Algal bloom
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). Algal
blooms harm water systems. Algal blooms cause the water to become cloudy. They block
light from reaching plants further down in the water.
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.
Excess nutrients in the water can also lead to low oxygen levels in the water. As more
algae and plants grow, they run out of resources and begin to 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. Lack of oxygen
can kill fish and other aquatic animals.
When the dissolved oxygen content decreases, many fish and aquatic insects cannot
survive. This results in a dead zone in oceans. Dead algae can sink deep in the ocean,
below levels where light can reach. Without light, photosynthesis cannot take place. So
when the dead algae decay in these deep regions of the ocean, the oxygen that is used up
does not get replaced easily. The lack of oxygen results in an area where only few
organisms can live.
Algal blooms may also be of concern as some species of algae produce toxins. The
microscopic organisms Pfisteria is a type of alga that can produce quickly in warm,
nutrient-rich waters of estuaries. It produces a chemical that poisons fish. The chemical is
also harmful to humans who breathe or touch it. People may become dizzy, get
headaches, or develop sores on their skin from Pfiesteria poisoning. Pfiesteria blooms
have occurred in the estuaries of North Carolina and other eastern states.
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.