The July 14 imagery from NOAA shows the cyanobacteria is still in about 80 percent of the lake, but the concentrations have lessened as indicated by the reduction in the red and orange areas. (Courtesy photo/NOAA)

The latest images from space show the blue-green algal bloom on Lake Okeechobee is decreasing.

Imagery from July 2 showed signs of cyanobacteria in 90 percent of Lake Okeechobee.

Since the satellite images are only available at set intervals, NOAA was unable to get usable photos July 6-13 due to cloud cover.

The July 14 image shows algae in less of the lake, and more importantly, that the concentrations have decreased in areas where there is cyanobacteria.

The NOAA imagery does not indicate what kind of cyanobacteria is present. There are thousands of types of cyanobacteria, although only about a dozen have been documented in Lake Okeechobee. Some — not all — of the types of cyanobacteria known to live in the lake can produce toxins under certain conditions. However, even cyanobacteria that can produce toxins does not always do so.

Dr. Edward Phlips, a professor with the University of Florida’s Department of Fisheries and Aquatic Sciences, explained the casual observer may not even notice the bloom.

“Massive blooms don’t always start at the surface,” he explained. “It can be in the water column.”

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.

“Just because you don’t see it, doesn’t mean there’s nothing going on,” he said.

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.

Oceanographer Michelle Tomlinson of the NOAA National Ocean Service explained they have been monitoring satellite photos of the lake since the start of June. They do similar studies of other lakes in the United States. “The algorithm we developed for the imagery is showing cyanobacteria blooms,” she explained. “So it is separating out the cyanobacteria from any other background algae in the lake. There may be some non-harmful phytoplankton mixed in there but what you are seeing is the concentration of the cyanobacteria.”

Oceanographer Rick Stumpf with the NOAA National Centers for Coastal Ocean Science explained: “The satellite is seeing the concentration up to 1-2 feet deep into the water. So even when the water will not show the scum everyone notices, the satellite can see how much bloom there is at that level.

He said the areas that show as “red” on the imagery are higher concentrations and those are the areas people are more likely to actually see scum on the water surface.

“Also, each satellite pixel covers the area of a stadium. In contrast, someone on the field can only make out maybe 30 yards across,” he added. Mr. Stumpf explained that NOAA also uses different wavelengths of light, including red and near-infrared, that the satellite detects that can’t be seen with the human eye.

Dee Ann Miller, of the Florida Department of Health, explained: “Some — not all — blue-green algae can produce toxins that can contribute to environmental problems and affect public health. Little is known about exactly what environmental conditions trigger toxin production. Over time, these toxins are diluted and eventually break down and disappear.

Persistent blooms are routinely monitored and retested. Because you cannot tell if algae is producing toxins by looking at it, the Florida Department of Environmental Protection (DEP) coordinates with the water management districts and the Florida Fish and Wildlife Conservation Commission to routinely sample observed and reported algal blooms and test for algal identification and toxin levels.”

So far, samples taken from the lake have shown no toxins or levels of less than 10 micrograms per liter. The World Health Organization considers levels below 10 micrograms per liter to be safe for recreational contact.

There have been no fish kills reported on Lake Okeechobee this summer.

Area anglers report the fishing has been very good this summer, especially early in the morning when the water is cooler.

According to Florida Sea Grant, it is impossible to tell the difference between a bloom on the surface and a bloom in the water column from satellite images alone.