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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.
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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.
Journal
Geophysical Research Letters