It looks dirty and muddy, a brown mass of weeds with gas-filled berries that allow it to float on the Gulf of Mexico's waters. Sometimes it washes ashore, getting caught in the toes of barefoot beachgoers or stuck to the bottom of flip-flops.
It appears to be just another sea plant.
But these Sargassum algae — sometimes called sea holly or Gulf weed — are key to dozens of species of marine life in the Gulf. Now, the oil is threatening to suffocate them, dealing a blow to fisheries and the ecosystem that scientists say may take years to recover. And as the algae die in the Gulf, less of the vital plant will reach the Sargasso Sea — some 3,000 miles away through the loop current — potentially harming that ecosystem as well.
Already, oiled sea holly has washed ashore in Orange, Alabama, and scientists are seeing larger patches of it mingling with the offshore oil slicks.
"We've seen Sargassum mats from the air co-occurring with oil slicks. They're in the same spot," said Sean Powers, a marine scientist at the University of South Alabama, who is using a National Science Foundation grant to track the seaweed and its surrounding marine life.
Sea holly washes up on Gulf of Mexico and East Coast beaches throughout the summer, jam-packed with tiny shrimp and crabs, little shells and sediment, a treasure trove for children. On this sandy barrier island, clumps of sea holly wash up, forming patches of brown on the white sand.
Like underwater coral reefs, these algae mats are critical habitats for marine life. Tuna, Mahi-mahi, dolphin fish, Billfish, shrimp, crabs and sea turtles all use the algae to spawn, sunbathe or hide from predators, often while noshing on them. The algae's own exclusive community — brown or yellowish fish with weed-like tails, unusual tiny shrimp and crab and unique seahorses — have adapted in color and behavior to live only there.
"Once it's oiled, from everything we know of the effects of oil, all of those animals that live in the Sargassum will die," Powers said.
Similar to phytoplankton — the nearly invisible floating plant life — sea holly is at the base of the marine food chain, said Dennis Heinemann, a fishery scientist with the nonprofit, Washington-based environmental group Ocean Conservancy.
Sea holly attracts so much marine life to it, fishermen congregate around the long weed lines formed by the algae, knowing they could increase their catch.
But experts say oil can kill the Gulf weed either by poisoning it or by restricting its ability to breathe or get sunlight.
Relying on the weed are 145 species of invertebrates, 100 fish species, 5 types of sea turtles and 19 different seabirds, said Ellycia Harrould-Kolieb, a marine scientist with the Washington-based nonprofit Oceana.
"They're trained to cue in on that Sargassum," Powers said, pointing specifically to younger fish and animals. "It's the only structure out there that provides them any refuge from predators."
Unlike land plants, Sargassum has few seeds and propagates by splitting off, creating new growth. When it dies, it leaves little behind. Powers estimates it would take at least three years to recover to pre-oil spill Sargassum levels, possibly longer.
While animals are resilient, habitat is not, said Bob Shipp, chairman of the Department of Marine Sciences at the University of South Alabama.
Past experience, including the 1989 Exxon Valdez oil spill in Prince William Sound, shows that if a habitat is harmed, the ecosystem will never recover in the same way. The herring that had once been a mainstay of the Alaskan sound never returned after the spill, partly because its foraging habitat had been destroyed, he said.
"We could see a whole new system created following the spill, and not a good one," Shipp said, noting a great deal of the Gulf economy relies on robust fisheries of red snapper, grouper, trout, flounder, bluefin tuna and other seafood.
"The ripple effect is going to be very extensive," Shipp said.
Sargassum is also awash in legend, including stories about vessels getting stuck in the Sargasso Sea's thick algae mats, some covering acres of the water's surface. Gulf of Mexico tourists sometimes view it as trash, annoyed it is not cleared off beloved white sandy beaches. Recently, some people have mistaken dead strands of Sargassum for oil washing up on Gulf beaches.
Until about two years ago, it was believed the Sargassum found in the Gulf originated in the North Atlantic.
Satellite images and research have shown, however, that the Sargasso Sea actually gets its algae mats from the Gulf, where the seaweed grows and propagates before getting pushed east through the loop current, around Florida and into the central North Atlantic.
"That would mean that the Sargassum that's lost in the Gulf will impact the weed in the North Atlantic, the tuna, the fisheries," Powers said. "This could have a larger effect."