A government research chemist has isolated what he calls "a suite of toxins" on seaweed eaten by the 112 manatees that have died in Florida's Indian River Lagoon.

Some of the toxins may be previously unidentified by science, and flourished because of sewage-fueled algae blooms that killed sea grass.

"These animals are swimming in some highly toxic water," said Peter Moeller, a chemist with the National Ocean Service. However, scientists say that doesn't explain why 52 dolphins and about 300 pelicans died there, since they ate fish, not seaweed.

The manatees filled their bellies with the reddish seaweed called Gracilaria because their normal food, sea grass, had been wiped out by a series of huge algae blooms fueled by nutrient pollution in the lagoon.

Initially, scientists thought fertilizer was the source of the pollution, but tests by Brian Lapointe from Harbor Branch Oceanographic Institute have found that the culprit is actually sewage. The sewage could be coming from leaks in the estimated 300,000 septic tanks scattered around the lagoon on the state's Atlantic coast, he said, or it could be migrating into the lagoon from the deep-well injection of treated sewage into the aquifer.

Vero Beach switched to deep-well injection two years ago after the state Department of Environmental Protection said the city had to stop dumping treated sewage directly into the lagoon. The DEP subsequently approved the $11 million deep-well injection system. Last year, one DEP official called it "one of the best deep wells I have ever seen."

Whether the sewage source is the wells or the septic tanks or some combination of the two, Lapointe said, "Basically the Indian River Lagoon is being used as part of our sewage treatment system."

The Indian River Lagoon has long been hailed as the most diverse ecosystem in North America. Its 156 miles of waterways boast more than 600 species of fish and more than 300 kinds of birds, attracting anglers and tourists to the towns along its shore, such as Titusville, Cocoa, Melbourne, Vero Beach and Stuart.

But a series of algae blooms wiped out more than 47,000 acres of its sea grass beds, which one scientist compared to losing an entire rainforest in one fell swoop. Then, beginning last summer, manatees began dying, and they haven't stopped. Soon dolphins and pelicans began dying, too.

The pelican die-off apparently stopped in mid April, according to Kevin Baxter of the Florida Fish and Wildlife Conservation Commission, but the manatee and dolphin die-off is continuing. The most recent dead manatee turned up July 9, he said.

When Lapointe and an assistant, Laura Herren, collected Gracilaria from the lagoon for Moeller to examine, they noticed it was covered in fuzz. "Those are the microscopic algae that we think are producing the toxins," Lapointe said.

Moeller said he was able to extract what he called "novel toxins that had not been described before." When he exposed cells from mammals to the toxins, the toxins killed the cells, he said.

While that strongly suggests the toxins on the Gracilaria are what killed the manatees, Moeller said, "any direct link to the manatee deaths is a long way off."

Meanwhile, "we have not found a definitive cause" for the dolphin deaths, said Megan Stolen, a research scientist at the Hubbs-SeaWorld Research Institute. "We have been careful not to lump the manatee deaths with the dolphin deaths since they eat different things, although that doesn't mean there's no direct relation."

So far only one dolphin sickened by whatever is in the lagoon has been captured alive. However, Stolen said this week, that dolphin — found by a kayaker last month and now being cared for at SeaWorld — has not yielded any clues to what's killing the others.

Craig Pittman can be reached at craig@tampabay.com