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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.
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).
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 toxinA toxin is a substance that causes either permanent or reversible injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. The term is usually reserved for natus, which kill fishAtlantic herring, Clupea harengus one of the most abundant species in the world Photo A fish is a poikilothermic (cold-blooded) water-dwelling vertebrate with gills. There are over 27,000 species of fish, making them the most diverse group of vertebrates. and are accumulated by filter feederFilter feeders (also known as suspension feeders) are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized structure, such as the baleen of baleen whales. Filter feeding is one ofs, like shellfishShellfish is a term used to describe molluscs and crustaceans used as food. Molluscs include the clam, mussel, oyster, and scallop; some crustaceans are the shrimp, lobster, crayfish, and crab. snails, while similar to most shellfish biologically, are gen. This bioaccumulationTo bioaccumulate literally means to accumulate in a biological system. However, it is commonly taken to measure the uptake over time of toxic substances that can stay in a biological system. Everything in a biological system has a biological half-life, th of toxins is why one must be careful eating shellfish collected at certain times of the year.