OKEECHOBEE – Satellite images show cyanobacteria in about 30 percent of Lake Okeechobee.
Images acquired on Aug. 6 are partly obscure by clouds on the eastern side of the lake, according to Dr. Sachi Mishra of the National Centers for Coastal Ocean Science, with the National Oceanic and Atmospheric Administration.
“Blooms are clearly visible in the lake center. Wherever the satellite can see through the clear sky, we estimate the bloom to be covering 165 square miles, or about 30 percent of the lake surface area,” Dr. Mishra explained.
The imagery is not a photo of the lake as the human eye would see it. The satellite uses wavelengths of light the human eye cannot detect, and a computer uses the data collected to generate the imagery showing concentrations of cyanobacteria in the water column.
Since June, NOAA images have shown the cyanobacteria increasing to 90 percent of Lake Okeechobee, then dropping to 10 percent, then spreading again to 40 percent, before dropping back to 17 percent then expanding to about 30 percent.
Without more testing, scientists can’t be sure what is happening, according to Dr. Karl Havens of Florida Sea Grant. It is possible, Dr. Havens explained, that the original Microcystis aeruginosa bloom might have consumed all of the available nitrogen in the lake water, and then died off, illustrated by the decrease from 90 percent in the July 2 image to 10 percent in the July 22 image.
Another type of cyanobacteria — one that can “fix” nitrogen from the atmosphere, might be reproducing now, illustrated by the increase in the cyanobacteria in the images taken on July 25, July 26 and July 29.
While Microcystis cannot “fix” nitrogen from the atmosphere, other cyanobacteria known to be present in Lake Okeechobee, such as Anabaena, can use nitrogen from the air, and thus are not limited by the nitrogen available in the water.
The Aug. 2 image appeared to show that second bloom – whether it was a resurgence of the original bloom or a different one – was decreasing. However, the difference could have been due to problems with the satellite imaging system, which can be effected by winds and clouds.
Just because you don’t see an algal bloom on the surface does not mean it is not there, Dr. Havens noted.
Cyanobacteria can rise and fall in the water column, but otherwise it is moved by wind action and the flow of water.
According to NOAA, the satellite imagery cannot determine what kind of cyanobacteria is present. It does not show if toxins are present.
There are thousands of species of cyanobacteria and about a dozen have been documented in Lake Okeechobee. Some cyanobacteria can produce toxins, some do not.
Even cyanobacteria capable of producing toxins do not always do so.
Florida Department of Environmental Protection has sampled cyanobacteria in the lake this summer, responding to reports of blooms. The sampling process is sporadic, as bloom may move or disappear before the researchers can get there to sample it. The most recent samples were taken July 23.
So far this summer, Microcystis aeruginosa has been dominate in the blooms that had a dominant taxon. Some are listed as “mixed algae with no dominant taxon.” All of the tests on Lake Okeechobee this summer either had no toxins, or microcystin levels below 10 micrograms per liter, the level the World Health Organization deems safe for recreational contact.
Cyanobacteria, commonly called “blue green algae” is not technically algae. While both cyanobacteria and algae derive their energy through photosynthesis, cyanobacteria lack a nucleus.
About the images
The NOAA imagery uses data from Sentinel-3a, part of the European Union’s Copernicus program, built by the European Space Agency (ESA) and operated by EUMETSAT (EU Meteorological Satellite office). It was launched in 2016, data became available in 2017. A matching satellite Sentinel-3b was launched in April, and should start providing routine data in 2019.
The satellite looks at the amount of light in many wavelengths. To detect cyanobacteria they use 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. The differences between these bands detect pigments that are characteristic to cyanobacteria.
Red means high concentration, followed by orange, yellow, green, then blue. Black is “not detectable,” concentrations too low to see or be a risk.
The satellite can see details the human eye cannot see, so it can see high concentrations even when there is not a scum.
From a boat, a person can only see about 20 yards around, while the satellite pixel covers the area of a football stadium.
The cyanobacteria in Lake Okeechobee tends to float up in early day, and sink in late day. If winds are light or calm, it may form a scum. As wind picks up it prevents scum formation, and strong winds can mix the bloom down into the water, so it is less visible.