. In water, HgT concentrations ranged from 1.8 to 7.8 pM, with the highest levels in surface waters and the lowest in deep waters (1.8-2.9 pM). HgT concentrations in sediment pore waters were 10 times as high. with methyl mercury levels of 5.1-14.4 pM. Reactive Hg (HgR) generally accounted for around 20% of HgT in the water column and <30% in pore waters. The distribution of elemental Hg and HgR suggests that Hg(II) is reduced in surface waters and that inorganic Hg is mobilized during remineralization of organic matter at the sediment surface. In the biota, concentrations ranged between 0.05 and 0.89 nmol . g(-1) (wet weight). Feeding habits and habitats account for these observations. Results suggest that the sediment is the main source of Hg contamination for the biota. A mass balance calculation showed that the Hg flux entering the lower estuary from the St. Lawrence River is equivalent to the amount buried in estuarine sediments.