WASHINGTON – How dead is the Gulf of Mexico?

It is perhaps the most important question of the BP oil spill — but scientists don’t appear close to answering it despite a historically vast effort.

In the 2 months since the spill began, the gulf has been examined by an armada of researchers — from federal agencies, universities and nonprofit groups. They have brought back vivid snapshots of a sea under stress: sharks and other deep-water fish suddenly appearing near shore, oil-soaked marshes turning deathly brown, clouds of oil in deep water.

But, with key gaps remaining in their data, there is wide disagreement about the big picture. Some researchers have concluded that the gulf is being spared an ecological disaster. Others think ecosystems that were already in trouble before the spill are now being pushed toward a brink.

“The distribution of the oil, it’s bigger and uglier than we had hoped,” said Roger Helm, a U.S. Fish and Wildlife Service official and the lead scientist studying the spill for the Interior Department. “The possibility of having significant changes in the food chain, over some period of time, is very real. The possibility of marshes disappearing is very real.”

This research has mainly occurred in the background, as public attention has focused on the “open-heart surgery” at BP’s leaking wellhead.

The patient is a 600,000-square-mile sea that contains swirling currents, sun-baked salt marshes and dark, cold canyons patrolled by sperm whales. Even before the spill began in late April, the patient was sick.

In recent years, Louisiana has been losing a football field’s worth of its fertile marshes to erosion every 38 minutes. In the gulf itself, pollutants coming from the Mississippi’s vast watershed helped feed a low-oxygen “dead zone” bigger than Chesapeake Bay.

So far, even the simplest-sounding attempts to measure the spill’s impact have turned out to be complex.

The official toll of dead birds is about 1,200, a fraction of the 35,000 discovered after the Exxon Valdez spill in 1989. But officials can only count the birds they can find, and many think some oily birds have sought refuge in the marshes.

Other scientists have focused on more subjective measures of the gulf’s health — studying the behavior of wildlife, the movements of oil and the state of larger ecosystems. For them, answers are even more elusive.

For example: Is the oil killing off Louisiana’s coastal marshes? State officials have said in interviews that they’ve seen it coating the grasses that hold the region’s land in place.

“The marsh grasses, the canes, the mangrove are dying. They’re stressed and dying now,” said Robert Barham, secretary of the state’s Department of Wildlife and Fisheries.

But Paul Kemp of the National Audubon Society said he flew over the same area and saw a different picture: The oil’s damage was relatively small, at least in comparison with the marsh’s existing problems.

“Here, we have a patient that’s dying of cancer, you know, and now they have a sunburn, too,” Kemp said. “What will kill coastal Louisiana is what was killing it before this oil spill,” including erosion.

Further offshore, federal scientists and university researchers have disagreed about the existence of “plumes” or “clouds” of dissolved or submerged oil. Several educators have reported finding underwater oil dozens of miles from the spill.

Just around the leaking BP wellhead, a Texas A&M University scientist reported finding pockets of water with very low dissolved oxygen.

The government has presented a very different picture.

An official at the National Oceanic and Atmospheric Administration said his agency found evidence of significant submerged oil — 1 to 2 parts per million — from the spill only within six miles of the well.