Dead fish fill the end of a canal near the Sell Isle Marina off Snell Island during a Red Tide bloom last July. [ DOUGLAS R. CLIFFORD | Times ]

FL - Human link to Red Tide highlights need for better water monitoring

Scientists have long tried to understand the connection between nitrogen pollution and the infamous toxic algal blooms.

When the ominous rust-colored cloud of Red Tide begins to saturate coastal waters in Southwest Florida, it means beach closures. Asthma attacks. Itchy skin and watery eyes. Dead fish and a wretched smell that can spoil the salty breeze.

Now, scientists know it means pollution made the scourge worse.

New research from University of Florida scientists is “providing clarity in what was previously a muddied landscape,” said environmental engineer Christine Angelini, a co-author of the study.

While Red Tides occur naturally, scientists have long debated the degreeto which they are worsened by high levels of nutrients such as nitrogen from human sources agricultural and urban. Scientists previously had found a correlation between so-called nutrient loads and Red Tide. But the new research offers some of the strongest evidence yet that humans directly influence the severity of the toxic blooms.


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Safe To Fish in the SF Bay? 'Harmful' Algal Bloom Rages On, Killing Thousands of Fish, The San Francisco Standard / September 02, 2022

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Toxic Algal Bloom Spreads Along California Coast, Poisoning Sea Lions, NOAA Fisheries / September 01, 2022


The study focuses on the Caloosahatchee River in Southwest Florida, which carries water and pollutants from Lake Okeechobee as it flows west to the Gulf of Mexico.

In the past, scientists looked for a direct relationship between nitrogen and Red Tide. But excess nitrogen doesn’t cause Red Tide — it exacerbates it, an effect that can take weeks. Looking for a short-term correlation did not implicate pollutants.

The scientists tapped longer-term data from water-quality and Red Tide databases maintained by the South Florida Water Management District and Florida Fish and Wildlife Conservation Commission, respectively. By analyzing biweekly nitrogen and Red Tide patterns over nine years, they were able to detect a more complex cause-and-effect relationship.

The results show a clear link between increasing amounts of nitrogen coming out of the Caloosahatchee and the intensity of Red Tide on the coast, Angelini said.

“There has been this kind of uncertainty about whether there is this human link,” said Angelini, director of UF’s Center for Coastal Solutions.“For the first time, this paper really shows that.”

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