LAKE OKEECHOBEE – Algal blooms on Lake Okeechobee were in the news in April, causing some to worry 2021 would be a bad year for algal blooms. But the early blooms may instead be a positive sign.
At the May 13 meeting of the South Florida Water Management Governing Board, Division Director Water Resources Lawrence Glenn explained an early bloom can consume most of the available dissolved nitrogen, leaving less fuel for algal blooms later in the summer.
Glenn said expanded monitoring on the lake has begun with 32 stations sampled every two weeks May through October.
He said the dissolved inorganic nitrogen (DIN) in the lake are fairly high right now. That DIN is available to fuel cyanobacteria (sometimes called blue-green algae). While some cyanobacteria can “fix” nitrogen from the air, Microcysits – the species dominant in about a third of the lake – requires DIN in the water to thrive.
He said as the Microcystis reproduces, it will consume the DIN.
“We are fairly high and we expect to see a drop again,” said Glenn. “Maybe having his occur a month earlier in the season, it will consume that nitrogen source so that the bloom won’t last as long in the summer when the rains come and the lake starts to fill back up.”
Another factor that might help the lake this year is the increase in submerged aquatic vegetation (SAV) which competes with the cyanobacteria for nutrients/
In 2018 there were only 5,000 acres of SAV on the lake, Glenn explained. This year the lake has about 16,000 acres of SAV.
Cyanobacteria are microscopic organisms. Under the right conditions, they reproduce rapidly into a visible “bloom.”
“Blue-green algae look for increased daylight,” said Glenn. They want higher temperatures and they look for food source.
“As we move into summer, that is the time blue-green algae have the ability to bloom.”
A mix of Phytoplankton naturally occurring in Lake Okeechobee include diatoms, green algae, cryptophytes and cyanobacteria. The proportions of each type of Phytoplankton in various areas of the lake change with the seasons.
“Typically as the summer moves on, you have increase in cyanobacteria,” said Glenn. In October, temperatures have declined, the nutrient sources preferred by cyanobacteria have been consumed and diatoms dominate the Phytoplankton mix.
Water samples from all 32 sampling sites in the lake show about a third of the areas do not have a dominant taxa, and about 33% are dominant for Microcystis. Microcystis is most likely to be dominate in the pelagic zone -- the deep water in the center of Lake Okeechobee. Glenn said the organic build up in the center of the lake can be stirred up by wind, providing fuel for the cyanobacteria there.
The water tests show on the south end of the lake, the diatoms are dominant
Green algae is out there and it doesn’t produce a toxin, he said. Some of the different strains of cyanobacteria don’t produce toxins.
Cyanobacteria have gas vesicles by which they can regulate their buoyancy. “As they are going through photosynthesis, they are creating oxygen. That oxygen goes into the colony and increases their buoyancy, causing the “bloom” to rise up to the surface.
Glenn said of the species of cyanobacteria that are capable of producing toxins, some may not produce toxins and some individual cells may be producing toxins and not releasing it.
“That’s why we take a measure of what is the dominant type of algae, and also measure what is the toxicity,” he said.
When freshwater is released to the coastal estuaries, “we look at what is the salt tolerance of the species,” of cyanobacteria, he continued.
“It will hit a salinity level at which the cells die ... and that’s a good thing,” Glenn said. At a salinity level of about 10 Practical Salinity Units (PSU) algae stops reproducing. They usually start to die around 12 PSU. Microcysits dies at about 18 PSU, he said.
If they are producing toxins, when cyanobacteria die, they are going to release those toxins, he explained. Once released, toxins may be viable for about two weeks.