K-11 
however, eutrophication is likely to also affect benthic communities in this segment, 
as indicated by the samples with excessive abundance/biomass. 
The predominant source of stress in the lower Potomac River (POTMH) appears to 
be from low dissolved oxygen, as indicated by the high percentage of samples clas¬ 
sified as uncontaminated and having insufficient abundance/ biomass. 
Patuxent River 
An inadequate number of samples were collected in the upper Patuxent River 
(PAXTF) and middle Patuxent River (PAXOH) for assessing benthic community 
degradation using the benthic diagnostic tool. In the upper Patuxent River (PAXTF), 
two samples were classified as contaminated and one had excessive 
abundance/biomass without likelihood of contamination. In the middle Patuxent 
River (PAXOH), three samples were classified as contaminated and none had exces¬ 
sive or insufficient abundance/biomass. In the lower Patuxent River (PAXMH), 46% 
of the degraded samples were classified as contaminated, with an average posterior 
probability of contaminant group membership of 0.51. Of the remaining uncontam¬ 
inated samples, 50% had insufficient abundance/biomass while only 1.5% had 
excessive abundance/biomass. 
In summary, accurate assessment of benthic community degradation in the upper 
Patuxent River (PAXTF) and middle Patuxent River (PAXOH) requires additional 
sampling; however, available data suggest contaminants may be a source of stress in 
these segments. Degradation in the lower Patuxent River (PAXMH) is likely to be 
the result of a combination of contamination and low dissolved oxygen stress. 
Chester River 
Over 38% of the degraded samples in the lower Chester River (CHSMH) were clas¬ 
sified as contaminated. Of the remaining uncontaminated samples, 11% had 
excessive abundance/biomass and 33% had insufficient abundance/biomass. Benthic 
community degradation in this segment would appear to be the result of contamina¬ 
tion, eutrophication, and low dissolved oxygen effects. All other segments in the 
Chester River had low sample size. 
Choptank River 
Accurate assessment of benthic degradation the upper Choptank River (CHOTF), 
middle Choptank River (CHOOH) and Choptank River mouth (CHOMH1) will 
require additional sampling. In the lower Choptank River (CHOMH2), 67% of the 
degraded samples were classified as contaminated, with group membership proba¬ 
bilities >0.90. Of the remaining uncontaminated degraded samples, 22% had 
excessive abundance/biomass while 11% had insufficient abundance/biomass. Cont¬ 
amination appears to account for most of the benthic community degradation in the 
lower Choptank River (CHOMH2), but eutrophication and low dissolved oxygen are 
also likely to play a role. 
appendix k 
2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
