Remote Sensing of Harmful Algal Blooms
By Natalie Von Tress, Environmental Policy Intern
Each week, SCCF provides information regarding red tide and blue-green algae based on satellite imagery maps published by NOAA’s National Center for Coastal Ocean Science. The processed satellite images NOAA provides are powerful tools for understanding when and where algae occur in the Florida Gulf Coast and Lake Okeechobee. The images are captured by the European Space Agency’s Sentinel-3 Ocean Land Color Imager (OLCI) sensors that detect different wavelengths of solar energy reflected from the earth’s surface. The wavelengths characterize different properties—displayed through colors on the visual spectrum—which are used to identify harmful algal blooms (HABs) that reflect specific parts of the spectrum. There are two Sentinel-3 satellites orbiting the earth, each with a revisit time of two to three days. The two satellites working together can capture almost-daily images of Southwest Florida. Images taken on multiple days can be pieced together to make composite images, reducing the effects of cloud coverage that can obscure the ability to detect HABs.
Chlorophyll-a is the most-common pigment in algae. It absorbs certain parts of the light spectrum to create energy via photosynthesis. The light that is reflected back is in the green part of the visual spectrum, which is detected by the satellites’ sensors. As such, the imagery refers to all algae living near the surface of the water. NOAA’s red tide monitoring map displays the chlorophyll-a fluorescence under a layer of field-verified samples of the red tide causing organism Karenia brevis collected along the coast, collected by many contributors. The chlorophyll-a fluorescence displayed by the colorful pixels is calculated using a method called “red band difference” which serves as an approximation for relative chlorophyll-a. The more orange or red the pixels, the more chlorophyll-a is present in the water. If medium and high K. brevis concentrations align with high chlorophyll-a fluorescence, it likely indicates a red tide bloom.
The NOAA satellite imagery data for Lake Okeechobee consists of two side-by-side images: the left showing a true color image of the lake and the right showing a processed image of the lake. The true color image shows what we would see if we looked down on the lake from the satellite’s perspective and how clouds may be obscuring the data. The image on the right shows the concentration of cyanobacteria cells (cells/mL) calculated from the data captured from the satellite and where blooms are forming. The color bar represents different concentrations of cyanobacteria. Black pixels show that there were no clouds in the way of the satellite, but no cyanobacteria were detected, and gray pixels show that data was not able to be collected due to cloud coverage or similar obstacles.
In addition to chlorophyll-a, cyanobacteria contain phycocyanin, a pigment-protein complex that fluoresces at a different wavelength than chlorophyll-a. Unlike the red tide images, which detect all algae, these images refer specifically to cyanobacteria. The cell concentrations in the images of Lake Okeechobee are calculated using a method called the Cyanobacteria Index (CIcyano), which can differentiate cyanobacteria from other algae by ensuring phycocyanin is also detected. By detecting the intensity and location of cyanobacteria, researchers can see where blooms are occurring with respect to where water is released to the Caloosahatchee, informing water management decisions to prevent nutrients and HABs being transported downstream.
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