Red tide: why Florida’s toxic algae bloom is killing fish, manatees, and turtles

The whale shark is the largest fish in the world. It can grow to 40 feet in length and weigh tens of thousands of pounds. In July, a 26-foot whale shark was found dead on Sanibel Island, on Florida’s southwestern Gulf Coast, its body riddled with the neurotoxin produced by tiny algae in the sea.

Marine scientists don’t know for sure how it died, but they suspect the Karenia brevis algae — a single-celled organism that’s currently in a massive bloom cycle, called a red tide.

The red tide has claimed many many victims this year on the southwestern Florida coast, which has become a rotting marine graveyard. A hundred manatees, a dozen dolphins, thousands of fish, 300 sea turtles, and more have died or washed along shores in putrid-smelling masses. They were all likely felled by the red tide.

The red tide is a normal, seasonal occurrence in southwest Florida. But this year’s red tide has persisted since last November — 10 months — making it the worst bloom since 2006 (when the tide lasted 17 months).

It’s gotten so bad that earlier in August, Florida Gov. Rick Scott declared a state of emergency because the state’s tourism industry has been hit hard by the persistent bloom.

Where is the red tide in Florida?

You can’t blame would-be Gulf bathers for avoiding the smell of rotting fish, or the risk of skin and eye irritation from swimming in red tide waters. The neurotoxin produced by the red tide can go airborne and cause issues in people with asthma or other respiratory issues. Even lifeguards on the hardest-hit stretches of shoreline are wearing gas masks.

The red tide isn’t showing many signs of letting up as it moves northward up the coast. In its most recent assessment, the Florida Fish and Wildlife Conservation Commission found it’s currently impacting 145 miles of coastline.

Javier Zarracina/Vox

All told, from the red tide and a separate freshwater algal bloom on Lake Okeechobee, Florida businesses report $90 million in tourism losses.

The red tide isn’t Florida’s only serious algae problem

Red tides — named for the muddy brown hue the Karenia brevis turn the water — are common in southwest Florida. They occur most years for a few weeks to a few months after the algae start to bloom in the fall.

In a bloom, millions of these tiny organisms produce a paralyzing neurotoxin that prevents fish and marine life from respirating. It’s mildly dangerous for humans too: The toxin can be dangerous for some people with respiratory sensitivities. And it’s really dangerous to eat shellfish steeped in K. brevis. (Though this isn’t such a big deal. Commercial shellfish operations strictly monitor for red tide, and recreational shellfish gathering is banned during red tide events.)

Red tide isn’t the only harmful algal bloom occurring in Florida right now. There’s also a bloom of toxic blue-green algae in Florida’s fresh waterways, like Lake Okeechobee. Red tide is a separate phenomenon, caused by a different organism, but both sorts of algal blooms are often fueled by the same thing — agricultural runoff (i.e., fertilizer) and warm water.

This current red tide began back in November, two months after Hurricane Irma drenched the state. It’s possible that when the US Army Corps of Engineers released a mass of water from Florida’s Lake Okeechobee to prevent flooding, those fertilizer-rich waters flowed to the ocean and helped fuel the current bloom, the Washington Post reports. Still, it’s hard to know the exact cause of this bloom. Gulf temperatures (which, on average, have risen over the past decades), water salinity, sunlight, and Gulf currents all play a role.

Karenia brevis is a single-celled organism belonging to a group of algae called dinoflagellates.
FWC Fish and Wildlife Research Institute

Researchers have observed a long-term upward trend in the concentration of K. brevis since 1950 onward, likely due to increased agricultural pollution (K. brevis are kind of like plants in that they thrive on fertilizer), and increased stormwater runoff drainage into the Gulf. Increased water temperatures may play a role too (though, as the Washington Post points out, if the water gets too hot, then the K. brevis don’t reproduce as well, so continued severe red tides in a climate-changed world are not a given).

What can be done about the red tide? Well, not much currently. There are some experimental designs that involve killing the red tide-causing algae with ozone, or introducing competing algae that would steal food from the K. brevis. But these ideas are a long way from coming out of the lab and into the seas.

How might the red tide end? Winds could shift and push the red tide farther out to sea; as could a hurricane. Or Floridians can just wait. The longest red tide on record lasted 30 months in the mid-’90s. Pray for the fish.

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