What's the best waterfront restaurant in Florida? 17 favorites for food, drinks and fun!
SARASOTA

Study IDs source of red tide bloom

Scientists say ocean currents, not fertilizers, were the major cause of 2018 outbreak

Carlos R. Munoz
carlos.munoz@heraldtribune.com

SARASOTA — A University of South Florida study blamed ocean currents — not land-based fertilizers — for making 2018 the worst year for red tide in Florida for more than a decade.

Red tide, a plantlike organism known as Karenia brevis, caused breathing problems, stuffed local waterways with dead fish and was responsible for damaging tourism along the southwest coast of Florida during the 18-month bloom that ended in February.

Read more: Complete coverage of red tide in Southwest Florida

While traces of K. brevis are always present in Gulf waters, the harmful algae is currently at undetectable levels, compared with the much higher concentrations at this time last year, according to the Florida Fish and Wildlife Conservation Commission red tide status report. 

A team of scientists from USF and FWC Research Institute used an underwater drone to find the origin of last year's record red tide bloom. The study was paid for with leftover planning funds from USF.

The robotic yellow glider that looks similar to a torpedo was deployed for a month in the thick of the red tide outbreak to evaluate water that could have fueled ride tide. It picked up high concentrations of chlorophyll, a signature of red tide, near the bottom about 50 miles west of Tampa Bay.

The data was run through a computer model that found conditions ideal for underwater delivery of red tide cells to the coastline.

"When we see strong upwelling induced by the Loop Current during spring to early summer, we don't get a major K. brevis bloom," said study coauthor Dr. Yonggang Liu, who studies the physics of the ocean with Robert Weisberg at USF.

During an upwelling, the Loop Current and its eddies can interact with the West Florida Continental Shelf in a way that causes upwelling strong enough to move nutrient-rich water from the deep ocean onto the shelf. This allows other algae to outgrow K. brevis.

In the middle of July a persistent upwelling began, but too late to suppress the bloom that was already moving toward the coast.

Tropical weather, which some believe can be a red tide killer, temporarily disrupted the upwelling circulation, when Tropical Storm Gordon brushed past south Florida in early September on its way to Louisiana and deposited some of the red tide in the Florida Panhandle.

After Gordon passed, red tide cells, pushed to the surface by the upwelling, were eventually carried to Florida's East Coast by the Gulf Stream. This led the to the simultaneous rare occurrence of red tide in three different regions of the state.

"This further demonstrates that the ocean circulation is the major determinant of Florida's Karenia brevis harmful algae blooms, dispelling the myth that land-based fertilizers are to blame," Weisberg said.

While pollutants can exacerbate an existing red tide, "they are not the root cause," Weisberg said.

USF's red tide prediction model has accurately predicted 20 of the last 25 red tide blooms, according to Weisberg, who plans to release a 2019 red tide forecast for the fall in mid-June.

Weisberg said timing and the proper design for his experiment led to its success.

"We understand the reason why the West Florida Continental Shelf is the originating point," he said. "It has to do with the width of the shelf. The middle of the shelf the water is depleted of nutrients. It's in that area red tide gets its foothold. It can dominate over other plant life. Once it gets its start killing fish, it starts living off fish."

Weisberg said red tide, which is ecological in nature, requires system science — a study of its nature, biology, and science itself, before attempts advancements can be made to mitigate the toxic algae.

"We need to be observing all the time so we fully understand the conditions for when there will be a red tide bloom or come up with mitigation strategies," Weisberg said. "We are just beginning to learn how red tide works. We haven't approached the problem in a scientifically sensitive enough way to make rapid progress."

Oct. 2017: A red tide manifests off Florida’s west coast. It persists through spring and summer of 2018 in the epicenter region.

Mid-July-Sept. 2018: Loop Current upwelling carries the new 2018 bloom inshore along the bottom, where it joins with the remaining 2017 bloom.

Aug. 24-Sept. 17, 2018: USF deploys underwater glider, which detects chlorophyll about 50 miles west of Tampa Bay.

September-November 2018: Tropical Storm Gordon carries the bloom to the Florida Panhandle; the Loop Current carries it to the east coast of Florida.

February 2019: The red tide ends.

Red tide time line