Insect Community Structure as an Index of Heavy-Metal Pollution in Lotic Ecosystems

Abstract

Data from two streams suggest that macroinvertebrate community structure exhibits a predictable, graded response to heavy-metal pollution. In the more heavily stressed sections of both streams, macroinvertebrates, other than tubificid worms and chironomids, were virtually eliminated from rock-rubble, riffle habitats. Midge larvae comprised 75 to 86% of all insects collected from the most grossly polluted stations and less than 10% of the insect communities at the least polluted stations. The correlation coefficient for percentage of chironomids in relation to copper concentration was +0.93 (P ≤ 0.01). Changes in the coefficient of variation (CV) of percentage of chironomids per sample exhibited a consistent pattern. The lowest CV was at the most polluted stations and the CV increased along a gradient of decreasing metal concentrations, reaching maximum values at the least polluted stations. The number of species of chironomids was inversely related to the proportional density of the family in samples. In Elam's Run, the number of species of chironomids identified in samples increased from 15 at the most polluted station to 39 at the least polluted stations. Fifty-seven species of midges were collected along the 3-km stretch of this grossly polluted stream. Caddisflies were numerically important at the intermediately polluted stations in both streams. Mayflies occurred only at the least polluted stations. We hypothesize that heavily polluted habitats are dominated by midges, moderately polluted habitats by midges and caddisflies, and minimally polluted or unpolluted habitats by caddisflies and mayflies. Since there appears to be a direct relationship between the fraction of a community composed of chironomids and the degree of pollution, we suggest that the percentage of chironomids in samples may be a useful index of heavy-metal pollution.Key words: heavy metals, stream pollution, chironomids, aquatic insects