Here is the definitive word on the subject of “subsurface plumes”:
“No definitive conclusions have been reached by this research team about the composition of the undersea layers they discovered. Characterization of these layers will require analysis of samples and calibration of key instruments. The hypothesis that the layers consist of oil remains to be verified.”
“While oxygen levels detected in the layers were somewhat below normal, they are not low enough to be a source of concern at this time.”
http://www.deepwaterhorizonresponse.com/go/doc/2931/552671/
The “plumes” consist of water with a somewhat lower oxygen content than that which surrounds it. Such low-oxygen layers can be caused by many things, one of which is dissolved oil.
Unfortunately, the term “subsurface plume” brings an image of an amoeboid blob of black/brown fluid with tendrils oozing off, which is not the case.
I think that’s a bit understated at this point. I can understand a desire to downplay it. And they could be right. However, among the findings this far:
“Tests showed that about 30 percent of the oxygen in the plume has been depleted, which could threaten marine life — mussels, clams, crabs, eels, jellyfish, shrimp and even sharks.”
It’s also not known precisely what the benefit or damage of the dispersal agents will be.
More testing of the samples is still to be done, and very recently one of the plumes seemed to be heading away from the coast.
“No definitive conclusions” is accurate, but I don’t find it that comforting a statement.