If you heard west coast Florida "red tides" come from Lake Okeechobee discharges, you aren't alone. You're wrong -- but not alone. It's time to flush that misinformation down the hopper.
University of South Florida researchers now confirm what they have long known but so few of us have accepted: Red tides aren't born in the big lake, they start in oligotrophic offshore waters and are pushed into coastal waters by winds and tides.
After years of study, researchers and colleagues at the university's College of Marine Science have identified reasons why some years are worse than others for the harmful alga bloom (HAB) Karenia brevis, called "red tide,” when it occurs off the west coast of Florida.
In a recently completed study comparing data collected on the 2012 red tide season, which was particularly robust compared to the quiet 2013 season, scientists found that the coastal ocean circulation on the West Florida Continental Shelf -- highly dependent on the Gulf of Mexico Loop Current -- was a determining factor in the greatly differing red tide occurrences.
Their paper describing this research was recently published in the journal Continental Shelf Research.
K. brevis creates a toxin that is threatening to organism health. In years of the worst outbreaks, red tide is responsible for millions of dollars in losses in the shellfish, finfish, recreation and tourism industries.
The university tells us, red tide toxins that end up in the food web can be transferred to other forms of life, from tiny zooplankton to birds, fish, aquatic mammals and humans. Toxins may also be inhaled, causing respiratory distress.
USF researchers confirm that red tide occurs naturally in the Gulf of Mexico -- certainly not close to shore and never in lake water.
Perhaps the first official to point this out loudly and publicly was Kevin Ruane, mayor of the City of Sanibel. Writing an op-ed for the Feb. 5, 2016 Fort Myers News-Press, Ruane said, "While it may be tempting to blame all adverse water conditions on Lake Okeechobee releases, it is not accurate to do so. Red tide blooms are initiated offshore in the Gulf of Mexico and are not the result of Lake releases."
In fact, Ruane went further. "Water that appears brown in color is also not necessarily the result of Lake Okeechobee releases," he said. "Despite the initiation of increased Lake Okeechobee regulatory releases, over the last four days approximately 70 percent of the current water flow is runoff from the Caloosahatchee watershed."
USF researchers say the trick is knowing when and where a red tide threat may emerge and how it may evolve along the coast. A number of predictive tools are in development to investigate this natural phenomenon, which has both biological and physical dimensions.
The study's lead-author Robert Weisberg, distinguished university professor of physical oceanography at USF, and his co-authors and colleagues have developed tools for observing, tracking and forecasting red tides using a combination of moored instrumentation, robotic gliders, satellite imagery and computer models.
While biology and chemistry control the growth of the blooms, they say, it is ocean circulation that unites the nutrients with sunlight to make photosynthesis happen. Ocean circulation also transports offshore blooms to the coast, Weisberg says. If the circulation conditions are not right, then a red tide will neither bloom, nor manifest along the coastline.
In this study, the simplifying factor was identified to be the Gulf of Mexico Loop Current. When the Loop Current interacts with the shelf slope near the Dry Tortugas, it can set the entire shelf in motion, bringing new nutrients onto the shelf from the deeper ocean. This sequence of events suppresses red tide by favoring other, faster growing phytoplankton (microscopic sea plants). By and large, the position of the Loop Current in spring and early summer provides a predictor of fall red tide conditions.
As documented by the Florida Fish and Wildlife Conservation Commission, the 2012 bloom resulted in the mortality of 293 endangered Florida manatees, the greatest number of red tide-related manatee deaths recorded from a single bloom.
If you want more information, check out the USF College of Marine Science Ocean Circulation Group website.
