K-2 
1.0 INTRODUCTION 
To meet the requirements of the Clean Water Act, the States of Maryland and 
Virginia are using benthic biological criteria for reporting overall condition and iden¬ 
tification of impaired waters in Chesapeake Bay. The Chesapeake Bay benthic index 
of biotic integrity (B-IBI) is the basis for these biological criteria. Previous work 
conducted by Versar and Old Dominion University had two objectives: to develop a 
methodology for the assessment of benthic community status for 303(d) impairment 
decisions and to produce an assessment for each of the Chesapeake Bay segments 
and sub-segments containing benthic community data. A statistical procedure was 
developed that tests whether the distribution of B-IBI scores from probability-based 
samples collected from a Bay segment is significantly different from the distribution 
of scores from reference sites (Llanso et al. 2003). This procedure, a stratified 
Wilcoxon rank sum test, was evaluated and applied to the 2004 assessment data. The 
assessment resulted in 26 segments considered impaired based upon benthic 
community condition. The Wilcoxon approach, however, was sensitive to small 
shifts in B-IBI scores relative to the reference condition, even in some cases where 
a majority of the B-IBI scores in a segment met the restoration goals. For stratified 
data (i.e., the habitat types of the B-IBI, see below) it was not possible to estimate 
the magnitude of the shift, for example by using a Hodges-Lehman confidence 
interval. Thus, with the Wilcoxon approach we were unable to estimate the magni¬ 
tude of degradation: the difference between the segment and the reference condition. 
A small difference could be statistically significant but of little ecological relevance. 
It was recommended that alternative methods be evaluated, especially those that take 
into account magnitude of departure from reference conditions and whether this 
magnitude is above specific thresholds of protection that the States may wish to 
implement. For the 2006 303(d) report, we developed a new method that quantifies 
magnitude of degradation. We call this method “Degraded Area.” In the present 
report, we describe the Degraded Area method, apply this method and the Wilcoxon 
approach to the 2006 assessment data, and compare the results. 
In addition, a benthic diagnostic tool has been developed that can be used to identify 
potential sources of stress affecting benthic community condition in the Chesapeake 
Bay (Dauer et al. 2002). The tool can distinguish stress due to contaminants versus 
stress due to other factors (e.g., low dissolved oxygen, or unknown). This screening 
tool was used to identify which impaired segments have a high probability of sedi¬ 
ment contamination. These segments could then be targeted for additional sampling 
or evaluation. The B-IBI metric scores for abundance and biomass were also used to 
identify (1) insufficient abundance patterns consistent with a low dissolved oxygen 
effect and (2) excessive abundance patterns consistent with eutrophication effects. 
appendix k 
2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
