Water Pollution and Deforestation

Explained

Eutrophication and deforestation, as sequences of events

Both of these topics are marked as chains. Each step causes the next, and the credit is given for the links rather than for naming the problem. Write them as sequences and they become some of the most reliable marks in the paper.

Eutrophication, step by step

1. Fertiliser is applied to fields, or untreated sewage is released, and rain washes nitrates and phosphates into a river or lake.
2. Those mineral ions are what algae need, so the algae grow rapidly. This is an algal bloom, and it covers the surface.
3. The bloom blocks light from reaching the plants below the surface.
4. Those plants cannot photosynthesise, so they die.
5. Decomposing bacteria feed on the dead plants and algae and multiply rapidly.
6. The bacteria respire aerobically, using up the dissolved oxygen in the water.
7. Fish and other aerobic organisms cannot get enough oxygen, so they die. Biodiversity falls.

The oxygen is the point of the whole story. A common weak answer says the water becomes polluted or poisonous, which misses what actually kills the fish.

Deforestation

Four consequences are examinable, and each needs its mechanism.

Carbon dioxide in the atmosphere rises, for two reasons at once. Fewer trees means less photosynthesis removing it, and burning or decomposing the felled timber releases more of it.

Soil erosion increases because tree roots were holding the soil in place and the canopy was sheltering it from heavy rain. Once the topsoil washes away, the land is far less fertile.

Less water returns to the atmosphere, because evapotranspiration from the trees was supplying it, so rainfall in the region can fall.

Biodiversity falls as habitats are destroyed, and species with small ranges can be lost entirely.

Writing the answer

Count the marks and plan that many links before you start. Use words that show causation, such as so, therefore and because, because each of those words is where a mark usually sits.

Then check that your last sentence answers the question that was asked. A perfect description of eutrophication scores nothing if the question asked for the effect on biodiversity and you never mentioned it.