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September 15, 2005
WEB FEATURE: SATELLITES SPOT MIGHTY MISSISSIPPI—IN THE ATLANTIC
Scientists using satellite imagery found that at least 23 percent of the water released from the mouth of the Mississippi River from July
through September 2004 traveled quite a distance—into the Gulf of Mexico, around the Florida Keys, and into the Atlantic Ocean.
The researchers combined data from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra and Aqua satellites with
information collected from ships to study the water discharge, appearing as a dark plume that stretched from the Mississippi Delta, around Florida
and up to the Georgia coast. MODIS detects the color of the ocean due to changes in the amount of tiny ocean plants floating on the ocean’s
surface known as phytoplankton, or algae and other decaying materials.
“This is the first time we have been able to estimate the amount or volume of freshwater discharged and carried over such remote distances.
By combining the very detailed data from MODIS with observations from ships, we got a three-dimensional view of the Mississippi plume,” said
Chuanmin Hu, of the College of Marine Science, University of South Florida, St. Petersburg, Fla., and lead author of the study. By using MODIS data
with information on sea surface currents and sea salt levels (salinity), the scientists estimated that about 20 billion tons of Mississippi River
discharge reached the Florida Straits and Gulf Stream off the Georgia coast. This is equivalent to about four times the volume of Lake Okeechobee,
the largest lake in Florida.
In early July 2004, the dark water plume traveled south along the eastern edge of the Loop Current off southwest Florida, reaching the Florida
Keys by late July 2004. By early August, MODIS images showed that the plume had expanded along the Gulf Steam as far away as the Georgia coast. The
plume was typically 30 to 65 feet deep with a width of 6 to 12 miles; although occasionally was as wide as 30-37 miles, before dissipating in October
2004.
While many factors, like ocean eddies—that mix waters of varying characteristics—influence the evolution of such events, climate and
weather patterns also play a role. For instance, following the Great Midwest Flood of 1993, Mississippi River water also moved into the Florida
Current. In 2004, heavy summertime rainfall may have contributed to the plume’s large size and persistence. But “it’s still too
early to know if there is a concrete connection between climate and the occurrence of these events, as much further study is needed,” said
Hu.
Researchers try to go beyond mapping the dispersal of the river water, by combining satellite information with direct observations from ships and
ocean surface drifters, to get a better idea of how these events may affect marine life.
“Mississippi River water may have some impact on marine life in remote delicate ecosystems like the Florida Keys. But we are still not clear
about the potential impacts of pollutants and pesticides,” said Hu. “Not all effects will be bad; in fact, some light dark water events
might actually protect bottom ocean dwellers, like coral, by providing them with shade.”
The study is published in the July 2005 issue of Geophysical Research Letters under support of NASA, NOAA, and ONR as a contribution to the
SouthEast Atlantic Coastal Ocean Observing System (SEACOOS). Coauthors include oceanographers James Nelson from the Skidaway Institute of
Oceanography, Elizabeth Johns from NOAA’s Atlantic Oceanographic and Meteorological Laboratory, and Zhiqiang Chen, Robert Weisberg, and Frank
Muller-Karger from the University of South Florida.
For more images and information, please visit on the Internet:
http://www.nasa.gov/centers/goddard/news/topstory/ 2005/mississippi.html
http://www.nasa.gov/vision/earth/lookingatearth/ mississippi.html
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Contacts:
Rob Gutro
NASA Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-4044)
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This image taken August 14, 2002, by the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard NASA’s Terra satellite, shows
black water off Sanibel Island, Florida, the hook-shaped island on the western coast at the midway point of the image. Credit: NASA GSFC
This image, using data from MODIS, flying aboard NASA’s Terra satellite, shows blackwater off the coast of southwest Florida in February
2002. Credit: University of South Florida
This image includes data from MODIS on NASA’s Terra satellite and shows the Mississippi River plume extending from the river mouth to the
Florida Straits (a) and into the South Atlantic (b). The black line in (b) shows ship survey areas of August 11-12, 2004. Plume features in the South
Atlantic are marked with arrows. Overlaid on (a) is the track of an ocean instrument (surface drifter) from Dry Tortugas, Florida to the South
Atlantic (red line). Credit: University of South Florida
These MODIS images from NASA’s Terra satellite show the Mississippi River plume (dark feature) in the Florida Straits. Overlaid on the August
21, 2004, image (b) is the location of cruise stations (crosses). Credit: University of South Florida
This image shows a major red tide bloom, extending more than 100 miles along Florida’s Gulf coastline, taken December 22, 2001, by MODIS,
flying aboard NASA’s Terra satellite. Notice the dark reddish color of the ocean, especially around Estero Bay (toward the south) and Apalachee
Bay (toward the north). Credit: NASA
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