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Black Water Turns the Tide on Florida Coral

A patch of “black water”spanning over 100 kilometers [60 miles] in diameter formed off southwestern Florida in early 2002 and contributed to severe coral reef stress and death in the Florida Keys. The “black water” contained a high abundance of toxic and non-toxic microscopic plants. When scientists examined the data collected by divers from the dark water area in the Florida Keys, they discovered a 70 percent decrease in stony coral cover, a 40 percent reduction of coral species, and a near-elimination of sponge colonies at two reef sites after the dark water passed. By examining satellite images and field survey data, they concluded that the coral reef ecosystem was stressed by microscopic organisms and toxins contained in the dark water. From the American Geophysical Union:Black Water Turns the Tide on Florida Coral

WASHINGTON – A patch of “black water”spanning over 100 kilometers [60 miles] in diameter formed off southwestern Florida in early 2002 and contributed to severe coral reef stress and death in the Florida Keys. The “black water” contained a high abundance of toxic and non-toxic microscopic plants. When scientists examined the data collected by divers from the dark water area in the Florida Keys, they discovered a 70 percent decrease in stony coral cover, a 40 percent reduction of coral species, and a near-elimination of sponge colonies at two reef sites after the dark water passed. By examining satellite images and field survey data, they concluded that the coral reef ecosystem was stressed by microscopic organisms and toxins contained in the dark water.

Researcher Chuanmin Hu and colleagues at the Institute for Marine Remote Sensing of the University of South Florida, the Florida Fish and Wildlife Conservation Commission, and the University of Georgia, present their findings in the journal, Geophysical Research Letters, published by the American Geophysical Union.

The water appeared black in satellite imagery because the concentration of the microscopic plants and other dissolved matters in the red tide were high,” Hu said. Because plants and dissolved matter absorb sunlight, they reduce the amount of light normally reflected from the ocean. When a red-tide bloom occurs the water takes on various hues of red or brown. While not all microscopic plants contribute to red tides, the darker hue created by both the plankton and the harmful algal blooms made the water appear black when seen from the satellite.

The “black water” event had caused alarm among local fishermen, divers, and the public, as the color of the water was unusual and fish seemed to avoid this large area. Satellite-based measurements of the dark water led to a number of investigations to help clarify the issues and provide answers to the public’s concerns. Instruments such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) aboard Orbimage’s SeaStar satellite and the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra and Aqua satellites provide information on ocean color that allows scientists to monitor the health of the water and the shallow ocean bottom environment.

During January 2002, SeaWiFS detected the dark-colored water in the Florida Bight, just southwest of the Everglades. In fall 2001, SeaWiFS images showed an extensive red tide off Florida’s central west coast, near Charlotte Harbor.

Red tides occur every year off Florida and are known to cause fish kills, coral stress and mortality, and skin and respiratory problems in humans. They are caused by high concentration of microscopic plants called dinoflagellates. Other microorganisms called cyanobacteria can also cause harmful algal blooms. The waters containing this red tide migrated to the south along the coast.

Winter storms caused large amounts of fresh water to drain from the Everglades into Florida Bight (the curve in the shoreline from the Keys north to Everglades National Park on the mainland), carrying high levels of nutrients such as silicate, phosphorus, and nitrogen to the sea. These caused a bloom of the microscopic marine plants known as diatoms in the same patch. The bloom turned the water dark and the “black water” patch re-circulated for several months in a slow clockwise motion off southwest Florida in the Florida Bight. Slowly, the dark water drifted farther south and toward the Florida Keys. By May 2002, the “black water” had moved through passages in the Florida Keys, dispersing into the Atlantic in the Gulf Stream.

Co-authors of the Geophysical Research Letters article included Serge Andrefouet and Frank E. Muller-Karger of the University of South Florida; Keith E. Hackett, Michael K. Callahan, and Jennifer L. Wheaton of the Florida Fish and Wildlife Conservation Commission, St. Petersburg, Florida.; and James W. Porter of the University of Georgia, Athens, Georgia.

The research was funded by NASA’s Earth Science Enterprise mission, the U.S. Environmental Protection Agency and the National Oceanic and Atmospheric Administration.

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Notes for journalists:

Reporters may obtain a pdf copy of this article, upon request to Harvey Leifert: [email protected]. Please provide the citation below, your name, title, name of publication, and email address. Neither the article nor this press release is embargoed.

Title of article: “The 2002 ocean color anomaly in the Florida Bight: A cause of local coral reef decline?”

Citation: Hu, C., K. E. Hackett, M. K. Callahan, S. Andrefouet, J. L. Wheaton, J. W. Porter, and F. E. Muller-Karger, The 2002 ocean color anomaly in the Florida Bight: A cause of local coral reef decline?, Geophys. Res. Lett., 30(3), 1151, doi:10.1029/2002GL016479, 2003.

Contact information for authors:
Chuanmin Hu: [email protected]
Frank Muller-Karger: [email protected]
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