The thick globs of BP oil that washed ashore on beaches along Florida's Panhandle in 2010 never reached Tampa Bay, to the relief of hotel owners, restaurateurs, anglers, beachgoers and local officials.

But oil from the Deepwater Horizon spill, floating beneath the surface after being sprayed with dispersant, settled on a shelf 80 miles from the Tampa Bay region within a year of the spill's end, according to a scientific study published this week.

There is some evidence it may have caused lesions in fish caught in that area, according to John Paul, the University of South Florida oceanography professor who is lead author on the study, published in Environmental Science & Technology. However, research is continuing on that question.

Tests of the samples from those areas on bacteria and other microscopic creatures normally found in that part of the gulf found that "organisms in contact with these waters might experience DNA damage that could lead to mutation," the study reported.

The oil that landed on the shelf, which extends miles into the gulf, is likely to stay there a long time, Paul said.

"Once it's in the sediment, it's kind of immobile," he said.

BP spokesman Jason Ryan said scientists working for the company, as well as various government agencies, had "conducted extensive sampling to identify, track and map oil in the water column over time," and found no signs of BP oil on the shelf near the Tampa Bay area.

But Paul said the researchers looked for signs of the Deepwater Horizon spill on the shelf based on observations by a colleague, USF oceanographer Bob Weisberg.

Weisberg found a major upwelling — a swirling current of cool water from deep in the gulf — had begun in May 2010 and continued through the rest of that year. The upwelling could have caught hold of the underwater plumes of dispersed oil off the Panhandle and then pushed them southward onto the shelf that lies off the state's west coast, he said.

"It made its way southeast across the bottom and eventually it gets to the beach," Weisberg said. "A little bit probably got into Tampa Bay, and a little bit probably got into Sarasota Bay, and it exited the Florida shelf down around the Dry Tortugas."

When he put forward his theory in 2010, Weisberg called for sampling to be done along the shelf to test whether he was right, but that proposal did not get any funding, he said.

Eventually, though, as part of a series of 12 trips into the gulf for their own research, Paul and his colleagues collected samples along the shelf, as well as closer to the site of the Deepwater Horizon disaster off Louisiana.

They found nothing in 2010, but when they went back in 2011 and 2012, they found what Weisberg had predicted. The oil did not reach the southern end of the shelf until last year. Water samples collected off the shelf were toxic to bacteria, phytoplankton and other small creatures, the report said.

The USF discovery shows that scientists continue to grapple with measuring the full impact of the disaster, which began with a fiery explosion aboard an offshore drilling rig on April 20, 2010.

The disaster held the nation spellbound for months as BP struggled to stop the oil. To try to break up the oil before vast sheets of it washed ashore on the beaches and marshes along the Gulf Coast, the company sprayed the dispersant Corexit directly at the wellhead spewing oil from the bottom of the gulf — even though no one had ever tried spraying it below the water's surface before. BP also used more of the dispersant than had been used in an oil spill, 1.8 million gallons.

The Corexit broke the oil down into small drops, creating underwater plumes of oil, something no one had ever seen before in an oil spill. The discovery of the plumes raised questions about how they would affect sea life in the gulf.

Yet even before BP managed to shut off the undersea flow July 15, 2010, observers ranging from Time magazine to Rush Limbaugh said damage from the 4.9 million-barrel spill seemed far less severe than predicted. In the three years since, though, scientists have uncovered ongoing damage — deformed crabs, dying dolphins and other woes.

Getting this study published in a peer-reviewed journal was a long process, Paul said.

"Publishing anything about the oil spill is inherently more difficult than anything else because it's so contentious," he said.

BP agreed last year to pay $4 billion to settle criminal charges, including manslaughter, in connection the disaster, and rig owner Transocean settled civil and criminal charges for $1.4 billion.

BP is now locked in a civil court battle with the U.S. Justice Department and hundreds of businesses affected by the spill. If it loses, BP could face damages of $17.5 billion, although company officials have predicted the fines will be less than $5 billion.

Craig Pittman can be reached at craig@tampabay.com