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Ocean currents carried Red Tide close to shore, scientists say



During the 14 months that a massive Red Tide algae bloom was plaguing Florida’s beaches, scientists cited a number of factors for why it was so bad.

Dust blown over by the Sahara Desert played a role. So did pollution from the Mississippi River. Climate change was involved. So was nitrogen-rich runoff from leaky septic tanks, overloaded sewer plants and fertilizer washing off of yards and farms.

Now a new study led by a University of South Florida scientist has announced that another, subtler force was at work, too: deep sea ocean currents.

By deploying an ocean-going robot called a “glider” to collect data for a month during the algae bloom, the scientists confirmed a long-held hypothesis that while blooms originate 10 to 40 miles offshore, they are transported to the Florida coast by subsurface currents.

“The physics control the biology,” said Bob Weisberg, a physical oceanographer with the university’s College of Marine Sciences. He was lead author on the study, which was just published in the Journal of Geophysical Research: Oceans Red Tide algae live in the Gulf of Mexico yearround. No one knows what sets off an explosion of their population, causing what’s known as a bloom. Blooms usually begin in the early spring or summer 10 to 40 miles offshore.

In late August, the scientists dispatched the torpedo-like glider into the Gulf of Mexico to a spot where they suspected the Red Tide algae bloom began: near the bottom about 50 miles off the coast between Clearwater Beach and Sarasota Bay. The glider picked up a high concentration of chlorophyll, a signature of the Red Tide algae cells, near the bottom..

Weisberg’s team, which included scientists from the university and the Florida Fish and Wildlife Research Institute, used the data collected by the glider and ran it through a series of computer simulations. That’s how they determined that a type of current called an “upwelling” occurred at just the right time to sweep the harmful algae from the bottom up into the shallower water near shore.

There the poisons in the toxic algae killed fish, manatees, dolphins, sea turtles and sea birds. It also irritated the eyes and throats of anyone who tried to visit the beach. The bloom, which was first detected in July 2017, lasted until Thanksgiving 2018.

Weisberg said he hopes to use what they have learned to develop a prediction about the possible severity of any 2019 Red Tide blooms by mid-June.

A slurry of dead fish, the result of Red Tide, was visible in Clearwater Harbor on the north side of Sand Key Park as Red Tide lingered along the coast in September.


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