NOAA’s ‘Eyes in the Sky’ Monitor Gulf Coast Harmful Algal Blooms
NOAA is using sophisticated satellite imagery as its “eyes in the sky” to advise Gulf of Mexico officials of the approach of harmful “red tide” blooms. With advance notice, officials are better able to take the necessary steps to protect human health, and mobilize for the coming cleanup effort.
At the root of the Gulf Coast “red tides” issue is a species of algae, the toxic dinoflagellate Karenia brevis, often referred to simply as K. brevis. In the Gulf of Mexico, particularly along Florida’s southwest coast, red tide blooms have become a yearly occurrence. When they happen, large expanses of ocean surface become discolored, in shades that actually vary from mahogany to yellow. Powerful toxins from the algae combine with reduced water quality to kill massive numbers of fish. Marine animals, including reptiles, mammals and birds, are also affected. In 2003 K. brevis was implicated in the unusually large number of sea turtle and manatee deaths reported in Florida.
When released into the air by wave action, K. brevis toxins can cause asthma-like symptoms in humans. The presence of dead and decaying fish in the waters and on the beach poses an additional health risk. Eating shellfish from contaminated areas can cause a condition in humans called neurotoxic shellfish poisoning (NSP). This can result in debilitating gastrointestinal and neurological symptoms, although NSP is rarely fatal to humans. To protect human health, states monitor areas known to have been contaminated in the past. When K. brevis cell concentrations exceed 5,000 per liter, appropriate state agencies are to close these areas to shellfish harvesting.
Harmful Algal Bloom Bulletins
Since 1999, Harmful Algal Bloom (HAB) Bulletins issued by NOAA’s National Centers for Coastal Ocean Science (NCCOS) have helped Gulf Coast state and local managers proactively confront HAB events. The HAB bulletins have helped local agencies anticipate those harvesting areas most likely to be affected by developing HABs. The NCCOS bulletins also alert officials to blooms in areas not previously sampled.
When state and local agencies receive advance warning of impending shore landings of the HABs, they can mobilize for the cleanup effort and alert the public of the potential health risk. These proactive steps also help protect marine mammals and help reduce adverse economic impacts to fisheries and tourism.
The HAB Bulletins are issued frequently (once or twice a week) with the onset of a bloom. Updates report on significant movements or other changes, helping local, state, and federal resource managers get a headstart in their efforts.
Knowing the potential extent and likely trajectory of the blooms helps managers best deploy employees and volunteers during an actual HAB event. In Florida, for instance, where hundreds of miles of shellfish beds are potentially at risk, monitoring the coastline can be daunting. The bulletins help state and local agencies direct crews to collect samples in areas most likely to be affected.
Florida and Alabama routinely monitor for the presence of K. brevis. Other Gulf Coast states take samples in response to reports of fish kills, discolored water, and respiratory distress in humans. In Texas, remote sensing and data analysis is the only practical way to monitor the state’s extensive coastline.
What Do the Bulletins Provide?
Each NOAA HAB Bulletin includes maps of the region showing surface water chlorophyll concentrations obtained from satellite sensors, along with field data on bloom location and intensity obtained from state agencies. The bulletins chart wind speed and direction, drawing attention to the presence of easterly and northeasterly winds. Information for the HAB Bulletins on wind speed and direction is derived from the NOAA Coastal Meteorological Automated Network, which has stations located approximately 200 km apart throughout the Gulf of Mexico.
Wind speed and direction are important because they affect the movement of HABs. Winds blowing surface waters away from the coast, explains Oceanographer Richard P. Stumpf, Ph.D., lead to an “upwelling” of bottom waters, which then replace the surface water. K. brevis blooms start near the bottom offshore, so the upwelling brings them shoreward, says NCCOS’s Stumpf.
Each HAB bulletin also includes expert NOAA analyses of what satellite data, field observations, and weather observations and forecasts say about the last known position of the red tide bloom and the path it is likely to follow. NOAA scientists who have expert knowledge of algae species ecology, satellite imagery for ocean color, and oceanographic processes and ecology in the Gulf Coast area conduct these analyses.
