Decision Process for Identification of Estuarine Benthic Impairments in Chesapeake Bay, USA

Roberto J. Llansó 1, Jon Vølstad 1, and Daniel M. Dauer 2

1 Versar, Inc., Columbia, Maryland 21045
2 Department of Biological Sciences, Old Dominion University, Norfolk, Virginia 23529

Measuring the health of benthic macroinvertebrate communities is often considered as a measure of support for a balanced population of aquatic organisms. This measure is often derived as an index of benthic condition based on species abundance, biomass, feeding guilds, and life history strategies. The States of Maryland and Virginia are interpreting aquatic life use support in the estuarine waters of the Chesapeake Bay using benthic community data assessed with respect to key stressors such as dissolved oxygen and toxic contaminants. The Chesapeake Bay benthic index of biotic integrity (B-IBI) is the basis for this decision process. Working together with the States and the US EPA, we evaluated alternative approaches for 303(d) impairment decisions based on the B-IBI. The evaluation took into consideration reference frequency distributions, multiple habitats within assessment units (segments), balance of Type I and Type II errors, and sample size. The assessment data consisted of 1,525 random samples collected from 1998 to 2002 throughout the Chesapeake Bay. B-IBI scores were grouped into condition categories and the distribution of scores within a segment was compared to reference distributions, treating the scores in each population of samples as ordered categorical responses. A stratified Wilcoxon rank sum test was found to be the most suitable method for assessing impairments given the complexities of the data. Using this method and a set of recommended requirements, 26 of the 90 Chesapeake Bay segments were considered impaired. We discuss the relative merits and limitations of the methods and provide recommendations for future assessments.

Keywords: Clean Water Act report, impairment, benthos, degradation, biotic integrity, and statistical methods