Satellite images show algal bloom changing; researchers still in ‘wait and watch’ mode

OKEECHOBEE — Satellite imagery continues to show changes in the cyanobacteria in Lake Okeechobee. Due to the lack of a broadbased regular sampling program, researchers can only speculate about what the images mean based on their knowledge of the lake and experience with algal blooms in similar bodies of water.

The imagery from July 2 showed cyanobacteria in 90 percent of the lake. According to the National Oceanic and Atmospheric Administration (NOAA), the cyanobacteria was in the water column, so it was not always visible on the surface. According to satellite imagery from July 6 through July 18, the area and concentration of the “bloom” decreased to about 30 percent of the lake. More recent images show cyanobacteria dropping to 10 percent and then growing again to 30 percent of the big lake. The most recent imagery — based on satellite images taken July 29 — shows cyanobacteria in 40 percent of the lake.

Varying colors in the NOAA image show concentrations of cyanobacteria. Areas in red are the highest concentrations and the areas most likely to have an algal bloom visible on the water surface.

Images taken by satellite since July 18 are open to interpretation.

The cyanobacteria might have moved since July 18 due to wind action. Or the original cyanobacteria, which according to the July 2 imagery was in 90 percent of the lake, might have died off and been replaced by a new “bloom” of a different species.

Winds and cloud cover can also affect the imagery.

Sachi Mishra, Ph.D. is a satellite oceanographer, under contract to the National Centers for Coastal Ocean Science, a division of the National Oceanic and Atmospheric Administration (NOAA).

On Friday, Dr. Mishra shared three new images of Lake Okeechobee, showing 10 to 30 percent of the lake containing measurable concentrations of cyanobacteria in varying levels.

“I would like to update you that we have three images (07/22, 07/25, and 07/26) showing bloom condition in Lake Okeechobee,” wrote Dr. Mishra. “The first two images were somewhat cloudy and therefore we waited for a clear image, to confirm the bloom condition we have been seeing recently, before releasing the images.

“Out of the three most recent images, the one acquired on July 26 shows the most accurate bloom scenario due to clear sky condition and low wind speed. We estimate the bloom area coverage to be about 30 percent,” Dr. Mishra stated.

Dr. Mishra explained that cloud coverage and wind might have interfered with images from July 25, which showed cyanobacteria in 20 percent of the lake, and from July 22, which showed cyanobacteria in 10 percent of the lake.

“Overall, images since last week look promising. Bloom areal extent has not changed since July 18, and confirms that areal extent number from that date are accurate. However since our observation on July 17, the bloom appears to be senescing (deteriorating),” Dr. Mishra stated.

On Monday, Dr. Mishra shared an image that shows cyanobacteria in about 40 percent of the lake.

“There is a 10 percent increase in bloom area as compared to July 26, which could be due to calmer winds helping the algae accumulate more near the surface and making them detectable by the satellite,” stated Dr. Mishra.

The July 29 NOAA imagery shows cyanobacteria in about 40 percent of Lake Okeechobee.

Dr. Karl Havens, director of Florida Sea Grant, has a different idea on what is happening in the lake. He noted the images show the bloom decreasing to just 10 percent of the lake on July 22 and then increasing again.

“NOAA satellite images from July 22, 25 and 26 indicate a progressive expansion of the bloom on Lake Okeechobee from 10 to 20 to 30 percent coverage of the lake. We do not know whether or not this indicates a recovery of the bloom, a second phase where another species of cyanobacteria is forming a new bloom, or just natural variability over a few days,” he wrote in an update to “Watching and Waiting: Uncertainty About When Algae Blooms Will End,” on the Florida Sea Grant website.

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.

Just because you don’t see an algal bloom on the surface does not mean it is not there, Dr. Havens added.

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 can not 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 cyanobaceteria 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 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.

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