The National Oceanic and Atmospheric Administration reported Friday that the algal bloom on Lake Okeechobee has decreased to about 30 percent of the lake. In addition, about half of the bloom has decreased significantly in concentration. At its peak on July 2, the bloom had been in about 90 percent of the lake, according to NOAA data.
The image acquired by NOAA on July 18 shows that the bloom area has decreased to about 30 percent (184 square miles) of lake area, down from about 60 percent on July 17. The concentration in the bloom has also decreased. About half (54 percent) of the bloom area on the July 18 image has cyanobacteria concentration of less than 100,000 cells per milliliter (cells/ml). Concentrations in the “red” areas are more than 1 million cells per milliliter.
“The massive algae bloom that covered nearly all of Lake Okeechobee a week ago appears to be undergoing a change based on satellite images from July 17 and 18. Those successive images suggest a progressive decline in the spatial extent of the bloom. At this time it is unclear what is happening due to a lack of data,” stated Karl Havens, Florida Sea Grant director.
Based on research conducted in other lakes, there are a couple of reasons why the bloom may be shrinking. First, the Microcystis bloom in Lake Okeechobee might have used up all of the dissolved inorganic nitrogen in the water and now it is figuratively starving. Second, and somewhat related, it might be that the bloom is going through a replacement cycle, where Microcystis will be replaced by a different kind of algae that can get its nitrogen from the atmosphere, such as Anabaena. That species of blue-green algae has formed large blooms in Okeechobee in past years.
“Because the state of Florida does not have a systematic and comprehensive algae bloom monitoring program in the lake, with sampling frequent enough to determine what is happening, we may never know the cause of the observed changes.”
The satellite images are only available at set intervals. Due to cloud cover, no usable images were available July 6-13. The July 14 image showed algae in about 80 percent of the lake, and also that the concentration was decreasing. July 17 imagery showed the bloom continue to decrease, as did the July 18 imagery.
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.
The NOAA computer imagery uses scans in a spectrum of light the human eye cannot detect. Cyanobacteria also 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. According to oceanographer Michelle Tomlinson of the NOAA National Ocean Service, NOAA does similar studies of other lakes in the United States. “The algorithm we developed for the imagery separates out the cyanobacteria from any other background algae in the lake,” she explained. There may be some non-harmful phytoplankton mixed in there, she added.
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.
According to Dr, Havens, algae blooms could be controlled by cleaning the water before it enters the lake and other waterways.
“The solution to the algal bloom problem is to clean up the nutrient sources north of Lake Okeechobee and in the land around the two estuaries,” he explained in a July 20 commentary on the Florida Sea Grant website. “Control of dispersed sources of nutrients in those watersheds will be a huge challenge and while projects are underway by the state to accomplish them, it could take decades before substantive results are seen.