Flexi Says: In some parts of the world, the red tide is an annual event. During the red tide, the sea changes color as algae take over the surface. Scientists call these events harmful algal blooms (HAB). During a HAB, algae can double its population once, twice, or even three times a day. This population growth is an example of an exponential function. In a very short time, these microscopic organisms can completely cover the surface of the water.