At first glance, the marshy, muddy coastline of Bay Jimmy in southeast Louisiana appears healthy three years after the nation’s worst offshore oil spill. Brown pelicans and seagulls cruise the shoreline, plucking fish and crabs from the water. Snails hold firm to tall blades of marsh grass.

Underneath the surface, environmentalists and scientists fear there may be trouble, from tiny organisms to dolphins. Yet the long-term environmental impact from the spill is still not fully known and will likely be debated for years to come.

The British oil company BP PLC has spent billions of dollars on cleanup efforts since the Deepwater Horizon rig exploded and a well ruptured April 20, 2010, spilling 200 million gallons (760 million liters) of crude.

The oil fouled 1,110 miles (1,800 kms) of beaches and marsh along Louisiana, Mississippi, Alabama and Florida. Fishing waters were closed and thousands of people who depend on the Gulf’s deep blue waters wondered if the coast would ever be the same. Crews continue to find oil buried underneath beaches whenever a tropical storm stirs up the Gulf.

“Visually, the coast looks great, and I think most of what was visible is gone,” said David Muth, director of the National Wildlife Federation’s Mississippi River Delta Restoration Program.

Still, oil sheens penetrated deep into marshes, worrying Muth.

“The micro-organisms and the smallest invertebrates, they’re all eating the grasses and eating each other,” he said. “Some of those persistent chemicals just get built up, and as each creature comes along and eats it, the toxins can be amplified right up the food chain until you get to the top predators, like dolphins and sea turtles.”

More than 650 stranded dolphins have been found since the spill, Muth said.

But those deaths started two months before the disaster and it’s not clear what is causing them _ or how much the spill may have contributed. Federal biologists have said the one consistent thread was a bacterial infection.

Turtle deaths also are being looked at, but the National Oceanic and Atmospheric Administration has said many probably drowned in shrimp nets.

Nearly every aspect of the spill’s environmental impact is under review, though much of the research cannot be released because it’s likely going to be evidence in court.

The trial’s first phase ended Wednesday without any rulings from the judge who heard eight weeks of testimony from witnesses for the federal government, a team of plaintiffs’ attorneys, BP, rig owner Transocean Ltd. and cement contractor Halliburton.

The first phase was designed to identify the causes of BP‘s well blowout and assign percentages of fault. The second phase, set to start in September, is supposed to determine how much oil spilled into the Gulf and examine BP and Transocean’s efforts to stop the gusher.

Damage can take years to show up. Herring populations looked normal after Alaska’s Exxon Valdez oil spill in 1989, but by 1993 there were only one-quarter as many spawning adults as in the late 1980s.

In Louisiana’s Bay Jimmy, erosion has been a problem, but that was case long before the spill. Different studies have come up with different answers about whether the spill increased the rate of erosion.

One found double the rate when heavily oiled parts of Barataria Bay were compared with more lightly affected areas, though the effect faded after 18 months.

However, scientists at the Louisiana Universities Marine Consortium laboratory in Cocodrie did not find such a stark contrast in the Barataria Bay marshes they studied, assistant professor Alex Kolker said.

“We’re still crunching numbers. I still want to dot all my i’s and cross all my t’s. But nonetheless I feel comfortable telling you we don’t see a large difference,” he said.

As the studies continue, so do cleanup efforts.

On the beach at Grand Isle, La., crews were still finding tar balls washing ashore. They were also drilling through the sand to find deposits of oil.

The spill, which fouled white-sand beaches along the Alabama coast, seems a distant memory at Sportsman Marina in Orange Beach, said general manager Brian Wells.

Located near the Florida line on a cove off Perdido Bay, the marina specializes in storing and fueling boats for private anglers. It’s adding a new bar and has about 50 more boats than this time last year, Wells said.

Nearby, restaurants are opening and condominium buildings are under construction.

“We’ve had a really good spring,” Wells said. “Boat counts are up, business is up.”

(Associated Press reporter Janet McConnaughey reported from New Orleans. Jay Reeves contributed to this report from Birmingham, Ala.)