Assessment framework for mid-Atlantic coastal plain streams using benthic macroinvertebrates

A collaborative study among 6 states along the mid-Atlantic seaboard of the USA developed a consistent approach for collecting and interpreting macroinvertebrate data for low-gradient streams of the coastal plain. The study had 3 objectives: 1) to evaluate the validity of aggregating reference site data into a single bioregion, 2) to select biological metrics that best discriminated reference sites from sites impaired by habitat disturbance and organic pollution, and 3) to combine these metrics into an index of biological quality. Macroinvertebrate, physical habitat, and water-quality data were collected in 106 streams during autumn 1995. Fifty-five sites were reference, 34 sites had habitat stresses, and 17 sites had water-quality stresses. Classification of reference sites divided the coastal plain into 3 bioregions, separated north and south by Chesapeake Bay and separated east and west by ecoregion. Five metrics were effective at discriminating impairment: number of taxa, number of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa, % Ephemeroptera, Hilsenhoff Biotic Index, and % clinger mode of existence. An aggregated index, the Coastal Plain Macroinvertebrate Index (CPMI), was developed using these metrics. The CPMI accurately identified 86% of impaired sites. The precision of CPMI scores was estimated to be ±10% (3 scoring units out of 30) at the 90% confidence interval. The CPMI accurately assigned both habitat disturbance and water-quality impairment indicating a similar degree of ecological impact from these 2 stressors. Guidance is provided for applying the CPMI to other macroinvertebrate data sets in the region.