Dark-colored river runoff includes nitrogen and phosphorus, which are used as fertilizers in agriculture. These nutrients cause blooms of marine algae called phytoplankton. During extremely large phytoplankton blooms where the algae is so concentrated the water may appear black, some phytoplankton die, sink to the ocean bottom and are eaten by bacteria. The bacteria consume the algae and deplete oxygen from the water that leads to fish kills.
Chuanmin Hu and Frank Muller-Karger, oceanographers at the College of Marine Science of University of South Florida, St. Petersburg, Fla., used fluorescence data from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard both NASA's Terra and Aqua satellites. MODIS detects the glow or phytoplankton fluorescence, from the plant's chlorophyll. The human eye cannot detect the red fluorescence.
The ability to detect glowing areas of water helps researchers identify whether phytoplankton are present in large dark water patches that form off the coast of Florida. Without these data, it is impossible to differentiate phytoplankton blooms from plumes of dark river runoff that contain few individual phytoplankton cells.
Because colored dissolved organic matter that originates in rivers can absorb similar amounts of blue and green color signals as plants do, traditional satellites that simply measure ocean color cannot distinguish phytoplankton blooms within such patches.