An algal bloom is a rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Algal bloom concentrations may reach millions of cells per milliliter. Algal blooms are often green, but they can also be other colors such as yellow-brown or red, depending on the species of algae. A 2011 record-breaking algae bloom in Lake Erie was triggered by long-term agricultural practices coupled with extreme precipitation, followed by weak lake circulation and warm temperatures, scientists have discovered. The Carnegie researchers also predict that, unless agricultural policies change, the lake will continue to experience extreme blooms.