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In the search to determine the cause of red tide, local researchers turned their focus to the possible contribution of fertilizer.

The consensus that scientists and representatives from the lawn care industry reached at a public forum Thursday was a familiar one when dealing with red tide: There is not enough evidence to firmly suggest that nutrient runoff causes red tide.

More than 75 residents, local leaders and scientists took part in the forum, which at times grew heated.

Members of the University of Florida Institute of Food and Agricultural Sciences extension office in Fort Myers organized the forum to discuss the issue because local leaders have talked about regulating the kind of fertilizers that stores can sell.

At the direction of the Bonita Springs City Council, Mayor Jay Arend recently sent a letter to Lee County commissioners urging them to adopt an ordinance that would limit the sale of domestic fertilizer in local stores to a type that is less harmful to the environment.

Such an ordinance should only allow the domestic sale of slow-release manufactured fertilizer, Arend said. A slow-release fertilizer doesn’t dissolve instantly, giving plants more time to absorb the chemicals.

There is no arguing that nutrients, such as nitrogen and phosphorus, found in most common fertilizers, contribute to the growth of red tide blooms. But there is not enough evidence to determine whether fertilizer and other nutrient runoff creates them, said Stephen Bortone, director of the marine laboratory for the Sanibel-Captiva Conservation Foundation.

MICHEL FORTIER / Daily News

TruGreen ChemLawn field manager Rick Beckstrom uses properly measured amounts of fertilizer on a lawn in Estero on Thursday, something the company strives to achieve based on seasonal conditions and the need to eliminate fertilizer runoff. Branch manager Keith Krueger sees his company as an environmental company as well as a lawn company. “Our job is to keep it green, but we’re also responsible for the environment,” Krueger says.

“Lawn fertilizers are part of the problem but they are part of a whole series of nutrients that come into the system,” Bortone said. “The problem is partitioning out what are the contributions of lawn fertilizer. We know it is a major contributor but not how much.”

An ordinance regulating lawn fertilizer is not the answer, said Erica Santella, a regional tech manager for TruGreen-ChemLawn.

The proper solution is for the lawn care industry to adopt best management practices, which are endorsed by the Department of Environmental Protection and will regulate all factors of growing plants, Santella said.

“Fertilizer ordinances miss the point,” she said. “Growing plants is complex and fertilizer is one part of it.”

Best management practices integrate the selection of plants, irrigation, fertilization and pest control in a way that minimizes environmental impacts, Santella said.

A big problem is individual homeowners who do not properly distribute fertilizers on their laws, causing much of it to runoff onto the street and into gutters, she said.

“For some reason people have a feeling of safety with slow-release fertilizer,” Santella said. “There is no feeling of safety if you get the stuff on the street.”

When fertilizer is properly distributed on turf grass, there is no runoff, she said.

“When you fertilize turf it’s like throwing a slab of meat to a pack of wolves,” nothing escapes, Santella said.

But her presentation drew groans from some in the crowd, who asserted that nutrients from Lake Okeechobee and agricultural runoff that flows into the Caloosahatchee River cause red tide.

One scientist said the Caloosahatchee is a major contributor to red-tide growth.

“If you put a dam across the mouth of the Caloosahatchee River and stop that flow, yes it will eventually reduce the nutrient load in coastal waters and make red tide go down,” said Larry Brand, a scientist with the division of marine biology and fisheries at the University of Miami.

On average, only 50 percent of the water in the river comes from the lake. The other 50 percent comes from local basin runoff, meaning water flowing from agricultural and urban areas within the river basin.

Fertilizer may be one factor contributing to red tide blooms, but not the only factor, scientists said.

Growth is one of the biggest culprits, they said. With more development comes more concrete covering the land. That means nutrients in runoff that would normally be absorbed into the ground flow directly into estuaries.

All evidence shows that Karenia brevis, which carries the neurotoxin that causes red tide, is increasing in frequency and abundance in the Gulf of Mexico, Brand said.

Where red tide used to occur only in fall months, it can now plague coastal waters all year round, he said.

“Tourists come down now and it’s still around and they don’t appreciate getting hit with a nerve gas,” Brand said, “and they check out of their hotel and leave.”