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The largest red tide bloom recorded in Florida waters since 2006 is lingering in the Gulf of Mexico and could move south over the next few days.

Red tide, caused here by Karenia brevis, can cause fish kills, make shellfish unsafe to eat and cause respiratory issues in marine mammals, sea turtles and humans. Thousands of dead fish were reported late last week about 40 miles off Hernando County, where the bloom stretches 80 miles north to south.

"The surface bloom is pretty stationary, but the deeper water portion of the bloom is projected to move south over the next three days," said Brandon Basino with the Florida Wildlife Research Institute. Species killed include snapper, grunts, various grouper, black sea bass, hogfish, triggerfish and crabs.

Upward of 236 manatees were killed in Lee County waters by a red tide outbreak in 2013 — causes of death for some animals was undetermined but thought to be related to the red tide bloom.

Predicting when and if the 4,000 square mile bloom will move into Southwest Florida waters is impossible, but water quality scientists say the region is ripe for an algae outbreak. Scientists with the Sanibel-Captiva Conservation Foundation tested the salinity, chlorophyll and turbidity of the water Tuesday at the Sanibel Causeway, looking for clues.

Algae feeds on nutrients such as nitrogen and phosphorus, and heavy loads of those nutrients were flowing through the Caloosahatchee River and coastal bays and beaches last summer as the region experienced record rainfall coupled with Lake Okeechobee releases.

"Right now nutrient levels are elevated and will probably support Karenia because of the lyngbya that's here," said water quality scientist Rick Bartleson, with the Sanibel-Captiva Conservation Foundation.

Lyngbya is a form of cyanobacteria found in the tropics and subtropics that's known to cling to certain sea grasses, forming a shiny clump of fiberlike hairs. Lyngbya can choke off oxygen from sea-grass roots, killing the grasses and lifting them from the bottom of bays and estuaries.

"It's almost as far as you can see in the shallow water along Sanibel," Bartleson said. "We're not seeing it offshore, so it's getting what it needs near shore."

Grass blades from various species were scattered along the causeway beach, along with the blob-like lyngbya.

Lyngbya is problematic in itself, but there's also a chance that the lyngbya that's here now will fuel a red tide in the near future as it releases nitrogen, a critical component for Karenia brevis — the organism that causes red tides in Southwest Florida.

"When it's out there and when it's that far away, I'm not sure that it would even get down here," Bartleson said of the current bloom. "We would probably have more patches that would show up if it comes our way. If it was closer to Sarasota, we're in line because we're sort of downstream from that area."

Red tide blooms in Southwest Florida typically start offshore of Sarasota before moving south to the Fort Myers area, which is where Lake Okeechobee releases stir with Gulf of Mexico waters. Lake Okeechobee and stormwater runoff across Lee County can feed red tide blooms, extending the duration and intensity of the outbreak by providing extra nutrients.

An algae bloom has also been reported in the Indian River Lagoon on the east coast, Basino said. The bloom in the Gulf of Mexico is about twice the size of Delaware.

"It's quite sizable," Basino said. "Most of the blooms form offshore in deeper water and are pushed by winds and currents inshore."

Bartleson said the bloom off Hernando County probably won't make it to Southwest Florida. Then again, there could be a bloom offshore now in the depths of the Gulf.

"We don't know when it's going to take off, it's difficult to tell," he said. "Sometimes it just pops up just west of us. In 2006, we didn't know anything until I saw red water at the beach. Nobody saw that coming. It can be in the lower portion of the water column where satellites can't see it."

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