K-13 
abundance/biomass, and 35% had insufficient abundance/biomass. Few samples in 
Tangier Sound were degraded. In Segment CB5MH, 18% of degraded samples were 
classified as contaminated and 82% of the uncontaminated degraded had insufficient 
abundance/biomass, indicating a low dissolved oxygen effect. In the lower main- 
stem, Segment CB6PH had 67% of the degraded samples classified as contaminated 
and 33% of the uncontaminated degraded samples classified with insufficient abun¬ 
dance/biomass. Segment CB7PHa had 63% of the degraded samples classified as 
contaminated, but none had contaminant group posterior probabilities above 0.90 
and the average probability for the segment was 0.58. Of the degraded samples not 
classified as contaminated in this last segment, 13% had excessive abundance/ 
biomass and 25% had insufficient abundance/biomass. Finally, none of the samples 
near the Bay mouth in Segment CB8PHa were classified as contaminated. 
In summary, contaminants were likely sources of stress to benthic communities in 
CB1TF and CB3MH, while a variety of stresses were likely in CB4MH. Low 
dissolved oxygen was the predominant source of stress in CB5MH, contaminants 
and low dissolved oxygen in CB6PHa and CB7PHa, and low dissolved oxygen alone 
in CB8PHa. 
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2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
