Features August 12, 2010, 5:00PM EST

The Coast Is Not Clear

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The spill could do its worst damage by exacerbating existing threats. Harm to the bluefin tuna, prized both as a gamefish and as a culinary delicacy retailing for $100 a pound, is the premier example. It ranges the Atlantic but spawns just once a year: precisely where and when the BP spill occurred. The floating beds of brown seaweed that shelter bluefin larvae and fingerlings soak up oil like a sponge. Ocean biologists worry that the spill might have wiped out most of the 2010 generation of Gulf bluefins.

Ordinarily, the loss of a year's worth of fish might be tolerable. The problem: Severe overfishing in international waters of the Atlantic Ocean has already endangered the Gulf-spawning population of bluefins, down by 80 percent since 1970. (American fishermen landed a little less than 800 tons of bluefins in the West Atlantic in 2008, while other nations' fleets landed about 1,200 tons, according to the International Commission for the Conservation of Atlantic Tunas.) The spill could possibly push the bluefin population into outright collapse, says Robert L. Shipp, chairman of the Gulf of Mexico Fishery Management Council as well as the marine sciences department at the University of South Alabama. The only other population of the species, which spawns in the Mediterranean Sea, is also threatened.

The single biggest challenge to the Gulf's ecosystem may be the ongoing loss of wetlands, estimated at 25 to 30 square miles' worth per year. Estuaries and marshes provide shelter for commercially important crabs and shrimp. They also buffer humans from the impact of hurricanes and soak up the nitrogenous compounds from fertilizer and manure runoff that are borne down the Mississippi. Nitrogen that the wetlands don't capture feeds algal blooms. Bacteria that feed on the algae use up oxygen in the depths of the Gulf, creating a seasonal "dead zone" that's hospitable only to jellyfish, bacteria, and some worms. This month the Louisiana Universities Marine Consortium announced that the low-oxygen zone extended for 7,722 square miles, the fifth biggest on record.

What's unknown is whether oil from the spill will significantly accelerate the destruction of the wetlands. The wetlands are sinking because levees along the Mississippi's ship channel prevent silt from replenishing them. Pipeline channels have diced up the wetlands, further weakening them. When an area of wetlands finally sinks beneath the waves, it exposes an adjoining area to the waves' action, speeding up losses. Says McKinney: "If the rate of loss accelerates to 35 or 40 square miles a year, it will give us less time to come up with a restoration plan."

Global warming subtly worsens many of the Gulf's problems. Warmer Gulf waters are conducive to the spread of the voracious lionfish, a tropical Pacific fish with poisonous spokes that displaces native species, and the equally aggressive Chinese tallow tree, which has infested Gulf marshes. Plus, the shores of the Gulf lie so low that a sea level rise of just inches can inundate huge swaths of fertile coastline. (Seas have risen 8 inches in the past century, NOAA says.) The BP spill could potentially give more of a toehold to invasive species by weakening native ones, McKinney says.

Scientists studying the Gulf's health emphasize that all damage assessments are strictly preliminary, so the bad news might not be over. A female crab lays about 3 million eggs, of which a handful grow up to be crabs. Many of the rest are eaten by fish and other crabs. So when oil droplets and chemical dispersant showed up in the larvae of blue crabs, it was a danger sign for the whole food chain. Judy Haner, marine program director for the Nature Conservancy in Mobile, Ala., says damage to fish populations could take three to four years to manifest itself.

Another stubborn unknown is the impact of the spill on small fish, such as menhaden, sardines, small jacks, and anchovies, that are food for creatures higher up the chain. Anchovies and menhaden are filter feeders that swim with their mouths agape, catching tiny food particles in their gill filaments. The tiny oil droplets suspended in subsea clouds could kill the fishes' food source, the near-microscopic crustaceans called copepods. The droplets could also clog the fishes' gills. At the same time, oil-eating bacteria could exhaust oxygen supplies in deep waters. Next unknown: If fish in the plumes do die, will others occupy their niche as the pollution clears and oxygen increases? Shipp, of the University of South Alabama, says he thinks the spill should continue to be regarded as Public Enemy No. 1 for the Gulf until those kinds of questions are answered.

BP has agreed to set aside $500 million for environmental study in the Gulf, many times the normal level of spending. That ought to be enough to get to the bottom of things. "If BP has put the money aside like they say and they don't renege on their promises and the government doesn't strip the money for other purposes—and those are big ifs—there should be money for studies of this spill," says Edward Overton, an environmental chemist and professor emeritus at Louisiana State University. After that, the far greater challenge will be to apply the newfound knowledge to helping the resilient Gulf survive all of its many man-made wounds.

Coy is Bloomberg Businessweek's Economics editor.