Chlorinated hydrocarbon cycling in the benthic nepheloid layer of Lake Superior

Abstract

The dynamics and composition of the benthic nepheloid layer (BNL) in western Lake Superior were studied with respect to the cycling of trace hydrophobic organic compounds. The BNL was enriched in t-PCB, p,p'-DDE, and hexachlorobenzene (HCB) but deficient in dissolved organic carbon relative to surface waters. Resuspension events in midsummer resulted in a 50% increase in the PCB burden in the water column. Seasonal cycling of PCB congeners was strongly dependent on their degree of chlorination, with heavier chlorinated congeners lost from the water column (t/sub 1/2/ = 17-28 days at two sites). Steady-state t-PCB concentrations in the BNL were maintained over the summer by transport of lighter chlorinated congeners from the underlying sediments. DDE is an effective tracer of sediment resuspension in Lake Superior. Recycling of trace organic pollutants in the BNL serves to increase their resident times in the lake.