NEW ORLEANS (AP) — Marine scientists have discovered a massive new plume of what they believe to be oil deep beneath the Gulf of Mexico, stretching 22 miles (35 kilometers) from the leaking wellhead northeast toward Mobile Bay, Ala.

The discovery by researchers on the University of South Florida College of Marine Science’s Weatherbird II vessel is the second significant undersea plume recorded since the Deepwater Horizon exploded on April 20.

The thick plume was detected just beneath the surface down to about 3,300 feet (1,000 meters), and is more than 6 miles (9.6 kilometers) wide, said David Hollander, associate professor of chemical oceanography at the school.

Hollander said the team detected the thickest amount of hydrocarbons, likely from the oil spewing from the blown out well, at about 1,300 feet (nearly 400 meters) in the same spot on two separate days this week.

The discovery was important, he said, because it confirmed that the substance found in the water was not naturally occurring and that the plume was at its highest concentration in deeper waters. The researchers will use further testing to determine whether the hydrocarbons they found are the result of dispersants or the emulsification of oil as it traveled away from the well.

The first such plume detected by scientists stretched from the well southwest toward the open sea, but this new undersea oil cloud is headed miles inland into shallower waters where many fish and other species reproduce.

The researchers say they are worried these undersea plumes may are the result of the unprecedented use of chemical dispersants to break up the oil a mile undersea at the site of the leak.

Hollander said the oil they detected has dissolved into the water, and is no longer visible, leading to fears from researchers that the toxicity from the oil and dispersants could pose a big danger to fish larvae and creatures that filter the waters for food.

‘‘There are two elements to it,’’ Hollander said. ‘‘The plume reaching waters on the continental shelf could have a toxic effect on fish larvae, and we also may see a long term response as it cascades up the food web.’’

Dispersants contain surfactants, which are similar to dishwashing soap.

A Louisiana State University researcher who has studied their effects on marine life said that by breaking oil into small particles, surfactants make it easier for fish and other animals to soak up the oil’s toxic chemicals. That can impair the animals’ immune systems and cause reproductive problems.

‘‘The oil’s not at the surface, so it doesn’t look so bad, but you have a situation where it’s more available to fish,’’ said Kevin Kleinow, a professor in LSU’s school of veterinary medicine.