Distribution and transformation of elemental mercury in the St. Lawrence River and Lake Ontario

Elemental Hg (Hg-o) is a volatile species that is responsible for water-to- air transfer of Hg in Lake Ontario and the St. Lawrence River. We conducted two cruises in 1998 to identify spatial and temporal patterns in Hg-o leve ls in these systems and performed field and laboratory experiments on redox transformations of Hg. Elemental Hg concentrations were higher in Lake Ont ario than in the St. Lawrence River. At stations in Lake Ontario, Hg-o leve ls were higher at the bottom of the water column than at the surface, where as they were homogeneous throughout the water column of the river. Elementa l Hg concentrations were generally higher in July than in September and, in contrast with experiments on inland lakes, were relatively constant during the day except for a narrow peak at sunrise. Field and laboratory experime nts showed that photoreduction of Hg(II) in St. Lawrence River water was su bstrate limited and was influenced by visible and ultraviolet radiation. Ps eudo first-order kinetics best described photoreduction, with k values betw een 1 and 2.h(-1).