Anglers who frequently fish Lake Okeechobee are often puzzled by media allegations the lake is “covered” in algae, or that algal blooms cover hundreds of square miles of the lake.
The confusion often comes from a misunderstanding of the National Oceanic and Atmospheric Administration (NOAA) color-coded satellite imagery. The imagery shows areas in black as no detectable algae potential, in blue as low potential, in green as moderate potential and in red or orange as high potential.
The key word here: POTENTIAL. Anglers visit an area shown in red as “high potential” often don’t see any visible algae.
The June 21 NOAA satellite imagery of Lake O showed some areas in blue and green, covering an estimated 160 square miles of the lake’s 730-square mile area. Yet visitors to the lake that same day in the area with the highest probability for algae didn’t see any visible algae.
NOAA satellite images of Lake Okeechobee only tell part of the story.
Algae and cyanobacteria (commonly called “blue green algae” but not true algae) are part of the lake’s natural ecosystem, just as they are part of every freshwater and marine ecosystem. They are the base of the food chain.
They are always present in the water, but most of the time they are not visible to the human eye. Under certain circumstances – hot weather, little water movement and plentiful supply of phosphorus and nitrogen for fuel – these microscopic organisms can reproduce rapidly into a visible “bloom.”
Not all algae produces toxins. Also, even algae capable of producing toxins may not do so.
The dominant species of algae in Lake O in recent years is Microcystis aeruginosa, which can produce the toxin, microcystin. Can – but doesn’t always do so.
Microcystin production requires a lot of nitrogen, according to South Florida Water Management District scientists.
Because Microcystis cannot "fix" nitrogen from the air as some other algae species can, they depend on dissolved inorganic nitrogen (DIN) in the water.
While Lake Okeechobee's water is high in phosphorus, which also feeds algae, the nitrogen levels in the water drop in the summer as the nitrogen is used by algal blooms.
A Florida Atlantic University study found the toxic algae bloom in the St. Lucie canal in 2017 was fed by nitrogen from septic tanks along the waterway.
A satellite image is a snapshot of one moment in time when the satellite is over the lake area. An algal bloom visible at that time may disappear shortly afterwards. Cyanobacteria are pushed around by wind and wave action. They can also move up and down the water column by inflating and deflating gas vesicles.
The NOAA images are often misunderstood by the public.
According to a satellite oceanographer with the National Centers for Coastal Ocean Science, NOAA National Ocean Service, the satellite detects spectra of light the human eye cannot see. The computer program creates a color-coded image with red in the areas most likely to have surface scum, with decreasing concentrations of algae indicated by areas in orange, yellow, green and blue. Black indicates areas where no cyanobacteria potential is detected.
The satellite looks at the amount of light different wavelengths. To detect cyanobacteria, NOAA uses several wavelengths of red and near-infrared light (this infrared detects brightness, not temperature). For water, only the light from the upper 1-3 feet is measured. Differences between these bands detect pigments characteristic to cyanobacteria.
Laboratory testing is required to determine what species of algae and/or cyanobacteria may be dominant in that area and if any toxins are present.
According to information shared at the Florida Blue Green Algae Task Force meetings, other biological matter in the lake can also impact the satellite imagery. The imagery gives scientists and water managers an idea where there MIGHT be an algal bloom. It’s a useful clue, but not proof of a bloom.
Florida Department of Environmental Protection (FDEP) responds to reports of algal blooms year-round. They take and test samples of any visible blooms they find. SFWMD also regularly tests water samples from multiple sites in Lake Okeechobee. During the wet season, they take samples from 29 routine monitoring sites every two weeks.
So far this year, only one water sample taken from Lake O has contained toxins – and those were barely detectable. In samples taken by SFWMD June 16-19, trace level of 0.10 parts per billion microcystin were found in an algal bloom at the Tory Island Marina Main Boat Ramp. NOTE: The Environmental Protection Agency considers toxin levels of less than 1.0 ppb to be safe for drinking water.
EPA considers microcystin toxin levels below 8.0 ppb to be safe for human recreational contact. And, according to explanations given at the Blue Green Algae Task Force meetings, “recreational contact” means children swimming in the water. EPA erred on the side of caution in setting that standard, assuming a small child might swallow some water while swimming and a child’s low body weight would make that individual more vulnerable to toxins.
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