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examBoard: Cambridge
examType: IGCSE
lessonTitle: Bioaccumulation in Food Chains
Water pollution doesn't just affect water quality it can have long-lasting impacts on entire ecosystems through a process called bioaccumulation. This occurs when certain pollutants, especially those that are fat-soluble and non-biodegradable, build up in the tissues of living organisms faster than they can be broken down or excreted.
Key Definitions:
Imagine a small lake contaminated with mercury from a nearby factory. Tiny phytoplankton absorb small amounts of mercury from the water. Small fish eat hundreds of these phytoplankton, collecting the mercury from each one. Larger predatory fish then eat many smaller fish, accumulating even more mercury. By the time a bird or human eats the large fish, the mercury concentration can be thousands of times higher than in the water.
Not all pollutants bioaccumulate. The most problematic ones are:
While related, these two processes describe different aspects of how pollutants move through ecosystems:
This refers to how chemicals build up within a single organism over time. For example, a fish absorbing mercury through its gills and from its food will have higher concentrations of mercury the older it gets. Bioaccumulation happens when uptake exceeds elimination.
This describes how concentrations increase as you move up the food chain. Top predators like eagles, sharks, or humans can have pollutant concentrations millions of times higher than what's in the water. Each trophic level (step in the food chain) can increase concentration by 10-100 times.
For a substance to bioaccumulate, it typically needs certain properties:
The substance must resist breakdown by natural processes or the organism's metabolism. Many synthetic chemicals are designed to be stable, which unfortunately means they persist in the environment.
Most bioaccumulative substances are lipophilic (fat-loving) and hydrophobic (water-repelling). This means they dissolve in and bind to fatty tissues rather than being excreted in water-based urine.
The substance must be in a form that can be taken up by organisms, either through direct absorption, respiration (like fish gills), or consumption of contaminated food.
Bioaccumulation creates serious problems for both wildlife and humans:
Top predators suffer the most severe effects of bioaccumulation:
Humans are typically exposed through consumption of contaminated fish and seafood:
One of the most infamous examples of bioaccumulation occurred in Minamata Bay, Japan, in the 1950s. A chemical factory released wastewater containing methylmercury into the bay for decades. This mercury bioaccumulated in fish and shellfish, which were then consumed by local people. The result was severe mercury poisoning, now known as Minamata Disease, which caused neurological damage, paralysis, birth defects and death. Over 2,000 cases were officially recognised and thousands more were affected. This disaster highlighted the dangers of bioaccumulation and led to improved regulations on industrial waste disposal worldwide.
DDT (dichlorodiphenyltrichloroethane) provides one of the clearest examples of bioaccumulation and its consequences:
Developed in the 1940s, DDT was hailed as a miracle pesticide that helped control malaria and typhus. It was widely used in agriculture and for pest control. However, scientists began noticing troubling patterns:
Rachel Carson's groundbreaking 1962 book "Silent Spring" brought attention to how DDT was causing eggshell thinning in birds of prey, leading to reproductive failure. DDT was eventually banned in the UK in 1984 and in most developed countries, though it's still used in some regions for malaria control.
Scientists use several metrics to assess a substance's tendency to bioaccumulate:
Substances with high BCF, BMF, or Kow values are more likely to bioaccumulate.
Several approaches can help minimise bioaccumulation problems:
The international community has recognised the serious threat posed by bioaccumulative pollutants:
This global treaty, which came into force in 2004, aims to eliminate or restrict the production and use of persistent organic pollutants (POPs). These are chemicals that persist in the environment, bioaccumulate and pose risks to human health and ecosystems. The convention initially targeted 12 chemicals (the "dirty dozen"), including PCBs, DDT and dioxins, but has since expanded to include more substances.
Killer whales (orcas) provide a striking example of bioaccumulation in action. As top predators that feed on seals and large fish, orcas accumulate extremely high levels of PCBs (polychlorinated biphenyls). Although PCB production was banned in the 1970s, these chemicals persist in marine environments. A 2018 study found that some orca populations had PCB concentrations 100 times higher than the threshold known to cause health problems. These high levels are linked to reproductive failure, immune suppression and population decline. Researchers predict that many orca populations may disappear entirely due to PCB contamination, even though the chemicals haven't been produced for decades a sobering reminder of how long-lasting the effects of bioaccumulation can be.
While bioaccumulation presents serious challenges, there are steps we can all take to reduce risks:
Bioaccumulation represents one of the most complex and persistent challenges of water pollution. Unlike immediate toxic effects that might kill fish directly, bioaccumulation works silently and gradually, often taking decades to reveal its full impact. The process demonstrates how pollutants can have far-reaching consequences beyond their initial release point, affecting organisms far removed in both space and time.
Understanding bioaccumulation reminds us that ecosystems are interconnected and that what affects one part of a food chain eventually affects the whole system including humans. By recognising these connections and working to reduce the release of persistent, bioaccumulative substances, we can help protect both environmental and human health for generations to come.
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