An algal bloom is an accumulation of algae (typically microscopic) or a rapid increase in their amount in a body of water. Algal blooms may occur in fresh water as well as salt water. Algal bloom concentrations may reach millions of cells per milliliter of water. Algal blooms are often green, but they can also be other colors such as yellow-brown or red, depending on the species of algae. So-called algal blooms are often caused by Cyanobacteria such as "Aphanizomenon flosaquae", which are no longer considered to be algae. Some kinds of algal blooms, such as red tides, are poisonous.