92 
BENTHIC INDEX OF BIOTIC INTEGRITY ASSESSMENT 
The benthic community health assessment is conducted in three phases to support 
the states’ tidal waters listing decisions (Llanso et al. 2005) (Appendices J and K). 
Phase I evaluates the sample size from the segment during the five-year assessment 
window. An impairment assessment based on benthic community health is not 
possible if the sample size requirement is not met. The data, however, may still prove 
useful as an adjunct to other aquatic life use data. If the sample size satisfies the 
requirements of the statistical method (n > 10), a formal assessment of status (i.e., 
impaired vs. supports aquatic life use) is determined using the “percent degraded 
area” statistical methodology (Phase II). 
Phase II assesses aquatic life use impairment based on a comparison of the Chesa¬ 
peake Bay benthic index of biotic integrity or benthic-IBI scores (Weisberg et al. 
1997). This assessment is possible only when the number of benthic-IBI scores 
within a segment is sufficient to meet the sample size requirement of the approved 
statistical method (n > 10). Phase II can result in one of two possible outcomes: 1) 
the segment is not impaired for aquatic life use due to benthic community status 
(note that the segment may still be impaired due to failure of the other aquatic life 
use subcategories or criteria); or 2) the segment fails to support aquatic life use due 
to benthic community status and is assessed as impaired (part 5). 
Phase III identifies the probable causes of assessed benthic impairment of the 
segment using a diagnostic tool that can pinpoint potential sources of stress affecting 
benthic community conditions in the Chesapeake Bay (Dauer et al. 2005). This 
methodology can also identify causes of stress and quantify the magnitude of degra¬ 
dation. In addition, it distinguishes stress due to contaminants from stress due to 
other factors (Appendix L). 
ASSESSMENT REPORTING FRAMEWORK 
A Chesapeake Bay tidal-water designated-use criteria attainment assessment spread¬ 
sheet has been developed to assist the states in reporting listing decisions for each 
designated-use segment (Table VIII-1). The assessment reporting framework effi¬ 
ciently documents relevant information as each segment goes through the listing 
decision flowchart described below. 
Table VIII-2 shows the example results of the Chesapeake Bay benthic analysis for 
the 2006 303(d) reporting cycle. The benthic-IBI assessments are separate from the 
Chesapeake Bay water quality criteria attainment assessment determinations and 
reported for the segments as stand-alone or supplemental information for the states 
to use in their 303(d) listing cycle decisions. 
chapter viii 
Framework (or Chesapeake Bay Tidal Waters 303(d) List Decision Making 
