Florida lagoon is an animal 'mass murder mystery' as 46 dolphins, 111 manatees and 300 pelicans die from unknown cause

Indian River Lagoon of Florida contains more species than anywhere else in the U.S.

Deaths of hundreds of animals, fish and birds have been occurring for more than a year

No specific cause can be found - biologists are perplexed

Theories range from fertilizer pollution to climate change

Hundreds of dead animals are washing up along the shores of one of America's most biologically diverse estuary.

More than 100 manatees, 300 pelicans and almost 50 dolphins were all found dead along the northern stretches of the Indian River Lagoon in Florida.

Biologists are now trying to work out what the problem is, but there is no doubt that it is a serious one.

Murder mystery: Two manatees out of 111 that have been found dead in the Indian River Lagoon in Florida

Dead in the water: Day after day, dolphins are floating up dead, emaciated down to their skeletons

Saving wildlife: Rescuers attend to a manatee affected by the unknown mystery killer. The manatees appeared to have abruptly sickened and drowned

The lagoon contains more species than anywhere else in the U.S. and to the towns along its edge — Titusville, Cocoa, Melbourne, Vero Beach and Stuart, it accounts for hundreds of millions in revenue from angling, boating, bird-watching, tourism and other waterfront activities.

Its 156 miles of water boast more than 600 species of fish and more than 300 kinds of birds.

The barrier island complex stretched across 40 percent of Florida’s coast, around Cape Canaveral, and consists of the Mosquito Lagoon, the Banana River and the Indian River Lagoon.

In the past, the lagoon has was constantly polluted by nutrients and fertilizers running off lawns and farms but now it appears some kind of tipping point has been reached.

Thousands of dead fish reported in Indian River Lagoon in southern St. Lucie County. Scientists believe it may be due to one or several causes from climate change to changes in water temperature and salt levels

No obvious cause: Florida's Indian River Lagoon is considered one of the most diverse ecosystems in North America

Investigation: Hubbs Sea World Research Institute research assistant Teresa Mazza during the rescue of a distressed dolphin in the Indian River Lagoon system in Florida

Manatees began dying in July 2012, 43 of them in just one month. A total of 111 have died.

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.

Biologists at a state laboratory in St. Petersburg examine every dead manatee that's found in Florida for a cause of death.

They appear to have abruptly fallen sick and drowned.

People have reported between 250 and 300 dead pelicans since January. The birds were emaciated.

Since January, the number of dead bottlenose dolphins has reached 46 - more than double the average recorded in previous years.

The dolphins also look emaciated, similar to the pelicans.

Perplexed: So far, biologists are at a loss to explain the cause of the untimely deaths

Tests: Two sub-adult Loggerhead sea turtles, netted from the water, sit in a boat in the Indian River lagoon with damp towels over their heads help to calm them

Recover: Scientists believe it could take up to a decade before the lagoon recovers

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.

This is not the first time that dolphins have perished in the Indian River Lagoon. Many died in 2001 and 2008 where again the cause of death was undetermined.

It is difficult for scientists to work out why the deaths are occurring when there is not an obvious cause.

With so much farmland, laws in Florida allow fertilizer to be poured into the coastal waters.

This can have effects on acidity, changes in water temperature and salt levels. Some are even suggesting global warming could be to blame.

It is ideal for algae which need high levels of nutrients and salinity to survive. That can also have an adverse effect on the ecosystem's food supply for other animals such as the dolphins.

There are few positive signs. Although the numbers pelican's dying appear to have stopped, now a new brown tide bloom has been spotted.