COCODRIE, La.—The tendrils of coastline here were some of the first shores to see oil after BP's Macondo well blowout last year. On May 7, 2010—two days before the start of the annual fishing season—oil bounced off Grand Isle and flowed into Terrebonne Bay, remembers Michel Claudet, Terrebonne Parish president. In fact, oil fouled 35 percent of the U.S. Gulf Coast's 2,625 kilometers of shoreline before the spill was done.

"The people of Terrebonne are still trying to recover from the spill," Claudet says. "No one knew and we still do not know what might be the long-term effects."

The murky waters of the Mississippi River Delta obscure a profusion of life, hence the abundant local commercial and sport fishing. They also do an excellent job of hiding the long-term impacts of last year's oil spill. The oil that reached shore has been absorbed into the sponge-like wetlands or drifted to the sediment bottom, impacting shoreline that serves as a nursery for sealife, coastal habitat and a stopover for migrating birds.

"This spill is significant and, in all likelihood, will affect fish and wildlife across the Gulf, if not all of North America, for years, if not decades," warned Rowan Gould, then acting director of the U.S. Fish and Wildlife Service last May. "We will recover a small number of oil-covered birds. The concern is what we can't see….We may never know the spill's impacts on many species of birds and marine life, given how far offshore they are found."

Six years after Hurricane Katrina, the storm's impact is still visible throughout New Orleans, as evidenced by the emptied neighborhoods or the new houses in the Ninth Ward that resemble fresh scar tissue, easily distinguished from the former housing stock. One year after BP's oil spill, however, its impacts are largely invisible, hidden by the deep, cold waters of the Gulf and dispersed in that vast volume of water or tucked away into the endless marshes of the Louisiana coast.

A massive scientific effort is ongoing to precisely quantify the environmental damage caused by the oil spill—whether measured in oily sediments or missing generations of sealife. This is part of the National Oceanic and Atmospheric Administration (NOAA) Natural Resource Damage Assessment Process to determine what and how much BP will have to pay as well as an undertaking to understand a unique oil spill: one that happened more than 1,500 meters beneath the sea surface, spewing roughly five million barrels of oil before it was plugged.

View a slideshow of Gulf Coast assessment efforts

As a result of this looming legal fight, much of what could be known about ecological impacts remains hidden. "Free and open access to scientific information concerning oil spills is not a given," noted the authors of a Congressional Research Service report (pdf) on the oil spill's ecosystem impacts last October. For example, dead dolphins that washed ashore earlier this spring have been seized by the U.S. government. "NOAA and other federal agencies came into every lab with a dolphin in the fridge and confiscated it," says Casi Callaway, baykeeper for Mobile Bay in Alabama. "All data, all studies, all work on dolphins was sequestered."

Known unknowns
Long-term impacts of the oil spill will not be known for years: After the Exxon Valdez spill in Alaska, it took three years before the local herring fishery collapsed. "A lot of species were spawning during the Deepwater Horizon [spill]," notes biological oceanographer Edward Chesney of the Louisiana Universities Marine Consortium (LUMCON). "We undoubtedly lost a lot of those fish and egg larvae—they can't move and are highly vulnerable to oil toxicity." The loss of entire generations of young marine life may also propagate up the food chain—over time. Already, scientists have found evidence of oil passing into plankton (pdf), which serve as the broad base of the food web.

Impacts to marine life range from outright death to reduced reproduction, altered development, impaired feeding as well as compromised immune systems. Even exposure to low concentrations of oil that fish embryos survive can alter the shape of their hearts as adults and reduce their ability to swim, according to research published April 11 in Proceedings of the National Academy of Sciences.

And simply because scientists had little information on certain species before the spill—such as the denizens of the deep that bore the brunt of the dispersed oil—its impacts may prove impossible to measure, although research continues into the array of nematodes, fungi, mollusks and other organisms that thrive on the seafloor. "We don't have a lot of information on deep water species in general," Chesney notes.

Known knowns
What is clear, however, is that the approximately five million barrels of Lousiana sweet (low-sulfur) crude that spewed into the Gulf of Mexico was toxic—a toxicity exacerbated by the use of 1.8 million gallons of dispersant both in the deep sea and at the surface. The oil itself sports an array of so-called polycyclic aromatic hydrocarbons (PAHs)—benzene, toluene and the like that are known to cause cancer. NOAA testing found more than 800 oil-related compounds in the water during the spill. "Those components are very toxic," says toxicologist Scott Miles of Louisiana State University. "Those are the ones you're sniffing when filling up the gas tank."

At the same time, they are compounds that fish and other organisms are efficient at not taking up into their tissues. "Accumulation of PAH is very difficult," notes toxicologist Joe Griffitt of the University of Southern Mississippi, who is studying how oil exposures that do not kill an animal can affect its reproductive success.

