ABOARD THE NOAA PISCES (Reuters) - Scientists above a U.S. research ship have started an around-the-clock search for elusive signs of oil lurking beneath the Gulf of Mexico's surface in what they jokingly call "Operation Dipstick."
As debate rages among scientists over how much oil remains in the water after BP Plc's massive oil spill, their research vessel circles above the blown-out Macondo well, some 40 miles southeast of the mouth of the Mississippi River.
Oil is not visible on the surface around the well, but as waters reopen to fishing, many question what the crude will do to this season's fish, shrimp and oyster catch, as well as its long-term effect on marine life.
The 35-member crew of the Pisces, operated by the National Oceanic and Atmospheric Administration, on August 18 started a three-week mission to collect sea water samples and study them for hydrocarbons or depleted oxygen levels that could indicate its presence.
"We're looking for hydrocarbons to see how things in the deepwater column are changing," Tom Weber, chief scientist aboard the Pisces, said on Friday. "Ever since the well has been capped, we haven't seen that much."
BP and the government have said no oil has leaked into the Gulf since BP placed a cap over the wellhead on July 15. Since then, the well was been plugged with drilling mud and cement. A relief well, seen as the final solution, is expected to be finished in September.
Traveling in a radial pattern around the spill, the Pisces collects about fifty water samples each day. Some are tested in an onboard chemistry lab, while other are ferried by boat to onshore NOAA labs.
The boat is also soon due to receive a more sensitive fluorometer, the device used to detect hydrocarbon in water using ultraviolet light.
The Pascagoula, Mississippi-based crew have so far only found small spots or "sags" of dissolved oxygen around the Macondo well, an indication that oil-eating microbes have been at work, they said.
The 208.6-foot (63.6- meter) Pisces is one of about 10 vessels -- some independent, some government and some hired by BP -- searching for hydrocarbons in the Gulf.
Just what is still out there is a hot topic. In an August report NOAA estimated 4.9 million barrels spilled after the April 20 explosion at the Macondo well, and that nearly three-fourths of that amount has been captured, dissolved or consumed by oil-eating bacteria.
But last week, oceanographers from the Woods Hole Oceanographic Institution said that the oil spill left at large plume of hydrocarbons in deep waters in the Gulf, and those chemicals could be there for some time.
While the Woods Hole researchers found the plume in June, it showed that subsurface oil droplets were not being easily degraded by microbes, as some had speculated.
In another reports, researchers at the University of Georgia said about three-quarters of the oil from the blown-out Macondo well was still lurking below the surface of the Gulf and may pose a threat to the ecosystem.
Even so, the head of NOAA told reporters at a briefing in Venice, Louisiana, last week that the government stands by their figures that others dispute.
"We believe those numbers remain to be the best ones that are out there," Jane Lubchenco, said, adding that the government's figures would be refined as more data becomes available.
And information collected from the Pisces will be used to help the government as it works to fine-tune its spill estimates, Weber said.
(Editing by Jackie Frank)