PORT FOURCHON, Louisiana (Reuters) - U.S. scientists will embark on a second mission on Tuesday to investigate whether a catastrophic Gulf of Mexico oil spill is damaging deepwater marine life and the surrounding environment.

Samantha Joye, a University of Georgia marine sciences professor who is part of the research team, said the two-week government-funded mission will focus on a plume of dispersed oil that she says is from the leaking BP undersea well.

The plume, which is roughly 20 miles long, six miles wide, and 100 feet thick, was discovered by the R.V. Pelican, a research ship, on its first mission.

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 appears to be radiating from the spill site, so that’s why we think it’s a mixture of emulsified or dispersed oil, little oil fragments that are generated by the actual eruption of the fluid from the sea floor,” Joye said in an interview with Reuters.

She said she was “99.8 percent sure” the plume, first spotted as “deep hydrographic anomalies”, was spill-related and said it was quite possible that other, smaller, plumes existed.

“We need to go out there and track and map the plume features and see how they are changing with time. Is oxygen dropping and if so, how fast? Is it microbial activity that is causing that,” Joye said.

Another unknown is the exact composition of the plume and the extent to which dispersants, sprayed and pumped into the water by BP to break up the oil, were changing its chemistry, she said.

London-based BP says it could try on Wednesday to shut off the well that has caused a major U.S. ecological disaster, threatening fishing communities in four states, and has also stirred a political storm since it blew out last month.

‘DEAD ZONE’ IN WATER?

Eleven scientists from universities in Mississippi, Georgia, North Carolina and California will embark on the Pelican, which will depart from Gulfport, Mississippi and join six other research vessels working in the Gulf.

The National Oceanic and Atmospheric Administration (NOAA) has five such vessels in the area.

NOAA, a federal agency, said the Pelican mission’s initial findings were premature, in what some critics said appeared to be part of a concerted effort to play down the environmental impact of the leak.

BP for weeks also maintained that determining the precise rate at which oil was gushing into the sea mattered little compared to its efforts to stop the flow, leading to criticism that it was trying to avoid accountability for what could be the biggest oil spill in U.S. history.

Images of oil washing onto beaches and coating entire islands in Louisiana’s fragile wetlands, as well as those of birds covered in oil, have highlighted the environmental threat posed by the leak.

But undersea damage, though it remains invisible, could be equally costly, according to Joye.

One likely cause of the depleted oxygen in the plume is the increased activity of microorganisms chewing up the oil and gas in the water, and the danger is the creation of an anoxic, or dead, zone in the water.

“That would kill anything that can’t run from it,” said Joye, who said the scientists are racing to catch up with what was effectively a lost first month after the leak began.

Little if any light penetrates at depths of 2,625-4,593 feet where the plume is located. A further planned voyage in August and September will try to determine how far the plume has affected the food web.