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An algal bloom or marine bloom or water bloom is a rapid 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 causative species. 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).
Some algal blooms are the result of an excess of nutrients (particularly phosphorus and nitrogen) into waters and higher concentrations of these nutrients in water cause increased growth of algae and green plants. As more algae and plants grow, others die. This dead organic matter becomes food for bacteria that decompose it. With more food available, the bacteria increase in number and use up the dissolved oxygen in the water. When the dissolved oxygen content decreases, many fish and aquatic insects cannot survive. This results in a dead area.
Algal blooms may also be of concern as some species of algae produce neurotoxins. At the high cell concentrations reached during some blooms, these toxins may have severe biological impacts on wildlife. Algal blooms composed of phytoplankters known to naturally produce biotoxins are often called Harmful Algal Blooms, or HABs.
Algal blooms are monitored using biomass measurements coupled with the examination of species present. A widely-used measure of algal and cyanobacterial biomass is the chlorophyll concentration. Peak values of chlorophyll a for an oligotrophic lake are about 1-10 µg/l, while in a eutrophic lake they can reach 300 µg/l. In cases of hypereutrophy, such as Hartbeespoort Dam in South Africa, maxima of chlorophyll a can be as high as 3,000 µg/l. 
Main Article Red tide
The so-called Red tide is an example of a naturally occurring estuarine or marine algal bloom. Red tide is caused by species of dinoflagellates, often present in sufficient numbers (thousands or millions of cells per milliliter) to turn the water a hue of red.
So-called black water is a dark discoloration of sea water, first described in the Florida Bay in January 2002. Although fishermen in Florida complained and requested that the government take action, scientists say that black water results from a non-toxic algal bloom, probably of diatoms. It dissipated within a few months by transport through the Florida Keys into the Florida Straits and by disruption by winds and wave action.
Algal blooms sometimes occur in drinking water supplies. In such cases, toxins from the bloom can survive standard water purifying treatments. Researchers at Florida International University in Miami are experimenting with using 640-kilohertz ultrasound waves that create micropressure zones as hot as 3,700 °C. This breaks some water molecules into reactive fragments that can kill algae.
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- Dead zone (ecology)
- Dinoflagellate (see "neurotoxins" and "red tide" under Ecology and fossils and see "phosphate" under Cautions)
- Domoic acid
- Red tide
- Zohary, T. (1990). "Hyperscums and the population dynamics of Microcystis aeruginosa". J. Plankton Res. 12 (2): 423–432. doi:10.1093/plankt/12.2.423. ISSN 0142-7873. Unknown parameter
- Bartram, J. (1999). "Chapter 1. Introduction". Toxic Cyanobacteria in Water: A guide to their public health consequences, monitoring and management. World Health Organization. ISBN 0-419-23930-8. Retrieved 2007-06-09. Unknown parameter
- "Black water status report". Florida Marine Research Institute. 2002, May 4. Retrieved 2007-06-09. Check date values in:
- Song, W. (2005). "Ultrasonically induced degradation and detoxification of microcystin-LR (cyanobacterial toxin)". Environmental Science & Technology. 39 (16): 6300–6305. doi:10.1021/es048350z. Unknown parameter
- Harmful Algal Bloom information from the Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute
- Harmful Algae and Red Tide Information from the Coastal Ocean Institute, Woods Hole Oceanographic Institution
- Toxic Blooms: Understanding Red Tides, a seminar by the Woods Hole Oceanographic Institution
- Gulf of Mexico Dead Zone and Red Tides
- Human Illness caused by Algae, an excellent summary.
- Red Tide Report A Compilation of citizen based, media and official reports of the locations and severity of current Red Tide Blooms.
- Florida Red Tide Coalition A citizen based group dedicated to raising awareness of Red Tide, debunking myths, educating the public and taking action to promote our oceans' health and help stop red tide.
- Red Tide updates for the Gulf Coast of Florida provided by Mote Marine Laboratory in Sarasota, FL
- Harmful Algal Blooms Observing System (HABSOS)