Algae are photosynthetic microorganisms that are found in most aquatic habitats. Algae love runoff nutrients, and an algae bloom occurs when nutrient pollution and lots of sunlight create a rapid increase in the density of the algae. Algal blooms may occur in freshwater as well as marine environments.
When an algae bloom does happen, the stream, river, lake, or ocean becomes covered with algae, creating a thick mat of surface scum. Bright green colored blooms develop from cyanobacteria, which are also known as blue-green algae.
Broadly, bloom species can be classified into three different groups:
• Those which produce harmless water discolourations, but the dense bloom on decomposition can cause anoxia and lead to indiscriminate mortality of marine life,
• Species which produce potent toxins causing a variety of gastrointestinal and neurological illness to humans and
• Species which are not toxic to humans but harmful to fish and invertebrates by damaging or clogging their fish gills. The high–biomass producers are linked with production of scums and reduction of habitat for fish and shellfish.
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.
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).
Freshwater algal blooms are the result of an excess of nutrients, particularly some phosphates. The excess of nutrients may originate from fertilizers, household cleaning products containing phosphorus. These nutrients can then enter watersheds through water runoff. Excess carbon and nitrogen have also been suspected as causes. Presence of residual sodium carbonate acts as catalyst for the algae to bloom by providing dissolved carbon dioxide for enhanced photosynthesis in the presence of nutrients.
A harmful algal blooms (HAB) is an algal bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms, or by other means. HABs are often associated with large-scale marine mortality events and have been associated with various types of shellfish poisonings.
People often get sick by eating shellfish containing toxins produced by these algae. Airborne HAB toxins may also cause breathing problems and, in some cases, trigger asthma attacks in susceptible individuals.
(HABs) involves toxic or otherwise harmful phytoplankton. Such blooms often take on a red or brown hue and are known colloquially as red tides.
Red tide is a term often used synonymously with HABs in marine coastal areas, however the term is misleading since algal blooms can be a wide variety of colors and growth of algae is unrelated to the tides.
Hence Impact of algal bloom are:
A. Public Health Concerns
• Exposure to algal toxins may occur through consumption of tainted water, fish, or shellfish; recreational activities; or inhalation of aerosolized toxins.
• Algal toxins are known to cause illness immediately (hours to days) after exposure. In addition, several algal toxins are believed to be carcinogens or to promote tumor growth, although more research on the effects of long-term exposure is needed.
B. Ecologic Concerns
• HABs may cause mortality of aquatic organisms because of low dissolved oxygen or algal toxins. Algal toxins also may cause mortality of terrestrial organisms using the water source.
• As more algae and plants grow, others 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. When the dissolved oxygen content decreases, many fish and aquatic insects cannot survive.This results in a dead area.
C. Economic Concerns
• Economic concerns associated with HABs include increased drinking-water treatment costs, loss of recreational revenue, loss of aquacultural and fisheries revenue, and livestock sickness or fatalities.
• Taste-and-odor compounds are of particular concern to drinking-water suppliers because of customer dissatisfaction with malodorous drinking water.