An algal bloom is a relatively rapid increase in the population of (usually) phytoplankton algae in an aquatic system. Typically only one or a few species are involved and the bloom is recognized by discoloration of the water resulting from the high density of pigmented cells. Although there is no officially recognized threshold level, algae are unlikely to be considered to be blooming unless more than 10,000 cells per millilitre occur. Algal bloom concentrations may reach millions of cells per millilitre. Colors observed are green, yellowish-brown, or red.

Cities and farms along the shores of lakes produce a variety of pollutants that get into the water. Improved laws and regulations have reduced the amounts of toxic chemicals entering lakes. However, even small quantities of toxic chemicals can enter food chains and create health hazards over the long term. Non-toxic chemicals also produce problems. For example, rivers and run-off from farmers' fields bring extra nutrients into a lake, allowing increased growth of algae in the water. The result is a population explosion of algae called an algal bloom. The algae cover the water with a film of green slime.

Too many nutrients in fresh-water lakes and ponds can cause populations of algae to increase rapidly. The algae reduce the oxygen supply available to other organisms in the water.

The excessive growth of algae may disrupt higher links of the local food web. Algae that die and sink to the bottom stimulate growth of decomposers, especially bacteria. Decomposition can result in the depletion of oxygen in the deeper water layers, and these conditions may result in fish kills or replacement with less valuable species more tolerant of higher phosphorus and lower oxygen levels. Deoxygenation also may cause chemical changes in the mud on the bottom, lowering the redox value of the sediment, releasing chemicals and toxic gases. All these changes further accelerate the eutrophication of the aquatic ecosystem.

Algal blooms may also be of concern as some species of algae produce neurotoxins. At the high concentrations reached during blooms, these may cause death if affected water is ingested.

Phytoplankton bloom in the North Sea and the Skagerrak - NASA

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 (Zohary and Roberts, 1990; Bartram et al., 1999).

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 red or brown.

Black water[]

So-called black water is a dark discoloration of sea water, first described in the Bay of Florida in January 2002 [1]. Although fishermen in Florida complained and requested that the "government do something", 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.

Water treatment[]

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 (Song et al., 2005).

See also[]

  • ciguatera
  • dead zone
  • dinoflagellate (see "neurotoxins" and "red tide" under Ecology and fossils and see "phosphate" under Cautions)
  • domoic acid
  • red tide


  • Bartram, J., Wayne W. Carmichael, Ingrid Chorus, Gary Jones, and Olav M. Skulberg. 1999. Chapter 1. Introduction, In: Toxic Cyanobacteria in Water: A guide to their public health consequences, monitoring and management. World Health Organization. URL: WHO. ISBN 0419239308.
  • Song W., Teshiba T., Rein K., and O'Shea K. E. 2005 (In press). Ultrasonically induced degradation and detoxification of microcystin-LR (cyanobacterial toxin). Environmental Science & Technology. Abstract.
  • Zohary, T. and R. D. Roberts. 1990. Hyperscums and the population dynamics of Microcystis aeruginosa. J. Plankton Res., 12: 423.

External links and further reading[]

Adapted from the Wikipedia article "algal bloom" http://en.wikipedia.org/wiki/Algal_bloom