SeaWiFS Image of black water in true color from Jan. 9, 2002. Credit: SeaWiFs Project and ORBIMAGE, Scientific Visualization Studio

In early 2002, a patch of "black water" spanning over 60 miles in diameter formed off southwestern Florida and contributed to severe coral reef stress and death in the Florida Keys, according to results published from research funded by NASA, the U.S. Environmental Protection Agency and the National Oceanic and Atmospheric Administration (NOAA). The "black water" contained a high abundance of toxic and non-toxic microscopic plants.

Chuanmin Hu and other colleagues at the Institute for Marine Remote Sensing of the University of South Florida (USF), St. Petersburg, Fla., and colleagues from the Florida Fish and Wildlife Conservation Commission (FFWCC) and the University of Georgia, co-authored an article on this phenomenon that appeared as the cover story of a recent issue of the American Geophysical Union's Geophysical Research Letters.

"The water appeared black in satellite imagery because the concentration of the microscopic plants and other dissolved matters 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.

When Hu and his colleagues 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, the authors concluded that the coral reef ecosystem was stressed by microscopic organisms and toxins contained in the dark water.

The "black water" event 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 of dark water. Satellite 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 benthic (ocean bottom) environment. The SeaWiFS and MODIS measurements of the dark water led to a number of investigations to help clarify the issues and to provide answers to the public's concerns.

SeaWiFS image of black water from Jan. 9, 2002 in false color visualization showing chlorophyll. Reds and oranges represent high concentrations of chlorophyll. Credit: SeaWiFs Project and ORBIMAGE, Scientific Visualization Studio

During January 2002, SeaWiFS detected the dark-colored water in the Florida Bight, just southwest of the Everglades. In fall 2001, the 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 and the Gulf Stream.

Co-authors on this research article included Serge Andrefouet and Frank E. Muller-Karger of USF; Keith E. Hackett, Michael K. Callahan, and Jennifer L. Wheaton of FFWCC, St. Petersburg, Fla.; and James W. Porter of the University of Georgia, Athens, Ga.

NASA funded part of this research as part of its Earth Science mission to understand and protect our home planet. NASA's Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather, and natural hazards using the unique vantage point of space.