Harmful algal blooms or HABs are algal blooms composed of phytoplankton known to naturally produce biotoxins, they can occur when certain types of microscopic algae grow quickly in water, forming visible patches that may harm the health of the environment, plants, or animals. HABs can deplete the oxygen and block the sunlight that other organisms need to live, and some HAB-causing algae release toxins that are dangerous to animals and humans. HAB can occur in marine, estuarine, and fresh waters.