Case Study 7 Red Tide Detection in the Eastern Gulf of Mexico Using MODIS Imagery

Many of the red tides (i.e., harmful algal blooms or HABs) in the eastern Gulf of Mexico (GOM) (24◦–31◦N, 90◦–80◦W) are caused by the toxic dinoflagellate, Karenia brevis (previously known as Gymnodinium breve or G. breve). Brevetoxins produced during K. brevis blooms can kill fish, mammals, and other marine organisms and cause respiratory irritation in humans (Hemmert, 1975; Asai et al., 1982; Landsberg and Steidinger, 1998; Kirkpatrick et al., 2004; Flewelling et al., 2005). K. brevis blooms can also adversely impact local tourism and commercial shellfish industries, leading to economic losses that have exceeded millions of US dollars during a single bloom event (Habas and Gilbert, 1974; Larkin and Adams, 2007). Although K. brevis blooms can change the water to many different colours (e.g., brown, red, or even black) depending on the bloom’s cell concentration and the concentration of other important optical constituents (Dierssen et al., 2006), they are commonly referred to as red tides. In the eastern GOM, red tides occur every year, mainly from late summer to early spring, yet their occurrence frequency, intensity, spatial extent, and duration all vary from year to year. Despite many years of community efforts, the mechanisms of initiation, maintenance and demise of red tides are still poorly understood and require further investigation. Data collected between the 1950’s and the 1980’s suggest that red tides are initiated offshore in nutrient-poor waters (Tester and Steidinger, 1997), and that they move toward shore by winds and currents, where they concentrate near fronts and utilize new nutrients from coastal runoff (Walsh et al., 2006). Several hypotheses that attempt to explain new nutrient supplies for these HABs have been proposed, including nitrogen fixation stimulated by atmospheric deposition of iron-rich Saharan dust particles