And these different compounds have different effects, some of which cancel each other out. "You have a very complex situation, very quickly," says Griffits. PAH can have impacts that don't kill the marine organism directly but reduce its reproductive success or promote tumors—even interfere with the process of copying the genetic code. "There is some evidence that PAH can affect methylation patterns," Griffitt says. "You stick a methyl group on DNA somewhere and then effect gene transcription. It's theoretically heritable."

Further, it is difficult to tell whether a decline in reproduction or an increase in cancer is a direct result of the BP oil spill, a natural oil seep, some combination of causes or another cause entirely—in addition to being difficult to detect in the first place. "We are starting to see some tumors and lesions in fish exposed to Deepwater Horizon [spilled oil]," Chesney notes. In fact, fish caught in the Gulf, such as red snapper, are showing signs of weakened immune systems that have allowed opportunistic infections. The cause may or may not be BP's oil spill.

Evidence from prior spills, such as the Exxon Valdez suggests further long-term effects. "Salmon embryos exposed to oil, when they grow up, their babies are compromised, through mechanisms such as messing with the [hormonal] system," says biologist Andrew Whitehead of Louisiana State University, who studied Louisiana marshes both before and after the oil spill. Alaskan shorebirds also did not breed as much, had smaller eggs when they did breed, and those chicks that did hatch died more frequently.

In addition, BP's Macondo well oil itself smothered birds; more than 8,000 such birds representing 102 different species were collected—2,263 of them already dead—by government workers. Of course, this is likely just a fraction of the birds impacted because an oil-coated bird at sea sinks. "It is this phenomenon that makes an accurate estimate of bird deaths extremely difficult," wrote the Congressional Research Service in an October report on oil spill ecosystem impacts. The Center for Biological Diversity estimates that the oil spill killed or harmed approximately 82,000 birds as well as more than 6,000 sea turtles and 25,000 marine mammals, such as various species of dolphins.

And, unfortunately, the oil that did reach the coast—nearly 700 kilometers of marshland and 235 kilometers of beach was oiled, according to the government's Shoreline Cleanup Assessment Teams—"is very persistent once it gets up in the marsh grass," Miles says. "We still have a lot of oil in the Louisiana marshlands." That oil killed the spartina marsh grass at times, reducing coastal wetlands and, ultimately, exacerbating coastal erosion. "If it does kill the grass in high enough concentrations and a big storm comes up, it's going to start eroding," Miles adds.

Deep clean?
At the same time, the closure of Gulf fisheries during the oil spill last year removed the enormous pressure from commercial fishing on populations ranging from shrimp to the tiny fish known as menhaden, the latter of which is caught to be ground up into meal. As a result, fishing this year is some of the best ever. "There are some fish species that are not as prolific as they have been. Others, there are millions, because we didn't fish them last year," Mobile Bay's Callaway says. "The food web has been touched and changed. We just don't know what that means."

And the fact that the spill occurred at sea—and beneath 1,500 meters of water—spared some of the most productive fisheries and spawning grounds in the world. "What is arriving at shore is much less toxic, much less difficult to deal with than what is coming out of the wellhead," says biologist Christopher D'Elia, dean of the School of the Coast and Environment at Louisiana State University. "If [the spill] had been closer, we would have been in much more trouble."

In the meantime, the Gulf shores enjoy a profusion of tarballs and tar mats in excess of the ones that are always present as a result of natural seeps. "I have spent every summer of my life in Gulf Shores, [Ala.,] and I have never seen anything that is remotely close to what we have here now," Callaway says. "You just run your fingers through the sand and you've got hundreds, depending on when the last time they did a deep clean."

Such "deep cleans" have their own impacts. "There weren't sand crab holes anywhere—those are a major chunk of the food web," Callaway adds. "I didn't see periwinkles or clams along the shoreline. I'm hoping that is temporary and not long-lasting."

But evidence from prior oil spills suggests that Macondo well oil will be a part of the Gulf Coast for a very long time. "Oil persisted for much longer in the environment than anyone expected," Whitehead notes of the Exxon Valdez spill. "The oil was gone from the surface pretty quickly but sediment-associated organisms were persistently exposed to oil over long periods of time—we're talking five to 10 years after the spill."

Only that kind of time will tell what the abundance of life in the Mississippi Delta and the Gulf of Mexico reveals about the long-term impacts of the oil that spewed from BP's Macondo well in 2010. "We are trying to link exposure to effect," Whitehead says. "We are asking the organisms themselves to tell us: 'Has there been a relevant exposure?'"

Editor's Note: Reporting for this article took place as a result of a fellowship from the Metcalf Institute for Marine and Environmental Reporting at the University of Rhode Island.