An algal bloom, also known as a marine or water bloom, is a dramatic upsurge in the population of aquatic microbes. This often happens in response to changes in ocean or lake chemistry, such as the introduction of fertilizer runoff.
Though the name implies the presence of algae, most algal blooms are the result of an explosion in the concentration of cyanobacteria. These microorganisms sometimes secrete dangerous toxins and kill off other marine life such as fish. Another problem caused by such blooms is the rapid depletion of dissolved oxygen in the water. This often creates "dead zones" that are inhospitable to animal life.