Certain substances, particularly those introduced by human activities, do not simply disappear. Instead, they can linger and build up within living systems. Understanding how these substances accumulate in organisms and move through ecosystems is important for recognizing their potential impacts.
Bioaccumulation Explained
Bioaccumulation describes the process where a substance gathers within an individual organism at a rate faster than it can be broken down or excreted. This means that over an organism’s lifetime, the concentration of the substance in its tissues increases significantly compared to the surrounding environment. Substances can enter an organism through various pathways, including direct absorption from water, air, or soil, as well as through the consumption of contaminated food.
Once absorbed, these substances are often stored in different tissues, particularly fatty tissues, due to their chemical properties. For example, aquatic organisms can accumulate fat-soluble chemicals, leading to body concentrations thousands of times greater than the chemical concentration in the water. This continuous intake and slow elimination result in a gradual build-up, and the concentration of the substance increases with the organism’s age.
Biomagnification Explained
Biomagnification refers to the increasing concentration of a substance in the tissues of organisms at successively higher levels in a food chain. This occurs because organisms at higher trophic levels consume many individuals from lower trophic levels, thereby accumulating the substances from their prey. As a result, the concentration of the substance multiplies at each step up the food chain.
Consider a simple food chain where microscopic organisms absorb a pollutant from the water. Small fish then consume many of these microscopic organisms, accumulating the pollutant. Larger predatory fish that eat many small fish will accumulate even higher concentrations of the pollutant, demonstrating how the substance’s concentration increases at each progressive trophic level.
Key Differences
The distinction between bioaccumulation and biomagnification lies in their scope of accumulation. Bioaccumulation focuses on the build-up of a substance within a single organism over its lifetime, where the rate of uptake exceeds the rate of elimination, meaning an individual organism’s body contains a higher concentration than its immediate environment.
Biomagnification, conversely, describes the increase in concentration of a substance as it moves up the food chain through different trophic levels. It highlights how substances become more concentrated in predators as they consume contaminated prey, leading to the highest concentrations in apex predators. Therefore, while bioaccumulation occurs within an individual, biomagnification describes the escalating concentration across an entire food web.
Common Contaminants and Their Effects
Several common contaminants are known to bioaccumulate and biomagnify, posing risks to wildlife and human health. Mercury, a heavy metal that enters ecosystems through natural sources like volcanoes and human activities such as industrial emissions. It primarily accumulates as methylmercury in aquatic food chains, leading to high levels in fish, especially larger predatory species like tuna, marlin, and swordfish.
Dichlorodiphenyltrichloroethane (DDT), an insecticide widely used after World War II but banned in the United States in 1972 due to its environmental persistence. Polychlorinated biphenyls (PCBs), used in industrial applications like electrical equipment, were banned in 1979 but also persist. These persistent organic pollutants (POPs) are fat-soluble and resist breakdown, allowing them to accumulate in fatty tissues and biomagnify up food chains. Effects on wildlife include reproductive issues, neurological damage, and thinning eggshells in birds, while human consumption of contaminated seafood can lead to various health concerns.