A dark band of water in the Pacific Ocean along the coast of El Salvador marks where a large colony of brown or red-tinted algae are growing near the surface of the water. Such blooms are occasionally visible in satellite images such as in the top image, taken by the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) on June 5, 2004. Since the dark streak is produced by microscopic plants, chlorophyll concentrations also show where a bloom may be occurring. In this case, high concentrations of chlorophyll, shown in red, are visible near the coast. In the areas where the bloom appears to be darkest in the natural color image (top), the chlorophyll algorithm has failed, leaving a black streak over the bloom. Some types of red-colored algae blooms can be toxic, but it’s not clear what sort of phytoplankton are blooming here.