NEW ORLEANS (AP) — The oil you can’t see could be as bad as the oil you can.

While people anxiously wait for the slick in the Gulf of Mexico to wash up along the coast, globules of oil are already falling to the bottom of the sea, where they threaten virtually every link in the ocean food chain, from plankton to fish that are on dinner tables everywhere.

‘‘The threat to the deep-sea habitat is already a done deal — it is happening now,’’ said Paul Montagna, a marine scientist at the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University-Corpus Christi.

Hail-size gobs of oil the consistency of tar or asphalt will roll around the bottom, while other bits will get trapped hundreds of feet below the surface and move with the current, said Robert S. Carney, a Louisiana State University oceanographer.

Oil has been gushing into the Gulf of Mexico at a rate of at least 200,000 gallons a day since an offshore drilling rig exploded last month and killed 11 people. On Wednesday, workers loaded a 100-ton, concrete-and-steel box the size of a four-story building onto a boat and hope to lower it to the bottom of the sea by week’s end to capture some of the oil. Crews also set fires at the worst spots on the surface Wednesday to burn off oil.

Scientists say bacteria, plankton and other tiny, bottom-feeding creatures will consume oil, and will then be eaten by small fish, crabs and shrimp. They, in turn, will be eaten by bigger fish, such as red snapper, and marine mammals like dolphins.

The petroleum substances that concentrate in the sea creatures could kill them or render them unsafe for eating, scientists say.

‘‘If the oil settles on the bottom, it will kill the smaller organisms like the copepods and small worms,’’ Montagna said. ‘‘When we lose the forage, then you have an impact on the larger fish.’’

Making matters worse for the deep sea is the leaking well’s location: It is near the continental shelf of the Gulf where a string of coral reefs flourishes. Coral is a living creature that excretes a hard calcium carbonate exoskeleton, and oil globs can kill it.

The reefs are colorful underwater metropolises of biodiversity, attracting sea sponges, crabs, fish, algae and octopus.

‘‘In my mind, they are at least as sensitive to contamination to oil as coastal habitat,’’ said James Cowan, an oceanographer at Louisiana State University. ‘‘They are in deeper water, so they are kind of out of sight, out of mind.’’

There are other important habitats in shallower waters, such as an ancient oyster shell reef off the Mississippi and Alabama coasts. It is a vital nursery ground for red snapper and habitat for sponges, soft corals and starfish.

Scientists are watching carefully to see whether the slick will hitch a ride to the East Coast by way of a powerful eddy known as the ‘‘loop current,’’ which could send the spill around Florida and into the Atlantic Ocean. If that happens, the oil could foul beaches and kill marine life on the East Coast.

‘‘Once it’s in the loop current, that’s the worst case,’’ said Steve DiMarco, an oceanographer with Texas A&M University-College Station. ‘‘Then that oil could wind up along the Keys and transported out to the Atlantic.’’

Engineers are racing to stem the flow of oil before the disaster escalates, mainly by getting ready to place a giant structure on top of the spill to funnel the crude into a tanker. BP is also exploring a technique in which crews would reconfigure the well that would allow them to plug the leak, but that effort is a couple weeks off.

The cause of the rig explosion is still not known, but investigators from multiple federal agencies are looking into the matter. The rig owner, Transocean, said in a filing with regulators Wednesday that it has received a request from the Justice Department to preserve information about the blast.

The Gulf ecosystem is already stressed by fertilizer and other farm runoff from the Mississippi River and the loss of wetlands to erosion and development. About 2,100 square miles of wetlands have disappeared since the 1930s in the southern Louisiana.

Every summer, algae caused by fertilizer runoff sucks up the oxygen in a large patch of the Gulf, creating a ‘‘dead zone’’ from which all sorts of sea creatures must escape. This year, they will be swimming into waters fouled by the oil spill.

‘‘We’re always wondering when we may reach the point where straw breaks the camel’s back,’’ Montagna said. ‘‘At some point you have to wonder if we will see catastrophic losses.’’