K-7 
contaminated. In the B-IBI, a score of 1 is assigned to total species abundance and 
total biomass if the value of these metrics for the site being evaluated is below the 
5th percentile or above the 95th percentile of corresponding reference values. A 
score of 1 is assigned for both insufficient and excessive abundance or biomass 
because abundance and biomass of organisms respond bimodally to pollution. An 
increase in abundance or biomass is expected at polluted sites when stress from 
pollution is moderate, such as at sites where there is organic enrichment of the sedi¬ 
ment. Excessive abundance and excessive biomass are phenomena usually 
associated with eutrophic conditions. A decrease in abundance and biomass is 
expected at sites with high degrees of stress from pollution; for example, sites 
affected by low dissolved oxygen. The insufficient and excessive abundance or 
biomass criteria can then be used to determine the likelihood of low dissolved 
oxygen problems versus eutrophic conditions for each of the Chesapeake Bay 
segments evaluated. 
4.0 RESULTS 
4.1. DEGRADED AREA 
Based on the bootstrap-degraded area procedure, 22 segments with sample size of at 
least 10 were considered impaired (Table 3). Impaired segments were sorted according 
to the lower 95% bound of the confidence interval of the difference between the 
proportion of sites in the segment below threshold (P) and the proportion of sites below 
threshold under the null hypothesis (Po), from high to low. The estimated P for the 
impaired segments ranged from 28 to 76%, and the average B-IBI score was below 3.0 
for most segments (Table 3). The estimates for CB4MH and CB5MH exclude the deep 
trough (>12 m) of the mainstem which is not monitored because this area is subjected 
to summer anoxia and has consistently be found to be azoic. 
Nineteen of the segments declared impaired in this assessment were also declared 
impaired by the Wilcoxon test in the 2004 assessment. Three segments (JMSMHb, 
PMKOHa, MOBPHa) were declared impaired in this assessment but not in the 2004 
assessment, and seven segments (LAFMHa, POCMH, POTOH, GUNOH, TANMH, 
NANMH, CB7PHa) were declared impaired in the 2004 assessment but not in the 
current assessment. Of the new impaired segments, the Nansemond River 
(JMSMHb) and Mobjack Bay (MOBPHa) were sampled with additional effort in 
2004. Previously, these two segments and the Pamunkey River (PMKOHa) had 
sample size <10. Of the segments that are no longer classified as impaired, only the 
Pocomoke River mesohaline (POCMH) had sample size <10 in the current assess¬ 
ment. 
4.2. WILCOXON 
The stratified Wilcoxon rank sum test identified 27 segments with sample size of at 
least 10 as impaired (Table 3). Segments impaired by the Wilcoxon test but not 
impaired by the Degraded Area method were the lower Bay meainstem (CB7PHa), 
Tangier Sound (TANMH), the Lafayette River (LAFMHa), Severn River (SEVMH), 
appendix k 
2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
