Until a couple of weeks ago, Mississippi's Clower-Thornton Nature Trail lured avid birders as well as small children, who wandered in fascination underneath its broad canopy of oak and dogwood trees. Now the trail's entrance sign warns: "Do Not Enter, Toxic," and the surrounding habitat is dying.

"Every tree is brown, every leaf is blown off," said Donna Yowell, executive director of the Mississippi Urban Forest Council, after touring the area. Hurricane Katrina, Yowell added, "has turned it into a toxic waste site overnight."

The scene of devastation in Gulfport, Miss., is just one of the ecological disasters to emerge as scientists, activists and state and federal officials have begun documenting how the hurricane damaged one of the nation's largest networks of estuaries, wetlands and cypress swamps -- a varied and watery ecosystem that sustains a wealth of birds, fish and vegetation. From polluted fisheries to battered forests, the Gulf Coast's habitat has suffered losses that will take years to restore, they say.

"It's as much a disaster for the places set aside to conserve wildlife as for the cities and the people who have been impacted," said Evan Hirsche, president of the National Wildlife Refuge Association. "This is what I would call catastrophic damage to our national wildlife refuges." There are 25 in the affected area.

In the aftermath of Katrina's unprecedented devastation, industrial toxins are seeping into coastal waters. Already-eroded barrier islands have washed away.

Federal authorities have devoted much of their attention so far to the contaminated water in New Orleans, where floodwaters are said to be laced with industrial toxins and untreated sewage. The city's flooded area includes 121 known contaminated sites and more than 1,000 that are possibly contaminated, according to Environmental Data Resources Inc., a firm that compiles environmental information on private and public property.

The polluted water is being pumped out into neighboring Lake Pontchartrain and the Gulf of Mexico, and is likely to affect areas far beyond the city's confines. Federal scientists are already investigating whether the contaminants have damaged valuable fisheries in the gulf, and some scientists and local activists are worried that Lake Pontchartrain is being sacrificed.

Yesterday, environmental activists released satellite images showing large oil slicks a few miles offshore, in the Gulf of Mexico, some stemming from known oil platform locations and stretching as far as 40 miles. Environmental Protection Agency Administrator Stephen L. Johnson said the agency has documented five oil spills in the New Orleans area.

Last week, the National Oceanic and Atmospheric Administration dispatched a research vessel, the Nancy Foster, to the coasts of Louisiana, Mississippi and Alabama to collect and test fish and shrimp, as well as water and sediment samples. The agency has also hired a commercial shrimp boat to take samples in the Mississippi Sound.

NOAA Fisheries Director Bill Hogarth said the agency will release its results in about a week, adding it would take "a minimum of two years" to restore the oyster industry.

"Obviously, we have to start paying attention to the potential of an environmental disaster," said Steve A. Murawski, NOAA Fisheries' chief science adviser. "This is a major fishing area."

The Gulf of Mexico ranks second only to Alaska among America's largest fisheries; coastal Louisiana alone produces 30 percent of the nation's domestic seafood. The Congressional Research Service estimated the hurricane may cost Louisiana's shrimpers $540 million in sales over the next year.

Experts suspect the hurricane has swamped everything from oyster beds to the sea grass that provides a critical nursery for fish, and the flush of nutrients from sewage-laden water into the gulf could spark massive algae blooms deadly to marine organisms.

"What we're looking at here is too much of a good thing," said Hans W. Paerl, professor of marine and environmental sciences at the University of North Carolina at Chapel Hill, citing the nutrient influx. "And what is the impact of those pollutants that are coming in, I don't think we know very well at all."

Congress plans to examine the question soon: Rep. Paul E. Gillmor (R-Ohio), who chairs the House Energy and Commerce subcommittee on environment and hazardous materials, will start hearings at the EPA's "earliest convenience," said his spokesman Brad Mascho.

Scientists and local advocates are particularly concerned about Lake Pontchartrain, which had begun to recover from decades of pollution. In recent years, it has become a commercial and recreational fishing locale, as well as a modest beach destination. Just this summer, lake visitors spotted manatees -- a large, endangered marine mammal that mainly lives in Florida -- in the lake's waters.

Rep. Bobby Jindal (R-La.), whose district encompasses the lake's entire north shore, said residents are worried the contaminants from New Orleans floodwaters will undo the progress made over the past decade.

"The tragedy is, as you drain the city, there is no good alternative to taking that water and putting it back into the lake," Jindal said.

Duke University environmental engineering professor Karl G. Linden said that while he understood that dumping polluted water into Lake Pontchartrain is the "lesser evil" -- compared with pumping it into the Mississippi River or the Gulf of Mexico, federal and state officials need to do a better job of treating the water first.

"There's got to be ways we can treat it without pumping it out raw," Linden said. EPA tests, he added, showed some of the industrial floodwaters are "as toxic as a toxic waste dump."

In addition to unleashing toxic and human refuse, the hurricane destroyed habitat critical to area wildlife. The storm hurt 25 national wildlife refuges that will cost at least $93 million to repair, according to preliminary estimates, a figure equal to a quarter of the entire federal budget for the refuges. Sixteen are temporarily closed.

In Mississippi's Noxubee National Wildlife Refuge, the hurricane felled pine trees crucial to the survival of the endangered red-cockaded woodpecker; Breton Island, a sanctuary for nesting and wintering seabirds and shorebirds, has largely washed away.

"It's going to damage things," said Cathy Shropshire, executive director of the Mississippi Wildlife Federation.

Steve Cochran, a Louisiana native who now works as a senior staffer at the advocacy group Environmental Defense Fund, said the hurricane dealt the final blow to flora and fauna that have declined for decades because of habitat loss.

"All of those things, entirely unique to that part of the world, have been disappearing since about, say, 1927, and now they've disappeared altogether," Cochran said, recalling swamp lilies he used to find right outside New Orleans. "Too few people have experienced them, and now, no one else will."

Researcher Meg Smith contributed to this report.

The Clower-Thornton Nature Trail in Gulfport, Miss., a haven for families and bird-watchers, now is cluttered with debris and contaminated by pollutants.