An algal bloom or marine bloom or water bloom is a rapi 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 discoloration 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 milliliter, depending on the severity. Algal bloom concentrations may reach millions of cells per milliliter. Colors observed are green, yellowish-brown, or red. Bright green blooms may also occur. These are a result of blue-green algae, which are actually bacteria (cyanobacteria). Blooms may also consist of macroalgal, not phytoplankton, species; these blooms are recognizable by large blades of algae that may wash up onto the shoreline.