NASA - National Aeronautics and Space Administration

Just like a homicide detective collecting clues from a crime scene, scientists are using NASA satellite images to figure out what caused a mysterious patch of "black water," seen in the aerial photograph below to form and spread over 60 miles off the southwestern Florida coast in early 2002. Scientists say this mysterious black mass contributed to severe coral reef stress and death in the Florida Keys.

SeaWiFS Image of Blackwater (True Color) Jan. 9, 2002
Courtesy: NASA

Scientists learned that the "black water" contained a high abundance of toxic and non-toxic microscopic plants that destroyed 70 percent of "stony coral," 40 percent of other coral species, and almost completely killed sponge colonies at two reef sites once the dark water passed. "Scientists were extremely surprised by the devastation," said Frank Muller-Karger of the Institute for Marine Remote Sensing of the University of South Florida (USF).

Fishermen were among the first to spot the black water and notice fish were avoiding the area.

Scientists first suspected red tide of contributing to this deadly incident. Red tides are blooms of toxic algae that are known to turn waters a shade of reddish brown. The water appeared black in SeaWiFs satellite imagery (Image 1, False Color Image 2) because the concentration of microscopic plants and other dissolved matters were high," said Chuanmin Hu also of USF.

SeaWiFS Image of Blackwater (False Color) Jan. 9, 2002
Visualization shows chlorophyll - reds and oranges represent high concentrations of chlorophyll
Courtesy: NASA

Red tides have killed huge numbers of fish, shellfish, marine mammals, and birds. They can also trigger skin and breathing problems in humans. They are caused by high concentration of microscopic plants called dinoflagellates, shown in image 3.

Microscopic image of a dinoflagellate, called Rhizosoleniacea, a cause for Red Tide. Rhizosoleniaceae blooms occur seasonally off Florida's Gulf coast due to freshwater outflow from the Everglades
Courtesy: Florida Marine Research Institute

A red tide occurred in December 2001, seen in image 4, just before the black water event. Scientists used satellite images from the SeaWiFs instrument on board the Orbimage 2 satellite and Terra to study the event. They theorized that the waters containing this red tide migrated to the south along the coast and mixed with nutrient-rich water from the Everglades.

Winter storms caused large amounts of fresh water and nutrients to drain from the Everglades into the water near the shoreline that stretched to Key West. The nutrients caused an intense bloom of the microscopic marine plants known as diatoms in the same patch. The "black water" area moved slowly clockwise for several months in that area, and by May 2002 had moved into the Atlantic Ocean.

NASA's Terra satellite acquired this image of a red tide bloom along Florida's west coast December 22, 2001. Notice the dark reddish color of the ocean.
Courtesy: NASA

Scientists are not done with their detective work. They are continuing to study satellite data and collect water samples in hopes of learning more about what caused the red tide to be so intense and make the water look black.

Rani Chohan
Goddard Space Flight Center, Greenbelt, Md.