BIRMINGHAM, Ala. — Tar balls washed onto Gulf of Mexico beaches by Tropical Storm Lee earlier this month show that oil left over from last year’s BP spill isn’t breaking down as quickly as some scientists thought it would, university researchers said Tuesday.

Auburn University experts who studied tar samples at the request of coastal leaders said the latest wave of gooey orbs and chunks appeared relatively fresh, smelled strongly and were hardly changed chemically from the weathered oil that collected on Gulf beaches during the spill.

The study concluded that mats of oil — not weathered tar, which is harder and contains fewer hydrocarbons — are still submerged on the seabed and could pose a long-term risk to coastal ecosystems.

BP didn’t immediately comment on the study, but the company added cleanup crews and extended their hours after large patches of tar balls polluted the white sand at Gulf Shores and Orange Beach starting around Sept. 6. Tar balls also washed ashore in Pensacola, Fla., which is to the east and was farther from the storm’s path.

Marine scientist George Crozier said the findings make sense because submerged oil degrades slowly due to the relatively low amount of oxygen in the Gulf’s sandy bottom.

“It weathered to some extent after it moved from southern Louisiana to Alabama ... but not much has happened to it since then,” said Crozier, longtime director of the state sea laboratory at Dauphin Island.

Crozier said remnants of the spill are “economically toxic” for tourism, but he doubts there is much of an environmental threat. The oil lingering on the seabed is of a consistency and chemical composition somewhere between crude oil and tar, he said.

The company refused a request by the city of Gulf Shores to expand the latest cleanup efforts to include heavy machinery.

Auburn analyzed tar balls dredged up by Lee at the request of the city of Orange Beach with outside funding from the city, the National Science Foundation and the Marine Environmental Sciences Consortium. The study wasn’t reviewed by outside scientists before its release.