Effect of Sediment Copper on the Distribution of Benthic Macroinvertebrates in the Keweenaw Waterway

Abstract

Benthic macroinvertebrates, sediment copper, and sediment particle size were examined in two areas of the Keweenaw Waterway. The north area is downdrift from deposits of copper tailings. South area sediment averaged 27% silt and clay and north area sediment 66%. South area sediment had an average copper content of 33 mg/kg and north area sediment 589 mg/kg. The number of invertebrates was 4.3 times greater in the south than the north. The average number of taxa at south stations was 20 and at north stations 8. Mollusks, mayflies, and crustaceans were common in the south but rare or absent in the north. Hexagenia was the most abundant animal in the south. A single individual was found in the north, at the only north station with low sediment copper. Three north stations with sediment particle size similar to south stations had the same restricted fauna of chironomids and oligochaetes as that found at other north stations. Equitability and Shannon-Wiener diversity values for most north stations did not indicate pollution because of the extremely low, and thus, even numbers of individuals collected in all taxa. Twenty-six taxa were common to both areas. Four taxa, found only in the north, were represented by five individuals. Twenty-five taxa, found only in the south, were represented by 1,007 individuals. The likely cause of the reduced invertebrate fauna in the north area is high levels of sediment copper.