A 50-yr Record of Pollution by Nutrients, Trace Metals, and Organic Chemicals in the St Lawrence River

Abstract

The deposition chronology of algal pigments, organic C, N, P, trace metals, and organic contaminants (PCB congeners, DDD, DDE, Mirex, hexachlorobenzene) is described for two sites in fluvial lakes (St. Francis and St. Louis) of the St. Lawrence River. Pigment concentrations rapidly decrease in the top 10 cm due to degradation, but show a broad secondary peak consistent with higher primary productivity between 1960 and 1975. In Lake St. Francis, deposition of organic C, N, and P peaked between 1950 and 1975. In both lakes, trace metal concentrations reached maximum values between 1960 and 1970 and decreased considerably between 1970 and 1980. At both sites, surficial trace metal concentrations are approaching (within a factor of 3) those observed in preindustrial sediments except Cd, which is still 5–6 times higher. Concentrations of Cr, Cu, Ni, and Zn are inversely related (p < 0.05) to river discharge whereas those of Pb are positively related to discharge. Concentrations of organic contaminants have declined by a factor of 5–10 between the mid-1960's and the early 1980's, except Mirex, which shows no obvious trend with time. Historic trends in pollution by organic chemicals appear to have been influenced not only by changing anthropogenic inputs, but also by river discharge.