Human Impact on the Environment

Sewage is the waste water from homes, businesses and industry, containing human waste, food scraps, detergents, and sometimes industrial chemicals. When raw or poorly-treated sewage enters a water body, it causes a similar oxygen-depletion problem to fertiliser-driven eutrophication.

How sewage damages aquatic ecosystems

1. Sewage contains lots of organic matter that bacteria can digest.
2. Aerobic bacteria multiply rapidly, breaking down the organic matter.
3. As they respire, they use up the dissolved oxygen in the water.
4. Oxygen-sensitive species (mayflies, stoneflies, trout) die or move away.
5. Only species that can tolerate low oxygen survive (sludge worms, blood worms, rat-tailed maggots, certain bacteria).
6. Biodiversity collapses.

Sewage also contains pathogenic bacteria and viruses that can cause disease in humans who drink the water, swim in it, or eat shellfish from it.

Bioindicators: using species to measure pollution

You can tell how polluted a water body is by looking at which species are living in it. Different species have different tolerances for oxygen and pollution:

Sampling a stream with a kick net and identifying what is there is a standard way to assess water quality. Bioindicator species are useful because they reflect conditions over weeks or months, not just a single moment (which a chemical test would).

Lichens as air-quality bioindicators

The same idea works for air. Different lichens (small symbiotic organisms growing on rocks and tree bark) have different tolerances for sulfur dioxide:

A walk through a city centre and a walk through unpolluted countryside will quickly show this pattern.