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.
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 in 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, like shellfish. This bioaccumulation of toxins is why one must be careful eating shellfish collected at certain times of the year.
Black water So_called black water is a dark discoloration of sea water, first described off the Florida coast in 2002. Although fishermen in Florida have complained and requested that the "government do something", scientists say that black water results from a non_toxic algal bloom, probably of diatoms.
References - 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 (http://www.who.int/docstore/water_sanitation_health/toxicyanobact/ch03.html)
- Zohary, T. and R. D. Roberts. 1990. Hyperscums and the population dynamics of Microcystis aeruginosa. J. Plankton Res., 12: 423.
External links - Gulf of Mexico Dead Zone and Red Tides (http://www.tulane.edu/~bfleury/envirobio/enviroweb/DeadZone.htm)
- Excellent summary of Human Illness caused by Algae (http://www.whoi.edu/redtide/illness/illness.html)
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