An overgrowth of algae in water that can shade out other aquatic plants and use up the water's oxygen supply as the plants decompose; blooms are often caused by pollution from excessive nutrient input..
An algal bloom is a relatively rapid increase in the population of (usually) phytoplankton algae in an aquatic system. Algal blooms can occur in coastal and marine waters as well as freshwater environments. Typically only one or a few species are involved. Although there is no officially recognized threshold level, algae can be considered to be blooming at concentrations of hundreds to thousands of cells per milliliter, though concentrations may reach millions of cells per milliliter. Algal blooms are often linked to eutrophication, a condition of water systems characterised by excessive concentrations of nutrients such as nitrogen and phosphorus compounds. Algal blooms are a problem for water environments because they often lead to conditions of low oxygen concentration after the phytoplankton die and begin decomposing. Algal blooms of certain specific species are considered to be Harmful Algal Blooms (HABs)