The Indian River Lagoon on Florida's east coast has long been known as the most diverse ecosystem in North America.
Its 156 miles of water boast more than 600 species of fish and more than 300 kinds of birds.
The lagoon is not just an ecological treasure. To the towns along its edge — Titusville, Cocoa, Melbourne, Vero Beach and Stuart, among others — it accounts for hundreds of millions in revenue from angling, boating, bird-watching, tourism and other waterfront activities.
But these days the Indian River Lagoon has become known as a killing zone.
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. As of last week, 111 manatees from Indian River Lagoon had died under mysterious circumstances. Soon pelicans and dolphins began showing up dead too — more than 300 pelicans and 46 dolphins so far.
How bad is it? In the past week, a dolphin a day has turned up dead in the lagoon, said Megan Stolen, a research scientist at Hubbs-SeaWorld Research Institute.
"When you lose the manatees, pelicans and dolphins, you know something is going on," said Marty Baum of Indian Riverkeeper, a nonprofit environmental group that tries to act as a steward for the lagoon and the Indian River that flows into it.
Yet so far nobody can name the killer. Biologists have some suspicions but are baffled about any connection among the species' problems. The diets are different: Manatees are vegetarians, while pelicans and dolphins eat fish. The symptoms are different: The manatees' stomachs are stuffed, while the pelicans and dolphins are emaciated.
Baum's family has lived around the lagoon since the 1860s, but he can't remember anything like this ever happening.
The lagoon has had algae blooms before. None of them were like the one that hit it in 2011. Experts called the explosion of the greenish Resultor species a "superbloom" because it covered nearly 131,000 acres and lasted from early spring to late fall.
Then came the "brown tide" algae bloom last summer, tinting the water a chocolate brown. The algae, Aureoumbra lagunensis, have been a recurring problem in Texas. Why it suddenly showed up in Florida is another mystery.
The algae blooms shade out sunlight needed by sea grass. By the time the algae was done, the lagoon had lost more than half its sea grass, essential to nurturing fish and other marine species.
Then came what Pat Rose of the Save the Manatee Club called "a cascade of events."
The mysterious manatee die-off began in the northern part of the lagoon last July, hit its peak around March and now produces another dead manatee about every two weeks, said Martine DeWit of the Florida Fish and Wildlife Conservation Commission.
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Biologists at a state laboratory in St. Petersburg examine every dead manatee that's found in Florida for a cause of death. But the Indian River Lagoon manatees have them stumped. The manatees appeared to have abruptly sickened and drowned.
Normally manatees eat sea grass. With much of the sea grass gone, the manatees turned to eating a red sea weed called gracilaria. But so far there is no sign that played any role in their deaths, DeWit said. The lab is continuing to test for viruses, pollutants or something else.
Similar tests are being run on the dead pelicans and dolphins. Stolen of Hubbs-SeaWorld said the dolphin die-off first became evident in January and has not let up since.
And the lagoon's 700 dolphins are already somewhat beleaguered. They tend to suffer from high levels of mercury. In fact, research by the Harbor Branch Oceanographic Institution at Florida Atlantic University has found that there's so much mercury in the lagoon's fish that people who eat them have higher mercury concentrations in their tissues than those who eat imported fish.
Scientists caught a break Friday when a kayaker discovered an ailing, sunburned, underweight dolphin stranded in the shallows, Stolen said. Rescuers were able to capture it for rehabilitation, and they hope it offers clues to what killed the others.
When it comes to naming the cause, the list of suspects rivals a game of Clue. Stormwater runoff filled with fertilizer and other nutrient pollution has been blamed for fueling the algae blooms. Other theories point to the U.S. Army Corps of Engineers dumping polluted water from Lake Okeechobee, changes in water temperature or salt levels, overflow from contaminated mosquito-control ditches, even climate change, which is boosting the acidity of the world's oceans.
The Harbor Branch Oceanographic Institute had hoped for $2 million in state money this year for a study of the lagoon's water chemistry, but Gov. Rick Scott vetoed the appropriation.
There are a few hopeful signs. The pelican die-off appears to have ended. As for the sea grass, "we're starting to see some regrowth in certain areas, but not as much as we'd hoped," said Tony Rice of the Indian River Lagoon Estuary Program, a government-sponsored partnership among local and state agencies.
Meanwhile, a new brown tide bloom was spotted last month. If the lagoon hasn't hit a point where it's sliding toward oblivion, said Rose, of the Save the Manatee Club, a return to normal is a long way off. "I'm thinking it's seven, eight, nine years," he said. "We could be looking at a decade before it recovers."
Craig Pittman can be reached at firstname.lastname@example.org.