LAKE OKEECHOBEE -- Florida Department of Environmental Protection (FDEP) tests on Lake Okeechobee over the past week in areas that a satellite image indicates concentrations of cyanobacteria found no toxins in most cases with one test showing extremely low levels of toxins.
A single test on a windblown blue-green algal bloom trapped in the Rim Canal side of the S351 structure at Belle Glade (in an area where the NOAA image does not indicate an algal bloom) did test positive for toxins.
The July 17 satellite image from the National Oceanographic and Atmospheric Administration indicated there was cyanobacteria in the water on the northwest side of the lake. The July 21 NOAA image shows cyanobacteria concentrations off the northwest and northeast shores. The NOAA images show concentrations in the water column. They do not show a surface bloom. On the imagery, areas in red or orange are most likely to have surface scum. There were no red or orange areas on the lake images for July 17 or July 21. The images show a reduction in the area containing cyanobacteria over the one week period between images.
For the past three weeks, researchers have been running an experiment at the Moore Haven lock, removing water from the lake and running it through a process to cause algae and cyanobacteria to float to the surface so it can be skimmed off. Throughout the test period, they also checked for toxins in the water. No toxins were detected.
On July 24, FDEP checked the areas on the lake where cyanobacteria concentrations were indicated on the NOAA map, and found no algae blooms. Even though they could not see any cyanobacteria, they took water samples for testing. Most of the water samples they took contained mixed algae with no dominant species. All but one sample had no toxins. The one sample that had detectable toxins had extremely low levels, lower than the standard the Environment Protection Agency sets for drinking water.
On July 24, FDEP sampled water just off the north shore of the lake. No algae was visible. Tests on the water sampled found a mixture of algae with no dominant species. Another sample taken about 10 miles south of the first sample showed mixed algae with no dominant species and no toxins. A sample just offshore of the Harney Pond Canal had mixed algae with no dominant species and no toxins. A sample taken about five miles southwest of that sample also found mixed algae and no toxins.
FDEP salso sampled water about 10 miles southeast of Lock 7. No algae was observed. A water sample found Microcystis in the water. Toxin level was extremely low — between the laboratory method detection limit and the laboratory practical quantitation limit at 0.46 micrograms per liters. (EPA considers levels under 1 microgram per liter safe for human consumption and levels under 8 micrograms per liter safe for recreational contact.)
FDEP also sampled an algae bloom within the S-351 water control structure on Lake Okeechobee near Belle Glade. This test showed the presence of Microcystis sp. Toxin level was 32 micrograms per liter.
While researchers have not determined all of the factors that cause blue-green algae to release toxins, past history has shown it is more likely to become toxic when the algal bloom is trapped in a contained area, such as a water control structure with no flow. Once algae has been blown against the back wall of a water control structure, it is trapped there until it dissipates or until there is water flow to move it.
Lake Okeechobee was at 11.54 feet above sea level on July 26, up slightly from July 19’s level of 11.47 feet. Average outflow for the past seven days at the Moore Haven lock was 72 cubic feet per second (cfs), This was due to flows of 200 cfs for 8 hours a day during the experiment period. Some flow through the Moore Haven Lock was needed for that experiment which ended this week. Average flow at the Franklin Lock was 1,739 cfs; this means about 95 percent of the flow to the Caloosahatchee estuaries was from local basin runoff.
There has been no flow from the Lake at Port Mayaca since March 30. Over the last seven days, the average flow at the St. Lucie Lock was 126 cfs, all from local basin runoff.
The NOAA imagery does not indicate what kind of cyanobacteria is present. There are hundreds of types of cyanobacteria, although only 28 have been documented in the Lake Okeechobee waterway. About 25 percent the types of cyanobacteria known to live in the Lake Okeechobee Waterway (including Lake Okeechobee, the Caloosahatchee River, the St. Lucie Canal and the St. Lucie river) can produce toxins under certain conditions. However, even cyanobacteria that can produce toxins does not always do so.
Cyanobacteria have gas vesicles which act as buoyancy control devices. The vesicles can be expanded and filled with gas, causing the cyanobacteria to float on the surface, or deflated, which causes the cyanobacteria to descend into the water column.
The NOAA image does not show what the human eye sees. It’s computer-generated imagery using data the satellite collects to locate concentrations of cyanobacteria in the water. Different colors on the imagery show areas of varying cyanobacteria concentration. The NOAA image does not show how many different types of cyanobacteria are present, or which types of cyanobacteria are present. It does not show whether or not toxins are present..
FDEP encourages the public to report algae blooms when you see them. You cannot tell if a bloom contains toxins without laboratory tests.
To report algae blooms on a lake or freshwater river, use the FDEP website floridadep.gov/AlgalBloom or call the agency at 1-855-305-3909.