Bioaccumulation and Biomagnification
The Monarch Butterfly is poisonous to eat. It does not make its own poison, however! As a caterpillar, it feeds on a plant called milk-weed. The caterpillar ingests toxins from the milkweed, which are stored in the tissues of the butterfly. The toxins do not harm the butterfly, but ingestion of toxins at a faster rate that they can be eliminated is called Bioaccumulation.
In the Monarch Butterfly example, bioaccumulation works to the advantage of the species. If fewer of the species are eaten because they are poisonous, more individuals will survive to reproduce. In contract, the bioaccumulation of toxins from human-made pollution can be devastating to a species. Man-made toxins can cause health problems and even death.
Biomagnification is a process that is related to bioaccumulation. Biomagnification is the increase in the concentration of a toxin as it moves from one trophic level to the next.
DDT
DDT (Dichloro-Diphenyl-Trichloro Ethane) the first of the chlorinated organic insecticides, was originally prepared in 1873, but it was not until 1939 that Paul Muller of Geigy Pharmaceutical in Switzerland discovered the effectiveness of DDT as an insecticide. Muller was awarded the Nobel Prize in medicine and physiology in 1948 for this discovery.
The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spread malaria and the lice that carried typhus. The World Health Organization estimates that during the period of its use approximately 25 million lives were saved. DDT seemed to be the ideal insecticide. It was cheap to make and as far as they knew at the time – it had relatively low toxicity to mammals.
Problems related to extensive use of DDT began to appear in the late 1940s. DDT was sprayed on crops, which then became part of the run-off into the water system. DDT was absorbed by the algae in the water. Microscopic animals and fish ate the algae. The concentration of DDT increased and was stored in the tissues of these organisms. The fish were then consumed by birds. At high concentrations, the DDT affected fish reproduction butthe effects of DDT were not recognized until the bird population was affected.DDT affected the bird’s egg shell. Normal eggshells are quite thick. Birds sit on their eggs to incubate them - but DDT made the egg shells thin, so when the birds sat on them the eggshell broke. This resulted in a dramatic decrease in the number of young produced, and so the populations declined.
Some bird populations declined more than others. These were more sensitive to DDT's harmful effects. The birds that were affected most by DDT were the ones at the top of the food chain - herring gulls and some of the other fish eating birds.
DDT is a very persistent chemical. It hangs around in the environment for a long time. For instance, DDT may persist in water for as long as 150 years! It stays in the tissues of all the animals that ingest it. That's why birds at the top of the food chain get a bigger dose. It's called bio-magnification or bio-accumulation - these refer to the buildup of a compound as you go up the food chain. DDT, for instance, is in the water and it goes into the small organisms, slightly bigger organisms, and it gets up into the top of the food chain... and by that time it's magnified, where it becomes a greater concentration than it was down at the bottom of the food chain.
The use of DDT was banned in Canada in 1972 and in the United States in 1973, although it is still in use in some other parts of the world. The buildup of DDT in natural waters is a reversible process: the EPA reported a 90% reduction of DDT in Lake Michigan fish by 1978 as a result of the ban.
PCB’S
PCBs stands for Polychlorinated Biphenyls. PCBs are extremely harmful and persistent. They last for many years because they do not break down easily on their own and they are difficult to destroy.
PCBs were first manufactured in 1929. For several decades, they were used widely as ingredients in many industrial materials, such as sealing and caulking compounds, cutting oils, inks and paint additives. PCBs were also used to make coolants and lubricants for certain kinds of electrical equipment, such as transformers and capacitors.
PCBs are still present in certain types of electrical equipment. Also, public concern over disposal practices has led to the storage of PCBs in many facilities around the country. In some instances, PCBs have been put into specially engineered landfills. Despite strict controls on the handling and storage of PCBs, there remains the potential for accidental releases into the environment.
Most of what is known about the human health effects of PCBs is based on exposures due to accidental releases or job-related activities. These exposures are much higher than the levels normally found in the environment. The adverse health effects include a severe form of acne, swelling of the upper eyelids, discolouring of the nails and skin, numbness in the arms and/or legs, weakness, muscle spasms, chronic bronchitis, and problems related to the nervous system.