Toxic effects of algal blooms
Algae are phytoplanktons and principle producer organisms of marine, estuarine and freshwater ecosystems. Their normal growth and photosynthetic ability support the life of many aquatic flora and fauna species. Algal bloom represent over growth of algae followed by accumulation of algal population on surface waters. Algal bloom may contain one or more species of algae living together independent of each other. Algal blooms can be formed in ocean water, lakes, rivers, ponds, artificial ponds, aquaria and even in home fish ponds. Excess algal growth in fish ponds results from over feeding and to avoid it we replace water regularly and limit food supply. The same thing applies in larger water bodies. Algal growth is increased beyond the limits when water body contains excess quantity of nutrients like phosphorus, sulfur, carbon and nitrogen. These nutrients are naturally present in limiting amounts which are scavenged by various flora and fauna species. But when they are abundantly available they flourish the growth of certain species like algae. What exceeds the natural limiting concentration of nutrients? You will be surprised to know that these nutrients come from our farmlands, industrial and domestic wastes. Rainwater carries fertilizer residues to water bodies, domestic wastes are disposed in the form of sewage to rivers and oceans and all types of industrial effluents are also discharged into oceans and rivers. Effluent waste and runoff waters enrich nutrients present in aquatic bodies to the state of eutrophication. Such more than sufficient nutrients support the lavish growth algae, which we term as algal bloom.
Freshwater algal blooms: In freshwater habitat individual algal species grow rapidly and die quickly resulting in accumulation of dead organic matter followed by its decay. Decaying consumes large amount of dissolved oxygen in water and increased biological oxygen demand (BOD). Such conditions lead to depletion of essential oxygen resulting in death of aquatic flora and fauna. Freshwater algal blooms are usually caused by green algae and cyanobacteria.
Marine algal blooms: Unlike freshwater algae, marine algae produce toxins which are harmful to other flora and fauna and hence marine algal blooms are highly toxic. Marine algae such as diatoms, dinoflagellates, cyanobacteria, coccolithophores and other algal associations are capable of forming toxic blooms. Different types of toxins such as microcystin, brevetoxin, saxitoxin, anatoxin, yessotoxin, exotoxin, neosaxitoxin, demoic acid, okadoic acid, nodularin, lyngbyatoxin, aplysiatoxin, cylindrospermopsin and ciguatoxin are produced by different algal species. Anatoxin, nodularin, lyngbyatoxin, aplysiatoxin, saxitoxin, microcystin, and neosaxitoxin are cyanotoxins. Algal toxins result in high mortality of fishes, seabirds, shell fishes, sea turtles and other oceanic flora and fauna. Algal poisoning can be categorized as CFP (Ciguatera fish poisoning), ASP (Amnesic shellfish), PSP (Paralytic), NSP (Neurotoxic), DSP (Diarrhetic) depending upon its toxicity and toxic effect. Complications like muscle weakness, lethargy, skin irritation, respiratory failure, toxicity to immune, digestive and reproductive systems are resulted from algal toxicity. Other harmful effects of algal blooms are oxygen depletion, mechanical damage to gills of fishes resulting in asphyxiation condition and death of fishes.
Important algal species forming toxic blooms: Heterosigma askshiwo, Chaetoceros spp., Aureoumbra lagunesis, Gymnodinium breve, Alexandrium tamarense, A. catanella, A. fundyense, Gyrodinium aureolum, Karenia brevis, Pseudonitzschia spp., Thalassia testudinum, Coccolithophore spp., Prorocentrum minimum and Pfiesteria piscicida. Some cyanobacterial genera responsible for toxic blooms include, Microcystis, Anabaena, Nostoc, Nodularia, Lyngya, Oscillatoria, Schizothrix, Umezakia, Cylindrospermopsis and Aphanizomenon.
Characteristic features and importance of algal blooms: Algal blooms show seasonal variation. Blooms are recognized by discoloration of water and pungent odor. Blooms can have characteristic colors depending upon pigment of phytoplankton involved in its formation. They are usually green but blue-green, brown, orange, purple, black, red and bright green have also been noted. Bloom contains high density of algal cells; millions of cells are occasionally present per milliliter of water which is very high as compared to normal water (100cells/ml). Poisoning by toxic bloom occurs by inhalation toxic aerosols generated by surfing or foaming, by consumption of poisoned fishes, shrimps or by contamination of drinking water supplies. Freshwater algal blooms may clog the piping of water purification plant; they are responsible for off odor and taste of water, thus affect water potability. Algal blooms represent unbalanced aquatic ecosystem and presence of external pressure such as excess fertilizer application of agricultural lands. It is indication of high productivity and can be a rich source of organic compounds which are obtained from algae. Algal blooms protect marine flora and fauna from UV radiation and also participate in natural sulfur cycle. Algae from blooms are important source of food for krills, shrimps and sea birds especially from Antarctica. An account of some world famous disastrous algal blooms is enlisted here:
Murray river, Australia (1878), French Polynesia (1960), Norway (1966), New England coastline (1972), Oman Gulf (1976), Masan-Chinhac Bay, Korea (1981), Indian Ocean(1983), Rhodes Island (1985), Elands Bay, South Africa (1980, 1997, 2002 till present), Darling and Barwan rivers, Australia (1991), St. Helena Bay, S. Africa (1994), Saldanha Bay, S. Africa (1997), Bering Sea, Alaska (1998), Cornwall, England (1999), Chesapeake Bay (2000), Celtic Sea, England (2001), Baltic Sea (2005), Brittany, France (2009) and Midwestern, US (2010).
Algal blooms are indicators of manmade aquatic pollution and it should be prohibited to save aquatic ecosystems and to prevent the economic loss resulting from contaminated and poisoned marine products.
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