NASA’s MODIS instrument captured this image of the southern tip of Florida curving into a teal ocean surrounded by white clouds. MODIS and a new generation of satellite-based sensors could help environmental managers track harmful algae in the Gulf Coast and beyond. Credit: NASA

Harmful algae can put public health, ecosystems, and coastal economies at risk. Advances in satellite imaging provide new ways to look at our living surroundings.

By the time they’re done, a series of massive algae blooms along Florida’s west coast in 2020 will be linked to about 2,000 tons of dead marine life around Tampa Bay. The human costs were stark as well, including a two-fold increase in asthma cases in Sarasota and Pinellas counties, and losses estimated at $1 billion across economic sectors from tourism to fisheries.

Using satellites to detect algae blooms

Earth-orbiting satellites have been used for decades to detect algal blooms from space, allowing repeatable observations over wider areas than is possible by directly sampling water. The most common monitoring technique relies on the visible spectrum to measure ocean color. However, this approach has been mostly limited to clear-sky conditions.

A recent study published in Geophysical Research Lettersled by scientists at NASAjet propulsion laboratory (Jet Propulsion Laboratory) in Southern California, showed how a space instrument called TROPOMI, or TROPOspheric Observing Instrument, was able to peer through cirrus clouds to reveal powerful evidence about… Karenia Brevis (or K.Previs), the microscopic algae responsible for the 2020 bloom.

TROPOMI’s enhanced ability to “see” and measure precise wavelengths of light could help federal agencies and local communities better predict and manage outbreaks of harmful diseases. (Tropomi flies aboard the European Sentinel 5P spacecraft, launched in 2017.)

K. Previs and West Florida

Scientists examined the West Florida shelf, a stretch of continental crust that extends from the Panhandle to the Keys. From its origins in other parts of the Gulf of Mexico, K. brevis is transported toward the coast by strong winds and ocean currents. Recent research has shown that West Florida, like many coastal communities, may be increasingly vulnerable to disease outbreaks because these algae thrive in warm, nutrient-rich conditions fueled by runoff, fertilizers and climate change.

Monitoring systems and risks of K. brevis

Systems for monitoring and forecasting harmful algal blooms are available in many states and coastal regions, including the Gulf of Mexico and the Great Lakes. National Oceanic and Atmospheric Administration (NOAA) works with academic, government, tribal, and local partners to develop and issue forecasts of harmful algal blooms — similar to weather forecasts — during seasons when algal blooms occur.

K. brevis events are particularly concerning because the algae produces a powerful neurotoxin that in high concentrations can kill large numbers of fish and poison marine life such as turtles, manatees and birds. The toxin can also be inhaled, causing respiratory illness in humans, and can accumulate in shellfish, leading to gastrointestinal illness in people who consume it.

Satellite detection mechanism

Algae also feed and grow on… PhotosynthesisK. brevis emits a faint red glow called solar-induced fluorescence (red SIF) that can be detected by some satellite instruments including TROPOMI, which is actually designed to measure air pollution.

By sifting through TROPOMI data acquired along the West Florida coast between 2018 and 2020, the team found that the instrument collected about twice the fluorescent information emitted by the algae as observed with previous methods based on ocean color. TROPOMI’s advanced imaging capabilities — including four spectrographs that measure light ranging from ultraviolet to shortwave infrared — enable it to penetrate thin cloud cover and obtain repeatable measurements of the ocean surface.

Space based feature

The scientists said their findings demonstrate the value of using TROPOMI alone or in coordination with other Earth observation instruments such as NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer) instrument, aboard the Aqua and Terra satellites. MODIS measures ocean color and is currently used by environmental managers in Florida and elsewhere to detect algae on clear days.

Another ocean coloring tool – scheduled to launch in early 2024 – will look at the world’s marine ecosystems like never before. NASA’s PACE mission, or Plankton, Aerosol, Cloud, and Ocean Ecosystem, will study phytoplankton and other ocean biology, aerosols, and clouds at much longer wavelengths than previous sensors. These observations will help predict harmful algal blooms, as well as the boom-bust cycle of fisheries and other factors that impact commercial and recreational industries.

“For nearly 20 years, ocean color sensors have been key to monitoring harmful algal blooms via satellite,” said Kelly Lewis, lead author and a NASA postdoctoral fellow at JPL. “This application of TROPOMI red SIF demonstrates how a combination of satellite technologies can enhance early warning systems beyond clear-sky conditions.”

Satellite early warning has been shown to have real-world benefits. A recent NASA-funded case study found that early detection of cyanobacteria in Utah Lake resulted in significant health care savings, lost work hours, and other economic losses. A few days of advance warning can translate into quicker action on the ground, including public alerts and even beach closures.

Reference: “First-light demonstration of red solar-induced fluorescence for monitoring harmful algal blooms” by Kelly Lewis, Philipp Kohler, Christian Frankenberg, and Michel Gerach, 10 July 2023, Geophysical Research Letters.
doi: 10.1029/2022GL101715

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