Samantha Joye of the University of Georgia says studies using a submersible discovered a layer of dead animals and oil in the Gulf of Mexico as thick as 4 inches in some places, the BBC reported Monday.
Joye and colleagues used the submersible to survey the lowest layer of water around the destroyed wellhead, known as the benthos.
"The impact on the benthos was devastating," she says.
"Filter-feeding organisms, invertebrate worms, corals, sea fans -- all of those were substantially impacted -- and by impacted, I mean essentially killed."
Joye says she disagrees with an assessment by BP's compensation fund that the gulf waters will recover from the effects of the spill by the end of 2012.
"The gulf is resilient," she said. "I do believe that it will recover from this insult, but I don't think it's going to recover fully by 2012."
The impact at the bottom of the aquatic food chain will inevitably have long-term effects on species nearer the surface, she says, including some humans depend on as a food source.
"I think it's going to be 2012 before we begin to really see the fisheries implications and repercussions from this," she said.
The spill was caused by an April 20, 2010, explosion on the Deepwater Horizon oil drilling rig that killed 11 men working on the platform and released an estimated 4.9 million barrels of crude oil over three months until the well was capped. It was the world's largest offshore oil spill.