These effects are:

1. Fertiliser runs into the water from the fields.

2. The nitrates and phosphates increase the growth of algae in the water.

3. The algae grow very rapidly until light becomes the limiting factor

4. Many algae die, because they don't get enough light.

5. Bacteria break down the dead algae using oxygen from the water.

6. The concentration of dissolved oxygen in the water drops and aquatic animals can die.

7. Sometimes oxygen is bubbled into the the water to restore the oxygen level.

8. The measure of the seriousness of the eutrophication can be estimated using increased biochemical oxygen demand (BOD).

Explore the examples on the map (click to enlarge).

Read more about these four examples.

Around 1960 the sparrowhawk population decreased throughout Britain because pesticide use contaminated its prey. Once inside the body of the sparrowhawk, DDT concentrated in fatty tissues and badly affected the deposition of calcium in the egg shells. High concentrations of DDT cause thin-shelled eggs which break and don't survive.

The Nile crocodile in Lake Kariba illustrates a substantial biomagnification of DDT through food-web accumulation from herbivores to top carnivores. The use of DDT in agriculture was banned in Zimbabwe in 1982, but DDT continues to be used to control mosquitoes and tsetse flies, insects that spread malaria. The concentration of DDT in the water of Lake Kariba was less than 0.002 ppb, but planktonic algae contained 2.5 ppm. A filter-feeding mussel had 10 ppm, while two species of plant-eating fish contained 2 ppm, and a bottom-feeding fish contained 6 ppm. A predatory fish and the great cormorant each contained 10 ppm. The Nile crocodile, the top predator in Lake Kariba, had 34 ppm of DDT.

Orcas / killer whales eat sharks and other predatory fish.This puts them at the top of the food chain. Algae contain low levels of PCBs but are consumed in huge numbers by ocean herbivores, then eaten by predatory fish and sharks.The concentration of PCB increases in the fat tissues of each animal. Orcas collect the highest PCB concentrations.

DDT affects the thickness of Bald Eagle eggshells, causing them to be fragile, and often crushed by the parents in the nest. Although DDT levels to which the birds had been exposed were low, their bodies were found to contain one million times the concentration that was present in the environment. This is bioaccumulation, the steady increase in concentration of a contaminant with increasing level in the food chain. DDT is not metabolized, and does not break down in the body.It is much more soluble in fat than in water. So it accumulates in body fat. Since DDT is not excreted, the carnivore accumulates all the DDT that was present in all of the prey organisms in their food chain.

2. Why does a predator accumulate a higher concentration of DDT than the prey which it eats?

3  Name two examples of Bioaccumulation.

Watch the short Biomagnification video and the historical DDT footage (10 minutes)  Pesticides DDT Rachel Carson Silent spring

1. What do you think could be the next case like DDT? 

2. Are there any chemicals used widely in the world today which could be having a bad long-term affect on human health?

3. How would you communicate today your concerns about this 'silent danger'?