Bioaccumulation

• Land, water and air are polluted by toxins.
• Some toxins that pollute the environment build up in higher levels of food chains, causing more harm.

Pollution is the release of harmful or poisonous chemicals called toxins into the environment.

Pollution on land

It is extremely important to recycle as much as possible, especially things such as batteries. If batteries are put into the rubbish bin they can end up in landfill where their toxic chemicals may leak out and cause pollution.

Pollution in water

Some factories have been caught illegally pumping waste into rivers and seas. Oil tankers have had accidents and spilled oil into the oceans.

Pollution in the air

Factory chimneys and vehicle exhausts release carbon dioxide and other pollutants into the atmosphere, causing climate change and harm to many forms of life, including humans.

Polluting toxins can be taken in by living things if breathed in, consumed in food or water, or through their skin. Some toxins break down into harmless substances quickly before they can affect living things. Other toxins do not do this and are absorbed by them. Many species can deal with exposure to these toxins in small levels. They do not stay in the organisms and most areexcreted.

Some polluting toxins cannot be excreted. These can be taken in by the organisms at the bottom of the food chain in low levels and so the harm is limited. The build up of these toxins is called bioaccumulation. These organisms are almost always plants or algae.

The primary consumer which eats the plants or algae, also eats the toxins within it. The toxins in this stage become concentrated.

Eventually the apex predator eats the animal below it in the food chain as well as the concentrated toxins within it.

Because the toxins cannot be excreted they are found in much higher concentrations in the organism at the top of the food chain, and the effects are much more severe. This build-up of toxins that cannot be excreted at the tops of food chains is bioconcentration.

DDT is an insecticide that can pass up the food chain from insects to small birds, and then from the small birds to birds of prey, like hawks. DDT increases in concentration within animals, as it moves up the stages of a food chain.

For example, an insect consumes a little DDT when it eats a plant that DDT has been sprayed on. Then a mouse eats lots of insects. All that DDT in the insects, stays within the one mouse. A hawk then eats many mice. All the DDT within the mice stays within the one hawk. We call this bioaccumulation and as the concentration increases, so do the harmful effects.

Due to its damaging effects to the environment, DDT is banned in many countries including the UK.

Mercury was used until recently to make insecticides and special paints that stop barnacles growing on the hulls of ships.

Unfortunately, when mercury gets into a food chain, it damages the nervous systems and reproductive systems of mammals, including humans. The diagram shows how mercury can concentrate in the food chain.

In the sea, tiny animals and plants called plankton absorb the mercury compounds. When the plankton are eaten by small fish, the mercury they contain stays in the fish because it cannot be excreted. As the fish need to eat a lot of plankton, the concentration of mercury in them becomes higher than the concentration in the plankton.

Larger fish then eat the small fish, and larger ones still (such as tuna) eat them. This creates a high concentration of mercury in the tuna. People eating contaminated tuna may get mercury poisoning. Mercury is now banned from many chemical products and mercury use in industry is carefully regulated.

Find out more about food chains and webs.

Microplastics are tiny fragments of plastic smaller than 0.5 cm. These are often formed from larger pieces of plastic, such as drinks bottles and plastic bags, which break down slowly. Plastic does not decompose and so remains in the environment forming smaller and smaller pieces. Some rivers in England have more than half a million of these in each square metre of riverbed.

Eventually these become small enough to be accidentally consumed by living things. They bioconcentrate in food chains because they cannot be excreted. The effects of this build-up are not fully understood yet but seem to cause other chemicals to bioconcentrate as well.

It is so important to recycle all plastics we have to avoid this and live more sustainably.