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Red tides sparked by human activity and effects of climate

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red tide dead fish

A noxious red tide has descended on the U.S. East Coast, killing fish and ruining spring break before it’s even begun.

The red tides have been plaguing the Florida coast for the past few months, but this week has been particularly bad, with 89 out of 157 samples from the waters along Florida’s Gulf Coast testing positive for the algal bloom, according to the Florida Fish and Wildlife Conservation Commission.

This has resulted in around 20 tons of dead fish and debris washing up on Florida beaches since December 12, the Tampa Bay Times reports, as well as dead turtles and manatees being found, and the BeachFest festival held annually in Indian Rocks Beach, Florida, being canceled a month in advance.

Red tides, so-named for the rust-red color they often cause in coastal waters, are blooms of algae, also known as harmful algal blooms.

red tide dead fish Thousands of dead fish float in the Boca Ciega Bay located near the mouth of Madeira Beach on July 21, 2021 in Madeira Beach, Florida. Photo by Octavio Jones/Getty Images

“The Florida red tide is a harmful algal bloom caused by the algae Karenia brevis,” Larry Brand, a professor of marine biology and ecology at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, told Newsweek. “It occurs naturally in the Gulf of Mexico at low concentrations but occasionally develops into dense blooms. The hot spot for these blooms is along the coastline from around Tampa Bay to Naples.”

Karenia brevis produces a neurotoxin called brevetoxin, which is released into the water and air from the algal cells when they are disturbed and broken open. These toxins primarily affect vertebrate neurons, and at high concentrations it can kill fish, dolphins, manatees, sea turtles, and sea birds.

The Tampa Bay Times reported that on March 1 a 10-foot manatee was found dead, floating in Boca Ciega Bay, the seventh manatee to have died from the red tide so far this year.

The red tides can also cause ill-effects to humans that are exposed to the toxins, either by breathing in the toxic fumes or by eating fish and shellfish that have themselves consumed the algae.

“It can affect humans in two ways. Consuming seafood contaminated with brevetoxin can lead to Neurotoxic Shellfish Poisoning. More commonly however, the toxin gets into the air and can cause respiratory distress,” Brand said.

Those who breathe in the toxins may experience symptoms including coughing, eye irritation or worsening asthma.

The red tide blooms can also impact other species indirectly, by using up much of the dissolved oxygen in the water or blocking light from reaching photosynthesizing plants on the seabed.

dead fish red tide A sign for depositing dead marine life from the Red Tide bacteria into a dumpster is seen at Bay Vista Park on July 21, 2021 in St. Petersburg, Florida. Photo by Octavio Jones/Getty Images

Exactly why these blooms occur is not yet known, and predicting when and where they will happen is difficult. However, it’s thought that a major factor in their occurrence is nutrient levels in the waters.

“Blooms result from a complex sequence of events starting with conditions that favor their growth more than 80 miles offshore in the Gulf, currents that move them toward shore, and pollution that intensifies and prolongs the blooms,” Don Boesch, a professor of marine science at University of Maryland Center for Environmental Science, told Newsweek.

“A particularly important contributor to this land-based pollution is runoff flowing down Florida rivers of excessive nitrogen, a nutrient which stimulates the blooms. This mostly comes down the Caloosahatchee River which receives flow from Lake Okeechobee and thus the Kissimmee River,” he said, adding that the problem “results largely from the intense agriculture.”

red tides Dead fish have washed ashore in the thousands due to a red tide on January 4, 2021 in Captiva Island, Florida. Photo by Andrew Lichtenstein/Corbis via Getty Images

Brand agreed, saying that across 50 years, there has been a 15-fold increase in the occurrence of red tides, and this may be due to the increased use of artificial fertilizers in agriculture, and the effects of climate change.

“Being an algae, it needs nitrogen and phosphorus to grow,” Brand said. “I attribute [the 15-fold increase] to the large increase in human activities in South Florida over the past 50 years, primarily fertilizer and sewage runoff. It is not clear if higher temperatures from climate change are increasing red tide, but the more intense rainstorms associated with it will be leading to more nutrient runoff.”

Climate change is expected to increase the occurrence of algal blooms like the red tides, and the huge mats of sargassum seaweed that form in the Sargasso sea before making their way towards the Caribbean and Florida. This is because climate change may increase rainfall in certain areas, increasing the run-off of fertilizers into rivers and subsequently oceans, and will also increase ocean temperatures, making it easier for the algae to grow.

“There are a number of ways that climate change could be playing a role in making matters worse, including changes in the currents and temperature of the Gulf, but at this point there is no scientific consensus on the degree to which change may be a factor,” Boesch said.

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about red tides? Let us know via [email protected]


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