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 macroi nvertebrate data for low-gradient streams of the coastal plain. The study h ad 3 objectives: 1) to evaluate the validity of aggregating reference site data into a single bioregion, 2) to select biological metrics that best dis criminated reference sites from sites impaired by habitat disturbance and o rganic pollution and 3) to combine these metrics into an index of biologica l quality. Macroinvertebrate, physical habitat, and water-quality data were collected in 106 streams during autumn 1995. Fifty-five sites were referen ce, 34 sites had habitat stresses, and 17 sites had water-quality stresses. Classification of reference sites divided the coastal plain into 3 bioregi ons, separated north and south by Chesapeake Bay and separated east and wes t by ecoregion. Five metrics were effective at discriminating impairment: n umber of taxa, number of Ephemeroptera, Plecoptera, and Trichoptera (EPT) t axa, % Ephemeroptera, Hilsenhoff Biotic Index and % clinger mode of existen ce. An aggregated index, the Coastal Plain Macroinvertebrate Index (CPMI), was developed using these metrics. The CPMI accurately identified 86% of im paired sites. The precision of CPMI scores was estimated to be +/-10% (3 sc oring units out of 30) at the 90% confidence interval. The CPMI accurately assigned both habitat disturbance and waterquality impairment indicating a similar degree of ecological impact from these 2 stressors. Guidance is pro vided for applying the CPMI to other macroinvertebrate data sees in the reg ion.