An algal bloom or marine bloom or water bloom is a rapid increase in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically only one or a few phytoplankton species are involved, and some blooms may be recognized by discolouration of the water resulting from the high density of pigmented cells. Although there is no officially recognized threshold level, algae can be considered to be blooming at concentrations An algal bloom or marine bloom or water bloom is a rapid increase in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically only one or a few phytoplankton species are involved and some blooms may be recognized by discolouration of the water resulting from the high density of pigmented cells. Although there is no officially recognized threshold level, algae can be considered to be blooming at concentrations of hundreds to thousands of cells per millilitre, depending on the causative species.

Algal bloom concentrations may reach millions of cells per millilitre. Colours observed are green, yellowish-brown, or red. Bright green blooms may also occur.. Some algal blooms are the result of an excess of nutrients (particularly phosphorus and nitrogen) into waters, and higher concentrations of these nutrients in water cause increased the growth of algae and green plants.

As more algae and plants grow, others die. This dead organic matter becomes food for bacteria that decompose it. When the dissolved oxygen content decreases, many fish and aquatic insects cannot survive. At the high cell concentrations reached during some blooms, these toxins may have severe biological impacts on wildlife.