The appearance of the oil is prompting restrictions of fishing in some waters and tests to determine whether the source is submerged oil from the BP explosion and rig sinking that killed 11 and inundated the Louisiana coast for months.

Experts had speculated that the first hurricane to hit the Gulf of Mexico since the April 2010 spill might stir up some of the estimated one million gallons of crude oil that remains.

“The discovery of oil spills on beaches in Louisiana confirms the severity of the Deepwater Horizon accident. This shows that the severity of the spill is much more complex than what we known today, said Bellona President Frederic Hauge, who was in Louisiana with other Bellona staff to follow the BP accident.

“These recent incidents also show very clearly that the oil industry’s claims that environmental problems after the Deepwater Horizon accident were resolved and no longer posed a danger were unfounded,” he added.

On Tuesday, the discovery of large areas of oil forced the closure of a 20 kilometer stretch of Louisiana coastline to fishing and other activities, reported the New Orleans Times Picayune. Recreational rod and reel fishing can continue in the area, but commercial and recreational shrimping, crabbing and commercial fin fishing is for now prohibited.

Jonathan Henderson, of the nonprofit Gulf Restoration Network, spent Tuesday earlier this week inspecting the East and West Ship Islands off the coast of Mississippi.

“What we saw yesterday was disgusting,” he told Bellona, recounting heavy deposits of tar balls that appeared to have washed in from offshore. He also reported that he saw oil below wind- and wave-battered sand.

“We saw all the telltale signs of BP oil resurfacing,” he said Thursday.

Henderson noted that samples collected for testing would eventually determine the toxicity and origin of the residues.

“There are numerous other companies doing business in the Gulf, and there are leaks every day of some shape or form,” Henderson said. “But these are areas that we know BP oil has hit and oil remains, and the weathered tar balls are consistent with ones that have been coming up since April 2010.”

Pieces of sea sponge soaked in oil have also been reported as far east as Florida, the Huffington Post reported.

Similar scenes are being relayed by crews who have taken to the skies and waters from Texas to Florida, evaluating the devastation left in Isaac’s wake.

Garrett Graves, Louisiana Governor Bobby Jindal’s top adviser on coastal issues, was not waiting for analysis of the oil to publicize his opinion on its origin.

“I’d say there’s a smoking gun,” he told MSNBC, adding “[the oil is in] an area that experienced heavy oiling during the oil spill.”

Karl Kristiansen, Bellona’s advisor on industrial waste, said that the oil that is washing up on Gulf of Mexico shores would scientifically be of a different kind than it was when it spilled because it has chemically modified in the period since the 2010 disaster.

Because of these modifications, identifying conclusively whether the oil products currently washing up in the Gulf originate from the Deepwater Horizon spill will depend on the investigative measures used.

Simple chemical fingerprinting techniques will probably not identify the source, said Kristiansen.

Some oil that’s been spotted in parts of the Gulf Coast has shown evidence of non-BP sources.

Petty Officer William Colclough, a spokesman for the U.S. Coast Guard, told the Huffington Post that a leaking platform, oil drums and barges have been spotted in the vicinity of oil that Coast Guard teams had detected.

“The investigation continues,” said Colclough. “And the clean up response is ongoing.”

For its part, BP also emphasized in a statement that it is too early to tell the origin of much of the surfacing oil.

“In Florida, Alabama and Mississippi, shoreline quick assessments were initiated on Saturday, Sept. 1, and there has been no indication thus far of Deepwater Horizon residual oil in areas outside of existing cleanup operations,” Melick said in a statement late Tuesday.

However, the company has by now acknowledged the appearance of residual BP oil along shorelines near Fouchon Beach and Grand Isle, Louisiana – accounts that were confirmed by the Mayors’ offices of both communities to Bellona by email.

“An important point in this situation is the fact that oil spills have accumulated at the sea floor as a compact layer that prevents any rehabilitation of ecosystems at the sea floor, which has been destroyed by the initial spill of oil. And it doesn’t move unless disturbed,” said Bellona’s Kristensen.

“Storms like Hurricane Issac cause these systems to move and create more spills and they may vortex up and damage water columns and fisheries and other ecosystems that are recovering from the initial [Deepwater Horizon] spill.”

Kristensen said that the Deepwater Horizon spill, or any large oil spill for that matter that is deposited on the sea floor, may create very risky situations and remain an environmental threat for a very long time to come.

‘This shows that we have large gaps in knowledge about how oil behaves and crumbles in the aftermath of any oil spill,” said Bellona’s Hauge. “In extreme weather, it will be reactivated meaning that oil spills pollute more than once.”