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Bioaccumulation and Biomagnification

     Bioaccumulation refers to the gradual accumulation of substances, such as pesticides or other chemical substances, in the organism over time. Bioaccumulation occurs when the rate at which the organism absorbs a substance is greater than the rate at which the substance is lost or eliminated through catabolism and excretion. Therefore, even if the levels of toxins in the environment are not very high, there is a greater risk when the biological half-life of a toxic substance is longer.

     The toxicity of metals is related to bioaccumulation and biomagnification. If the metal storage or absorption rate is greater than the organism's metabolism and excretion rate, it will lead to the accumulation of toxins.

     Living organisms can absorb chemicals through breathing, skin absorption, or swallowing. When the concentration of a chemical substance inside the organism is higher than its surrounding environment (air or water), it is called biological concentration. Biomagnification is another process related to bioaccumulation: when an organism rises from one trophic level to another, the concentration of chemical substances in the body increases. The process of bioaccumulation is necessary for the growth and development of an organism. However, the accumulation of harmful substances also occurrs at the same time.

     Under the combined action of bioaccumulation and biomagnification, fish at the top of the food chain (such as sharks) and fish with longer growth cycles (such as tuna) have higher levels of methylmercury.




Bioaccumulation and Biomagnification


DDT structure diagram


     A typical example of bioaccumulation occurs in an insecticide called dichlorodiphenyltrichloroethane (DDT). Before 1972, DDT was an insecticide used to control mosquitoes and insects in the United States. DDT was washed into the creek by the rain. Eventually, it was washed into lakes and oceans.

     DDT bioaccumulated in every organism. Over time, it had been amplified to a very high level by biomagnification through the food web, especially in carnivorous birds, such as ospreys, bald eagles, brown pelicans, and peregrine falcons, which are in the higher trophic levels of the food chain. The content of DDT in those birds is so high that the eggshells of the birds become unusually thin. Because the eggshells are too thin, the adult birds accidentally smash the eggshell of the unhatched young birds, and the young birds died. As a result, the number of these birds dropped sharply. In 1972, DDT was banned in the United States. Since then, the number of many predatory birds has increased significantly.



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