Application of two indices of benthic community condition in Chesapeake Bay

SUMMARY The Chesapeake Bay Benthic Index of Biotic Integrity (B-IBI) and the Environmental Monitoring and Assessment Program’s Virginian Province Benthic Index (EMAP-VP BI) were applied to 294 sampling events in Chesapeake Bay and the results were compared. These estuarine benthic indices are intended to identify benthic invertebrate assemblages that have been degraded by low dissolved oxygen concentrations or high concentrations of chemical contaminants. The B-IBI includes several community measures and weights them equally using a simple scoring system that compares them against values expected for undegraded sites. It includes 11 measures of species diversity, productivity, indicator species and trophic composition. The EMAP-VP BI uses discriminant function coefficients to weight contributions of species diversity and the abundances of two indicator families. The two indices agreed on degraded or undegraded classifications for benthos at 81.3% of the sites. This level of agreement is within the level of accuracy achieved during index development and, therefore, may approach the limits that can be achieved. The indices were strongly associated (Pearson’s r ¼ 0.75). The B-IBI was more conservative than the EMAP-VP BI, classifying 72.7% of the disagreements as degraded. The 55 sites where the indices disagreed were distributed in different habitats throughout the Bay except polyhaline sand. Many of the classification disagreements were at sites with index values close to, but on opposite sides of, the degraded–undegraded thresholds, with 49.1% of the B-IBI values within 0.5 units and 81.8% within 1.0 units; the corresponding values for sites where both indices agreed were only 23.4% and 62.7%, respectively. The pattern for the EMAP-VP BI was similar, with 61.8% and 74.6% of disagreements and only 18.8% and 38.9% of agreements within 0.5 and 1.0 units of the threshold. Although the close agreement suggests that either index is suitable for evaluating the benthic condition, the B-IBI offers some additional advantages. Copyright # 2002 John Wiley & Sons, Ltd.