POLYCHLORINATED BIPHENYL CONGENERS IN EMERGENT MAYFLIES FROM THE UPPER MISSISSIPPI RIVER

We determined polychlorinated biphenyls (PCBs) in emergent mayflies (H exagenia bilineata) from a 1250-km reach of the upper Mississippi Rive r (UMR). Total PCB concentrations (sum of 125 congeners) ranged from 0 .21 to 4.1 mu g/g of dry weight (1.2-29 mu g/g of lipid weight). Conce ntrations were highest in pools near the Twin Cities and the Quad Citi es metropolitan areas. Longitudinal movement of PCBs was extensive dow nstream from the Twin Cities (175-320 km) but was not apparent downstr eam from the Quad Cities. The PCB composition of mayflies was relative ly homogeneous throughout most of the river. However, the congener com position in mayflies from two distant locations differed markedly from the other samples and contained a greater abundance of lower molecula r weight congeners. Recent PCB discharges from point and nonpoint sour ces may account for these differences. Emergent mayflies seem to be a useful indicator of PCB contamination of the UMR.