Bio magnification (or bio accumulation) refers to the ability of living organisms to accumulate certain chemicals to a concentration larger than that occurring in their inorganic, non-living environment, or in the case of animals, in the food that they eat. Organisms accumulate any chemical needed for their nutrition.
Pesticides, even though present in exceedingly low concentration in the soil or the surrounding water are taken up by various microbes, plants and animals which may accumulate and concentrate them several thousand times. The concentration of 0.00001 ppm of DDT, for example, may get magnified to almost 70,000 times in oyesters within a period of 40 days.
One kg of soil may contain only 0.0001 mg of an organo-chlorine pesticide whereas carrots grown on this soil may contain as much 2-6 mg per kg and the rabbits feeding on these carrots may contain as much as 22-35 mg per kg of the toxicant.
Toxaphene may occur in lake waters only in the concentration range of 0.0002- 0.0006 mg per Itr but the water plants growing in the lake may contain as much as 0.2-0.4 mg per kg, invertebrates 0.5 – 1.5 mg per kg and trout and salmon may contain as much as 3.0 – 6.0 mgper kg. Toxaphene is decomposed very slowly and even after a period of six or seven years there is little significant change in its concentration. Similar bio-accumulation and bio-magnification have been recorded for a number of other pesticides as well (Gruzdyev et al 1980).
The process of bio-accumulation and bio-magnification makes exceedingly low quantities of pesticides or their toxic residues available to the living organism in a highly concentrated state. The symptoms of toxicity either appear in the animal accumulating the poisonous material or it makes the animal poisonous to other organisms which feed on it. The worst suffers are animals at the top of the trophic structure.
For example, a very low concentration of DDD, an organochlorine insecticide was bio-magnified 5000 times in fishes in Lake Clear of California soon after it was sprayed in 1957. Each fish possessed about 100 ppm of the pesticide and grebes which represented the top level of the trophic structure succumbed to its toxicity after eating 20 such fishes (Hunt and Bischoff 1960, Rudd and Herman 1972).
Another undesirable consequence of the phenomenon of entry, accumulation and bio- magnification of highly persistent pesticides is the transport and dissemination of these poisons too far off places in a highly concentrated state, within the bodies of living organisms.
If a bird carries 100 mg per kg of DDT within its body, with thousands of birds migrating from one place to another a large quantity of the pesticide, about 1 kg per 10,000 kg of bird-biomass, shall get transported to new regions where it is released after the birds die and their bodies decay. Thus the pesticide applied in Florida may appear in Polar Regions and cause toxicity there
