“It's an aquatic prairie,” said Gary Vitrano, a Louisiana state wildlife biologist.

But as the Deepwater Horizon oil spill laps at the Louisiana coast, its effects go far beyond what spreads across the surface to what can't be seen far below.

Not only do the toxic components of oil threaten the wildlife here, it's also the unprecedented amount of chemicals used to fight it. Already, oil and the chemical dispersant have invaded the complex web of life in the Gulf of Mexico, from tiny contaminated plankton and blackened sargassum drifting through the spill to thousands of dolphin known to live and travel there.

“My concern is with the entire food chain,” said Donald M. Baltz, chairman of the oceanography and coastal sciences department at Louisiana State University. “Adding toxic materials to the mess in the form of dispersants has its pros and cons ... it's a concern whether the largest harm will come from the oil or from what they use to ‘solve' the problem.”

Because of potential threats of contamination to fish and shellfish, the National Oceanic and Atmospheric Administration has closed a large area of the Gulf through May 17 and considered declaring a national fisheries disaster.

Many species of fish, shellfish, birds, sea turtles and mammals are considered at risk. Red snapper and spotted sea trout are among those that will soon begin to spawn in potentially contaminated offshore waters.

In the potential path of the spill are 20 national wildlife refuges currently filled with nesting herons, terns and brown pelicans, removed from the endangered species list in Louisiana just a few months ago.

Immense contamination

BP bought out the manufacturer's entire U.S. stockpile of dispersant to try to keep the spill from reaching refuges and beaches, as well as the largest coastal wetlands system in the United States. But the oil made landfall anyway late Wednesday on the Chandeleur Islands, part of the Breton National Wildlife Refuge, southeast of New Orleans.

Miles of boom have been laid to try to keep it from tainting shallow waters filled with sea grass, shorebirds and many other creatures that, if fouled by oil, might be impossible to clean or even locate.

Already, the offshore contamination from the spill is immense and unprecedented in the northern Gulf of Mexico.

This spill is more than a third as large as the devastating Exxon Valdez spill in Alaska in 1989, according to official estimates, and could get larger. And never before has so much dispersant — containing toxic heavy metals — been deployed to fight a spill in a single site.

So far, BP estimates that about 95,000 barrels of oil — 5,000 barrels a day — have spewed into the Gulf since the drilling rig exploded on April 20.

On top of that, more than 250,000 gallons of dispersant have been applied both by air and underwater at the Deepwater Horizon spill site, the U.S. Coast Guard confirmed.

Though considered by many experts as the lesser of two evils, the environmental impact of so much dispersant at one site remains “widely unknown,” Environmental Protection Agency officials admit.

The spill has fouled waters frequented by several species of dolphin. Bottlenose dolphins in the Gulf may remain in their home waters even during major environmental threats, like red tide and hurricanes, based on research conducted on a Florida community of those dolphins by Randall Wells, a Florida-based scientist with the Chicago Zoological Society.

“We are concerned they don't have the option to leave and they would be subject to whatever happened within the region,” Wells said.

NOAA has photographed whales and striped dolphins swimming through the spill in recent days. At least 38 dead sea turtles and two dolphins have washed up near the spill area, though the cause of death is still unknown.

Beneath the surface

It could take years of scientific study for the true environmental effects of this spill to be measured, researchers and scientists predicted.

“It's important to understand for every carcass that washes up on a beach, there may be many more carcasses that sink or are scavenged and are in the marshes out-of-sight. For every bird carcass there are probably another 10 that are not picked up,” said Stan Senner, director of conservation science for the nonprofit Ocean Conservancy. “Beyond that, there are a whole array of impacts that go on unseen.”

More than 21 years after the Exxon Valdez oil tanker ran aground on a reef, signs of that spill continue to show in the formerly pristine Prince William Sound in Alaska. On sunny days, oil still seeps up through the rocks on its shores, staining the fur of river otters. And the Pacific herring has never fully recovered.

The monitoring continues today.

Ed Overton, another Louisiana-based researcher who is part of NOAA's chemical hazard assessment team is analyzing the potential effects of the dispersant used by BP, called Corexit. It has worked well on smaller spills, but its safety limits are unknown in such volume.

“If you drink a gin and tonic, one is fine — a whole bottle is not fine,” he said.

Tissue samples taken

The Louisiana Department of Wildlife and Fisheries has focused on the top of the food chain out of concern for public health, taking tissue samples of spotted sea trout, red drum, flounder and other species popular among seafood lovers. The state already has closed areas east of the Mississippi River delta to fishing.

Even now, as the Deepwater Horizon's rust-colored oil stain moves and spreads, teams of spotters working for NOAA and for the U.S. Fish and Wildlife Service are doing rapid inventories by plane to produce rough estimates of locations and concentrations of sea life and birds that may be harmed by the oil and the dispersant.

Even now, as the Deepwater Horizon's rust-colored oil stain moves and spreads, teams of spotters working for NOAA and for the U.S. Fish and Wildlife Service are doing rapid inventories by plane to produce rough estimates of locations and concentrations of sea life and birds that may be harmed by the oil and the dispersant.

Sperm whales, which have teeth and feed on fish, as well as baleen whales, are likely to swim around the worst of the mess.

Yet those and many others species favor feeding grounds near the mouth of the Mississippi River just a few miles west of the spill site.

The Gulf has a tremendous capacity to reabsorb smaller amounts of oil, which naturally bubbles up from its bed. It appeared to consume even the 124 small spills, an estimated 17,000 barrels, created by the Hurricanes Katrina and Rita, according to U.S. government estimates.

Already, the spill is the biggest man-made threat to the Gulf in two decades.

“Even if the oil spill stops now, it's going to take a long time for the ecosystem to consume it,” said James H. Cowan Jr., an LSU professor and Gulf fisheries expert. “For me, this has the potential to become a chronic insult.”

Reporter Harvey Rice contributed to this report.

lise.olsen@chron.com

matthew.tresaugue@chron.com