Scientists at Woods Hole announced their discovery and detailed mapping of a large underwater plume of finely dispersed oil from the BP spill. Measuring 35 km long x 2 km wide x 200 m thick, it was about 900 meters (3,000 feet) below the surface and drifting slowly southwest from the leaking Macondo well. The team was tracking this plume in late June, up until Hurricane Alex chased them back to shore. The researchers said it appeared to be breaking down and dissipating much more slowly than expected, probably because of the very low water temperature at that depth.
The combined concentration of several key indicator hydrocarbons (benzene, toluene, ethylbenzene, and total xylenes) in the plume was at least 50 micrograms (millionths of a gram) per liter. That's very dilute, although it may have some toxic effects.
How much of the "missing" oil was in that plume? The scientists calculated that about 6-7% of the 2.2-2.6 million gallon daily flow rate from the well was represented in this plume during the 10 days they were measuring it. They conclude that
the total amount of petroleum hydrocarbons in the plume and the full extent of possible risks to marine biota remain uncertain.