Depletion of total PAH (%) of crude oil in sediment fraction (SED, circles) at after 18 days of incubation. The dashed arrow is interpreted as the direction of increasing biodegradation extent. A, All samples: Initial total PAHs are represented by squares. Samples are color-coded according to sampling water depths. Depletion in deep water samples are possibly due to off-gassing effect. B, Distinguishing biodegradation from off-gassing, using different isomer ratios of methylated-PAHs (MF: methyl fluorene m/z 180; MD: methyldibenzothiophene m/z 198; MP: methylphenanthrene m/z 192). Samples are color coded by pressures (MPa). Error bars represent one standard deviations from the means.