Dissolved trace metals in Lakes Superior, Erie, and Ontario

In spite of continuing large inputs from anthropogenic sources, average concentrations of many trace metals in the Great Lakes remain quite low: 2.8-4.5 ng L(-1) for Cd, 3.2-11 ng L(-1) for Pb, and 87-277 ng L(-1) for Zn. These metals are rapidly scavenged by the seston (suspended particulates) and have a rapid turnover rate in the water column. Factors that affect the distribution of dissolved trace metals include water depth, seston abundance, and biological processes. Higher concentrations are generally found in nearshore stations and especially near the urban centers and polluted river mouths. In summer, there is a marked depletion of Zn (and other bioactive metals) in the epilimnion of the offshore waters. The patchiness in the concentrations of the metals is attributed to spatial differences in the biological processes. With the exception of Cr, there is no systematic increase in concentration down the drainage basin from Lake Superior to Lake Ontario. Most of the Pb, Cd, and Zn loadings into the lakes are retained in the basin, but there is a significant export of dissolved Cu, Ni, and Cr via the St. Lawrence River.