K-36 
Summary (cont.) 
Thresholds are set for each of seven benthic habitats in Chesapeake 
Bay: tidal fresh oligohaline. low mesohaline high mesohaltne sand, high 
mesohaline mud. polyhalme sand, polyhaline mud. 
The threshold is set as the smaller of two values: 
1. 5 Th percentile IBI score for the good reference distribution (i.e, sites 
with low scores are unlikely to come from good reference conditions) 
2 Maximum observed IBI score for the degraded reference distribution 
(i.e, sites with low scores are likely to come from degraded 
conditions) 
See example next slide for two hypothetical habitats 1) Habitat A, the 
distributions of scores for the good and the degraded reference sites do 
not overlap, 2) Habitat B, the distributions overlap 
Habitat A 
EEE2 
IBI scores 
20 2.7 
IBI scores 
T 
2.2 30 
appendix k • 2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
