The Red Tide algae bloom now tossing tons of dead fish on Pinellas County's beaches has been fueled for months by many things — runoff from over-fertilized lawns, leaking septic tanks and sewage lines, even dust from the Sahara Desert.

Now add another ingredient to the mix making this the worst Red Tide bloom in a decade: pollution flowing from the Mississippi River.

A 2007 federal study concluded that the extremely large amount of nutrients flowing from the Mississippi into the Gulf of Mexico can stimulate Red Tide blooms growing on the continental shelf off the west coast of Florida.

The lead author of that paper, oceanographer Richard Stumpf of the National Oceanic and Atmospheric Administration, said in an interview this week that that's exactly what happened this year — and it's why this Red Tide resembles the horrific one from 2005.

The nutrients are not what spark the Red Tide algae to suddenly multiply by the thousands and become a bloom, he said. But they do feed the bloom and make it larger.

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"Every possible source of nutrients is feeding it," Stumpf said, noting the same thing happened in 2005.

That bloom, deemed one of the worst Red Tide outbreaks in Florida history, spent more than a year killing fish and shutting down beach tourism from the Alabama border to the Florida Keys. The impact on sea life was so catastrophic that it created a dead zone — an area of the Gulf devoid of oxygen and sea life — that stretched from New Port Richey south to Sarasota.

"There was just no oxygen on the Florida continental shelf," Stumpf said.

The Mississippi River connection to the two Red Tide blooms is another sign of what an incredibly complex mechanism the Earth is — and also the unintended consequences of pollution.

As the Mississippi rushes southward toward the Gulf, the river picks up a lot of passengers. That includes more than 900,000 metric tons of nutrient pollution from the farms and homes along its banks that use fertilizer to grow crops and keep lawns green.

Most fertilizer in the United States comes from phosphate mined in Florida and shipped through the Port of Tampa.

Most of the time the river flows south, and so the pollution feeds a type of plankton that dies and creates a massive "dead zone" near the mouth of the river. But sometimes currents in the Gulf push the flow eastward, toward Florida.

That's been happening for months on end this year, according to Robert Arnone, a University of Southern Mississippi professor who analyzes the river's flow into and through the Gulf.

"It's not just one plume," he said, "but it's like all the waters (from the river) are moving to the east." The Gulf's loop current "is pushing all the waters to the east."

The nutrients settle into deeper water, where the Red Tide algae first begin feeding off the bounty, the study found. As the blooms grow on the continental shelf, they're then pushed toward shore and concentrated by the prevailing wind patterns of late summer and fall.

The current Red Tide bloom was first detected last November and appeared to move offshore around February. Usually, Stumpf said, Red Tide fades away around March, but that didn't happen this year.

Instead, "it came back inshore in the spring," he said. And that time of year, there are no other types of algae around that might compete with Red Tide for the nutrients in the water, he said.

"That's why it's such a bad problem" this year, he said. "Everybody thought it was gone, but it wasn't."

Harmful algal blooms such as Red Tide occur in the waters of almost every U.S. coastal state, caused by numerous species. The direct economic effect in the United States is estimated to average $75 million annually, and scientists predict climate change will lead to more blooms that are more intense.

No one knows what starts a Red Tide bloom, and no one can predict how it will end. In the case of the 2005 bloom, Stumpf said, it took another major natural disaster to stop it.

"It was broken up by Hurricane Katrina," he said. "That's the only good thing we got out of Katrina."

Contact Craig Pittman at craig@tampabay.com. Follow @craigtimes.