Oil from the BP blowout is degrading rapidly in the warm waters of the Gulf of Mexico and becoming increasingly difficult to find on the water surface, the head of the National Oceanic and Atmospheric Administration said Tuesday.

“The light crude oil is biodegrading quickly,” NOAA director Jane Lubchenco said during the response team daily briefing. “Significant oil has been dispersed and broken down by bacteria.”

Lubchenco said, however, that both the near- and long-term environmental effects of the release of several million barrels of oil remain serious and to some extent unpredictable.

“The sheer volume of oil that’s out there has to mean there are some pretty significant impacts,” she said. “What we have yet to determine is the full impact the oil will have not just on the shoreline, not just on wildlife, but beneath the surface.”

But much of the oil appears to have been broken down into microscopic particles that are being consumed by bacteria. Little or none of the oil is on the sea floor, she said, but is instead floating in the gulf.

Her conclusions come from the work of several NOAA boats now collecting water samples, as well as the analysis of academics brought in to help study the spill effects. The goal, she said, is to get a scientifically sound assessment of the overall environmental effects of the spill. “We’re close to being able to put together a comprehensive picture of what is still out there — how much has been removed by skimmers and burned off and how much remains,” she said. “We’re getting close to an answer.”

NOAA has been at the center of several disputes about what has been happening to the oil from the BP well. Early reports by university scientists of large plumes of oil moving below the water’s surface were generally dismissed by NOAA, and the agency has also determined that sea turtles and sea mammals that washed up on shore after the spill do not appear to have died from the oil. These conclusions have led some to charge NOAA is underestimating the spill’s environmental damage.

A significantly more optimistic assessment of the environmental effects of the oil well blowout came Tuesday from Edward Owens, who worked with Exxon for four years on the Valdez spill in Alaska and who has been hired as a consultant to BP. Owens was quoted as saying the fragile Louisiana marshes would be close to pre-blowout condition within months and that the environmental impact on the gulf as a whole would be “quite small.”

Lubchenco rejected the sanguine conclusions of a BP restoration consultant. “Anyone who classifies the results of the accident as anything less than catastrophic has not been watching,” she said.