ST. PETERSBURG, Fla. — Scientists have found evidence that oil has become toxic to marine organisms in a section of the Gulf of Mexico that supports the spawning grounds of commercially important fish species.

Researchers from the University of South Florida said Tuesday that, in preliminary results, oil appears to reside in droplet form among the sediments of a vital underwater canyon where clouds of oil from the BP spill were found.

"So, indeed, the waters have a level of toxicity that needs to be recognized, and I think these were some of the first indicators that the base of the food web — the bacteria and the phytoplankton — may be affected," said David Hollander, chief scientist on a research vessel that just returned from a 10-day trip in the Gulf.

More than 200 million gallons of oil leaked into Gulf waters from BP's Deepwater Horizon well until it was capped last month. The oil company also used millions of gallons of chemical dispersant to break up the oil as it gushed from the runaway well off the Louisiana coast.

Researchers peering into the murk described what they saw using a process involving ultraviolet light.

"We were able to detect sediments that had oil covering them," said Hollander. "It wasn't like a drape, don't get me wrong, like a blanket of oil; rather, it looked like a constellation of stars that were at the scale of microdroplets. They seemed to be at every location we looked east of the wellhead, and interestingly and surprisingly, at the top of the DeSoto Canyon to the east."

He described the DeSoto Canyon as an underwater geologic feature that is thought to bathe the Continental Shelf with nutrient-rich waters.

In subsurface waters east of the wellhead, phytoplankton — microscopic, plant-like organisms that form the base of the marine food web — was found to be in poor health, Hollander said.

In those locations, phytoplankton was repressed, or "feeling a toxic response to those waters," he added.

Bacteria — microorganisms ubiquitous in nature — did not respond negatively under the surface; but at the surface, it was repressed, he reported during a conference call Tuesday explaining the results of the mission.

The field-based results are consistent with shore-based laboratory studies that showed phytoplankton more sensitive to chemical dispersants than bacteria, which was deemed more sensitive to oil, said USF spokesman Peter Howard.

In some of the Gulf's deeper waters, there was "very strong toxicity," with some sites more toxic than others, said biological oceanographer John Paul, who also worked on the project.

Oil from the spill has not gone away, scientists said, estimating that perhaps 25 percent or 30 percent of it has been accounted for in various ways, while 70 percent to 75 percent of it has not.

Paul said he had "good confidence" in the team's results, although it must still chemically "fingerprint" the oil to formally establish its source.