Factors influencing the accuracy of a macroinvertebrate bioassessment protocol in south carolina coastal plain streams

The multiple habitat sampling protocol (MHSP) is a bioassment method designed to assess the ecological health of South Carolina streams on the basis of macroinvertebrate samples collected from natural substrates. The MHSP is computed by averaging the EPT (numbers of Ephemeroptera, Plecoptera, Trichoptera taxa) and BI (a biotic index that reflects the pollution tolerances of individual taxa) to produce a bioclassification score. The MHSP produced low bioclassification scores that could falsely indicate environmental degradation in some undisturbed, high quality streams in the Sandhills ecoregion. This problem had two causes. The metrics (especially EPT) were significantly related to stream size, which potentially confounded stream size effects with environmental impacts, and the scoring criteria for EPT were too high for some Sandhills streams. We corrected these problems by developing new scoring criteria for Sandhills streams that utilized residuals from regressions of the metrics on stream width to normalize for stream size. The MHSP and related protocols are effective methods for assessing environmental quality, but allowances must be made for the effects of stream size and the potential ecological heterogeneity that naturally exists among streams in some ecoregions.