9.16B - Pollution indicators

9.16B(a,b) — Water-quality indicators

This part covers the Higher-only content in 9.16B. An indicator species is an organism whose presence, absence or abundance provides evidence about environmental conditions. It is evidence to interpret, not a device that directly reads a pollution concentration.

In water, different invertebrates tolerate different conditions. When biodegradable organic waste enters a stream, decomposer populations may increase and respire more, using dissolved oxygen. This can leave low-oxygen conditions that exclude sensitive species but allow more tolerant species to remain.

• Bloodworms and sludgeworms can tolerate polluted water and the low dissolved-oxygen conditions often associated with biodegradable organic pollution. Finding many of them, especially when oxygen-sensitive species are absent, supports the inference that water is polluted.
• Freshwater shrimps and stoneflies are associated with cleaner, well-oxygenated water. Stoneflies are particularly sensitive to pollution and low oxygen, so their presence supports the inference that water quality is good.

This does not mean that one organism proves a pollution level. Flow rate, temperature, pH, sediment, the riverbed, food supply, season and sampling effort can all change which species are found. Stronger water-quality evidence uses several species and their abundance, standardised repeated sampling at comparable sites, and direct measurements such as dissolved oxygen or the concentration of a suspected pollutant.

9.16B(c) — Air-quality indicators

Air-quality indicators require the same care.

• Different lichen species have different sensitivities. Some pollution-sensitive lichens decline as sulfur dioxide or nitrogen pollution increases, while some pollution-tolerant lichens remain or become more abundant. Identifying the species is therefore more informative than simply recording lichen present.
• Blackspot fungus on roses is inhibited by high sulfur dioxide. Blackspot on suitable rose leaves can support the inference that sulfur dioxide is low, while its absence can support the inference that sulfur dioxide is high. However, rose variety, leaf moisture, fungicide use and whether fungal spores reached the plant can also affect infection.