BREVARD COUNTY, Fla. -

A federal researcher in South Carolina has isolated the “smoking gun” — or “guns” — that likely killed some 111 manatees, 51 dolphins and 300 pelicans during the past year: toxins from microscopic algae that cling to the seaweed that manatees and fish eat.

“We have found a number of toxins, I think at least three varieties,” Peter Moeller, a research chemist at the National Ocean Service in Charleston, told Local 6 News partner Florida Today.

But while Moeller has zeroed in on the toxins, researchers still don’t know what algae is producing them, nor do they know how to eliminate it from the Indian River Lagoon.

His lab isolated the toxins from tiny algae that sticks to seaweed called Gracilaria, or red drift algae. Scientists gathered the drift algae in late May from just south of Minutemen Causeway in Cocoa Beach, a hotspot for the mysterious manatee deaths.

Manatees have been eating more of the thick Gracilaria, because algae blooms have choked out their usual seagrass diet. Scientists had suspected a toxin that affects the seacows’ nervous system was hampering the marine mammal’s ability to surface, causing it to drown.

Moeller’s lab tested the algae toxins on mice neurological cells and human breast cancer cells.

“We are killing mammalian cells so, yes, there is a mammalian toxin on that Gracilaria,” he said.

They also expose fish and brine shrimp. “These toxins kill them, too,” Moeller said of the three types of toxic molecules.

“What we see is three bands of activity,” Moeller explained. “There could be hundreds of toxins in there.”

The next step is to describe the molecular structure of the three “suites” of toxins, he said, then see if the same toxins exist in the manatee, dolphin and pelican tissues.

Moeller specializes in describing the chemistry of unknown algae toxins, some of which have pharmaceutical uses. It remains unknown what algae — one or many — is producing the toxins inthe Indian River Lagoon. Brown tide, which first bloomed here last year, is one suspect. Although not known to be toxic, it or some other algae might have become toxic as a competitive advantage over other species.

Excessive amounts of nutrients such as nitrogen and phosphorous are thought to be behind the lagoon’s excess algaein recent years.

Many point to fertilizers as the main source. But Brian LaPointe, a researcher with Florida Atlantic University’s Harbor Branch Oceanographic Institute in Fort Pierce, suspects septic tanks, sewer plants and reclaimed water might be feeding a toxic algae explosion. LaPointe gathered the Cocoa Beach drift algae samples for Moeller last month, about a mile southeast of the city’s sewer plant.

LaPointe’s tests have shown the lagoon’s drift algae harbors nitrogen in forms it typically takes after passing through a long digestive tract such as a human’s or throughthe biological processes at a sewage treatment plant.

“It’s certainly a viable place to look,” Moeller said of LaPointe’s hypothesis that human wastefrom septic tanks is a major cause of the problem.“And it’s certainly the first place to look.”

The Gracilaria explosion in the lagoon has been building up for years.In moderation, drift algae is good, providing food and cover for marine life and sponging up nutrients from the water. But too much can choke out seagrass.

Drift algae has entangled the lagoon in a boom-and-bust cycle in recent years, leaving the estuary with too much or too little.

In 2010, Nova Southeastern University used an acoustic sensor to survey the lagoon’s drift algae from Titusville to Sebastian Inlet. They found drift algae had increased by 46 percent in two years, to 102,162 metric tons over the 109 square-mile study area.

LaPointe saw Gracilaria and stingrays, not much else, during his recent sampling dive in Cocoa Beach.

“The prognosis is not good ...,” LaPointe said.