Bioaccumulation is a process where substances, such as pollutants or toxins, gradually build up within a living organism over time. This happens when an organism takes in these substances from its environment at a faster rate than it can eliminate them. The result is an increasing concentration of these harmful compounds in the organism’s tissues, which can have significant consequences for its health and for the wider ecosystem.
How Substances Accumulate
The chemical properties of a substance heavily influence how it accumulates within an organism. For instance, lipophilic substances, which dissolve readily in fats and oils, tend to accumulate in the fatty tissues of organisms, where they can be stored for extended periods.
The persistence of a substance, or its resistance to breaking down in the environment, also plays a large role. Substances that are not easily degraded remain in the environment longer, increasing their opportunity to be absorbed by organisms. An organism’s physiology, including its metabolic rate and efficiency in eliminating waste, further influences how quickly and to what extent substances accumulate. For example, fish can absorb pollutants directly from water through their gills in addition to ingesting them through their diet.
Journey Through the Food Web
A distinct, yet related, process is biomagnification, where the concentration of a substance increases as it moves up through different trophic levels in a food web. This means that substances that bioaccumulate in organisms at lower trophic levels become more concentrated in the tissues of predators that consume them.
Consider a simple aquatic food chain to illustrate this concept. Microscopic organisms like phytoplankton can absorb persistent organic pollutants directly from the surrounding water, accumulating these substances in their bodies. When small zooplankton feed on many of these contaminated phytoplankton, they ingest and accumulate even higher concentrations of the pollutants. As larger fish consume numerous zooplankton, the pollutant levels in their tissues become further amplified. This escalating concentration continues up the food chain, resulting in the highest levels of toxins in apex predators.
Common Bioaccumulating Substances
Several types of substances are known to bioaccumulate due to their chemical properties and persistence in the environment. Heavy metals, such as mercury and lead, are common examples. Mercury can be converted into methylmercury by bacteria, a toxic form that readily accumulates in aquatic organisms. Lead can contaminate soil and water, entering the food chain through plants and other organisms.
Persistent organic pollutants (POPs) also show a strong tendency to bioaccumulate and biomagnify. These synthetic chemicals are resistant to environmental degradation and are highly fat-soluble, allowing them to remain in an organism’s body for extended periods. Examples include polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT). Even though the production of many POPs was banned decades ago, their persistence means they can still be found in ecosystems and animal tissues today.
Impacts on Life and Ecosystems
The accumulation of toxic substances through bioaccumulation and biomagnification has wide-ranging negative effects on individual organisms and entire ecosystems. At the individual level, organisms exposed to bioaccumulated substances can suffer from various health problems. These include neurological damage, reproductive issues, and suppression of the immune system, making them more susceptible to diseases. For example, methylmercury can affect nerve cells, the heart, and kidneys in vertebrates, including humans.
At the ecosystem level, these impacts can lead to significant disruptions. Population declines in top predators are a common consequence, as they accumulate the highest concentrations of toxins. This can alter predator-prey relationships and lead to imbalances within food webs. Reduced biodiversity can also occur as species unable to cope with pollution levels face extinction. Humans are also at risk through the consumption of contaminated food, which can lead to neurological damage, developmental problems, and an increased risk of chronic diseases.