Red tide blooms show up in satellite imagery as areas of high chlorophyll, discolored water. Chlorophyll imagery alone, however, is not sufficient to distinguish between harmful and non-harmful algae. The presence of red tide algae ultimately must be confirmed by microscopic examinations of water samples from suspect areas.
“The NCCOS bulletins give us a reliable guide as to places to do sampling,” says Earnest Truby, Ph.D., with the Florida Fish and Wildlife Commission’s Florida Marine Research Institute. “Without the interpretive analysis NCCOS provides, the chlorophyll images just aren’t that helpful at all. By comparing current levels to previous months’ averages, the bulletins can pretty much tell you what is going to be a red tide and what isn’t.”
An important asset for NOAA’s HABs-abatement efforts is the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), a joint effort by Orbital Imaging Corporation (OrbImage) and the National Aeronautics and Space Administration (NASA). Since 1999, NOAA’s CoastWatch program has been acquiring near real-time SeaWiFS imagery data, with HAB monitoring as a primary application. NOAA integrates this data with other datasets to support states’ monitoring for HABs.
SeaWiFs monitors the color of the world’s oceans, collecting light intensities that are sensitive to subtle variations in color. Scientists have fine-tuned the algorithms used to distinguish between chlorophyll, phytoplankton, and other substances in the water. With a spatial resolution of one kilometer, this data is most reliable for offshore coastal areas and must be used with caution in confined areas like bays and estuaries.
Since NOAA began the satellite monitoring for HABs in 1999, its ability to correctly identify the scale of the blooms and species involved has steadily improved, Stumpf says.
“At one time, there was no other way to detect red tide other than when people were coughing at the beach or were seeing dead fish come ashore,” Stumpf recalls. “Now we have an eye in the sky, and we try to provide estimates of when and where a bloom will move. By not having to sample broadly and virtually at random, agencies save important time and money.”
“We started off looking for any kind of algae, with only an image of total chlorophyll to go by. Now, we have a whole number of other tools – information from those images, simple modeling of transport, and satellite and other optical characteristics of the water. These are things we couldn’t have even tried to do just a few years ago.”
The initial HAB Bulletins basically monitored previously detected blooms, and had limited capability to detect new blooms and predict bloom movement. In 2000, NOAA improved transport forecasts and detection capabilities, and began predicting conditions conducive to bloom development. With SeaWiFS data available on a daily basis, estimates of Gulf of Mexico chlorophyll concentrations now are better able to track bloom movements as they occur.
Predicting transport is important, not just to forecast where a bloom is likely to move, but also to show where it is on a given day. Even with daily imaging, cloud cover on some days obscures portions of the coast. In addition, it can take up to a week for field samples to be analyzed in the lab. NOAA models aim to overcome those obstacles by helping to successfully predict HAB transport from data on wind speed and direction.
Since 2000, NOAA scientists have also used chlorophyll anomalies detected by satellite to help direct local agencies to appropriate areas to sample or monitor. Stumpf points to habitat requirements particular to K. brevis that favor this approach. Generally, K. brevis blooms in late summer and fall, when there are few blooms of other phytoplankton species. In addition, K. brevis prefers water with higher salinity. So they begin offshore where they are more visible to satellite, rather than in the low-salinity estuaries where they are harder to see. Thus, the agencies can be more selective in sampling because they have the additional spatial information from satellite.
Stumpf cautions, however, that in some areas and at certain times of the year, using that the anomaly approach is inappropriate and ineffective in detecting K. brevis. In coastal regions that receive tannin-rich water from coastal swamps, for instance, highly discolored water in river plumes may falsely indicate a HAB. Other areas on the Alabama and Louisiana coasts with high river inflow also can result in false positives. The challenge, says Stumpf, lies in correctly identifying HABs on the edges of these areas.
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