Ocean Circulation Caused Last Year's Red Tide Event, Study Says

Apr 24, 2019

A new study shows that ocean circulation was a major cause of the toxic red tide bloom, which plagued Florida's West Coast for over a year. It expanded up to the Florida Panhandle and circled down around to the East Coast.


Earlier research showed nutrients from things like septic tanks and fertilizer make red tide blooms worse off Florida's coast. 

But University of South Florida College of Marine Science Physicist Bob Weisberg said when red tide forms on the continental shelf in the middle of the gulf, it thrives when there are fewer nutrients, which allows the organism to grow up and get stronger. 

"Once they arrive at the beach in very high concentrations, then they can dominate over all other species. And if you get them more nutrients at that point, it may exacerbate an existing red tide," he said. 

In July of last year, Weisberg said, the ocean water started circulating in an "upwelling state," meaning the water carrying red tide approached the shore along the bottom and left the shoreline along the top, dropping off more red tide.

Between August and September of last year, USF’s glider tracked red tide organisms from where they started—the “X” on the graphic—to where they ended up. You can see how far and deep red tide traveled along Florida’s west coast by following each colorful line.
Credit USF College of Marine Science / Courtesy

"The upwelling circulation was so strong that some of the cells were at the surface at the beach were transported offshore sufficiently far to get into the Gulf of Mexico loop current around the Florida Keys and up the east coast in the gulf stream, giving us a red tide bloom on the east coast, which is a very rare occurance," said Weisberg.

"And on top of that, in early September, Tropical Storm Gordon came through and those winds were strong enough to blow some of the red tide cells to the panhandle."

He and his colleagues determined that when the 2017 red tide stayed along Florida's west coast through 2018, more new red tide organisms from the continental shelf flowed in.

Weisberg said by June he should be able to predict how bad red tide will be this year, based on the concentration of nutrients on the gulf's continental shelf.