Plastics and eutrophication

Last time

Before we start: last time, in Food production, monocultures and sustainable resources, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Two ways to spoil water

A plastic bag can stay in the sea for hundreds of years. A farmer's fertiliser can kill all the fish in a lake. Both are pollution, but they work in very different ways. Today: why plastics last so long and harm wildlife, and how too many nutrients in water lead to eutrophication.

Plastics

Most plastics are non-biodegradable. Microorganisms cannot break them down, so they stay in the environment for a very long time. They pile up in landfill sites and in oceans, and sunlight only breaks them into smaller pieces called microplastics. Burning plastic is not a clean solution, because it releases carbon dioxide and sometimes toxic gases.

Harm to wildlife

Animals can become tangled in plastic, or swallow it and block their digestive system, which can kill them. Microplastics are eaten by small animals, and pass up the food chain to larger animals and people. Plastic also takes up space in habitats, and can carry harmful chemicals into the water.

Reducing the problem

Reduce the amount of plastic used, for example by using reusable bags and bottles. Reuse items, and recycle plastics so that the material is made into new products. Biodegradable alternatives and proper waste collection stop plastic reaching rivers and seas.

Water pollution

Sewage, fertilisers and slurry from farms add nutrients to rivers and lakes. Pesticides, oil and chemicals from factories can poison water life directly. Rain washes fertiliser from fields into streams. This is called run-off.

Eutrophication, step by step

Nitrate ions from fertiliser or sewage enter the water. Producers such as algae grow quickly: an algal bloom. The thick algae block light from reaching the plants below, which cannot photosynthesise, and so they die. Decomposers feed on the dead plants and algae. Their aerobic respiration increases and uses up the dissolved oxygen. With too little oxygen, fish and other animals die.

Preventing it

Use less fertiliser, and apply it at the right time, so rain does not wash it off the fields. Treat sewage before it enters rivers, and keep a strip of plants beside the stream to absorb nutrients. Do not spread manure when heavy rain is forecast.

Reading a polluted stream

A polluted stream has low dissolved oxygen, thick green algae, and cloudy water. The number of different species falls. Only species that tolerate low oxygen, such as some worms, survive. Scientists can use the species present as indicators of how clean the water is.