Fig. 3. Effect of exposure to saltmarsh sediments contaminated with mercury and polychlorinated biphenyls (PCBs) (approximately 10 µg/g each) over one tidal period in saltmarsh mesocosms on the bioluminescent ratio (BLR). (A) Comparison of BLR after exposure to contaminated () and uncontaminated (•) sediments. (B) Difference between the BLR of indigenous bacterial communities exposed to pristine sediments and the same communities exposed to contaminated sediments. Relative differences were calculated for paired samples from each sampling date as the difference between the BLR from the clean and contaminated samples divided by the BLR of the clean sample. Positive values indicate that the BLR of communities exposed to contaminated sediments was depressed relative to unexposed communities. One sample (24 Apr. 2000) was excluded from this analysis as an outlier.