Algal bloom

A "red tide" resulting from a dinoflagellate bloom discoloring the water on the right
A "red tide" resulting from a dinoflagellate bloom discoloring the water on the right

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.

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.

Missing image
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. The species responsible for red tides on the gulf coast of Florida is a dinoflagellate called Karenia brevis (formerly Gymnodinium breve.) It produces brevetoxins which produce respiratory irritation in humans.

Coastal pollution produced by humans appears to be a causal factor in red tides in some parts of the world, but red tides also occur in places where there are no associated human activities. Some red tides produce large quantities of toxins, which kill fish and are accumulated by filter feeders. This bioaccumulation of toxins causes bivalves – like oysters and clams – collected in areas affected by algal blooms to be potentially dangerous for human consumption.

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.


  • 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 (
  • Zohary, T. and R. D. Roberts. 1990. Hyperscums and the population dynamics of Microcystis aeruginosa. J. Plankton Res., 12: 423.

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