Physically and chemically (Corexit 9500) generated Macondo 252 oil dispersions, or emulsions (no
Corexit), were prepared in an oil-on-seawater mesocosm flume basin at 30–32 C, and studies of oil compound
depletion performed for up to 15 days. The use of Corexit 9500 resulted in smaller median droplet
size than in a physically generated dispersion. Rapid evaporation of low boiling point oil compounds
(C 6 15) appeared in all the experiments. Biodegradation appeared to be an important depletion process
for compounds with higher boiling points in the dispersions, but was negligible in the surface emulsions.
While n-alkane biodegradation was faster in chemically than in physically dispersed oil no such differences
were determined for 3- and 4-ring PAH compounds. In the oil dispersions prepared by Corexit
9500, increased cell concentrations, reduction in bacterial diversity, and a temporary abundance of
bacteria containing an alkB gene were associated with oil biodegradation.