Editing Swarm behaviour (section)

====Algal blooms====
Many single-celled organisms called [[phytoplankton]] live in oceans and lakes. When certain conditions are present, such as high nutrient or light levels, these organisms reproduce explosively. The resulting dense swarm of phytoplankton is called an [[algal bloom]]. Blooms can cover hundreds of square kilometres and are easily seen in satellite images. Individual phytoplankton rarely live more than a few days, but blooms can last weeks.<ref>Lindsey R and Scott M (2010) [http://earthobservatory.nasa.gov/Features/Phytoplankton/page1.php What are phytoplankton] [[NASA Earth Observatory]].</ref><ref name=NOAA_HAB>[http://www.glerl.noaa.gov/res/Centers/HumanHealth/docs/habs.pdf Harmful algal blooms in the Great Lakes] {{Webarchive|url=https://web.archive.org/web/20100616173236/http://www.glerl.noaa.gov/res/Centers/HumanHealth/docs/habs.pdf |date=2010-06-16}} 2009, [[NOAA]], Center of Excellence for Great Lakes and Human Health.</ref>