Bioaccumulation
🎯 In this topic you will
- Explore the chemical DDT and its historical uses
- Use a model to explain how DDT moves through a food chain
- Understand the concept of bioaccumulation and why it occurs
🧠 Key Words
- accumulate
- bioaccumulation
- biodegradable
- biomagnification
- insecticide
- persistent
- toxic
Show Definitions
- accumulate: To gradually gather or build up a substance in a particular place or organism over time.
- bioaccumulation: The build-up of toxic substances in an organism over time, often from its environment or food.
- biodegradable: Capable of being broken down naturally by microorganisms into harmless substances.
- biomagnification: The increase in concentration of toxic substances as they move up the food chain.
- insecticide: A chemical used to kill or control insects.
- persistent: Describes a substance that does not break down easily and remains in the environment for a long time.
- toxic: Harmful or poisonous to living organisms.
🧪 What is DDT?
DDT is an insecticide. This means that it kills insects.
🔬 Early Uses of DDT
DDT was first produced in the 1940s. It was used to kill insects that transmit diseases. It was especially useful for killing mosquitoes that transmit malaria, and fleas that transmit a disease called typhus. DDT was also used to kill insects that eat crops.
⚠️ Lack of Awareness
No one thought that DDT could harm organisms other than insects. This old picture was taken in the 1940s. It shows a beach being sprayed with DDT to kill mosquitoes. The people on the beach are being sprayed, too.
📚 Rachel Carson’s Discovery
DDT is very good at killing insects. But gradually, people began to realise that it was also harming animals that no one wanted to kill. In 1962, an American author called Rachel Carson wrote a book called Silent Spring. She described how DDT was killing not only mosquitoes, but also birds.
🌍 Environmental Impact
Her book made many people realise that some insecticides, including DDT, are very harmful to the environment. Scientists now understand how it causes harm to ecosystems.
🧪 Did you know?
DDT was once sprayed on public beaches — with people present — before scientists realised it could harm birds and entire ecosystems.
🌿 DDT and Environmental Persistence
We now know that DDT does not break down. It is a persistent chemical. It stays in the environment for many years. It is not broken down by decomposers.
💨 How DDT Spreads
When DDT is sprayed, some of it is carried up high into the air. It can be blown for very long distances, far away from where it was used.
🧬 DDT in Animal Bodies
When DDT gets into an animal’s body, it stays there for the whole life of the organism — it never breaks down.
🐣 Effects on Birds and Eggs
DDT is very harmful to many kinds of animal. It is toxic (poisonous). For example, it makes the shells of birds’ eggs very thin and easy to break. This old photograph shows some eggs of a bird called an ibis. The eggs did not hatch, because the female ibis that laid them had DDT in her body.
📌 Important Concept
Bioaccumulation of Persistent Pollutants: Chemicals like DDT do not break down in the environment and can build up in the bodies of organisms. Over time, they accumulate in food chains, causing long-term harm to animals, such as thinning eggshells in birds.
🧪 Bioaccumulation Explained
Imagine that DDT has been sprayed onto some water. Tiny algae take up some of the DDT. Shrimps eat the algae, and fish eat the shrimps. Cormorants (fish-eating birds) eat the fish.
📈 What is Bioaccumulation?
All the DDT in all of the algae that a shrimp eats over its lifetime accumulates, or builds up, in its body. The longer the organism lives, and the more DDT it takes in, the more DDT it gets in its body. This process is called bioaccumulation.
🐟 How DDT Moves Through the Food Chain
All of the DDT in all of the shrimps that a fish eats accumulates in the fish’s body. Eventually, all the DDT in all of the fish that a cormorant eats in its lifetime accumulates in the cormorant’s body.
📊 What is Biomagnification?
This means that the concentration of DDT in an animal’s body increases as you go up the food chain. This is called biomagnification.
📉 DDT Concentration in the Food Chain
The next diagram shows how the concentration of DDT in the bodies of species in a food chain increases along the chain. The concentration is measured in parts per million (ppm). This is the number of grams of DDT in one million grams of the organisms.
⚠️ Common Mistake
Don’t mix up bioaccumulation and biomagnification — bioaccumulation happens within one organism over time, while biomagnification happens across the food chain.
❓ QUESTIONS
1. How many times greater is the concentration of DDT in a cormorant’s body than in a minnow’s body?
👀 Show answer
2. Explain, in your own words, why the concentration in the cormorant is greater than in a minnow.