Detecting Pollution (WJEC GCSE Biology)

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Naomi H

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Naomi H

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Signs of Pollution

  • Several factors can be assessed when seeking to determine the level of pollution present in a habitat, e.g.
    • Indicator species, oxygen concentration and pH levels can be assessed when analysing pollution in a stream
    • Indicator species can provide information about air pollution

Water pollution

Stream indicator species

  • Water pollution occurs when, e.g. raw sewage or fertilisers are released into a body of water, such as a stream
    • This causes microorganisms in the water to increase in number
    • As these microorganisms respire they use up the oxygen in the water
  • Some invertebrate species, e.g. stonefly larvae and freshwater shrimps, are highly sensitive to the concentration of dissolved oxygen in the water, making them good indicator species for water pollution
    • E.g. the presence of stonefly larvae in a stream or river indicates that the water is not polluted
  • On the other hand, some other invertebrate species, e.g. mosquito larvae and blood worms, are unaffected by polluted environments
    • E.g. the presence of blood worms in a body of water, especially if stonefly larvae are absent, indicates a very high level of water pollution 

Freshwater indicator species diagram

Invertebrates species that are found at different levels of pollution

The presence or absence of certain species in a freshwater habitat can be used as an indicator of pollution levels

Oxygen & pH

  • Indicator species, as the name suggests, can be a useful indication of pollution levels, but as their presence can be affected by factors other than pollution, it can be useful to measure other variables when assessing pollution in a stream, e.g.
    • Oxygen concentration can be measured using an oxygen meter; low oxygen levels can indicate high levels of pollution
    • pH levels can be assessed by taking water samples and testing with an indicator; changes in pH can be a sign of water pollution

Air pollution

Lichen as an indicator species

  • Lichen can be used as an indicator species for air pollution
  • Lichens are highly sensitive to sulfur dioxide concentrations in the environment
    • Sulfur dioxide is one of the air pollutants released from car exhausts and power stations during the combustion of fossil fuels
  • The abundance and type of lichen species growing at a particular location can be monitored to determine levels of air pollution, e.g.
    • A high abundance of lichen indicates clean air
    • A high abundance of bushy lichens indicates very clean air
    • A high abundance of crustose lichens in the absence of leafy lichens indicates more polluted air
leafy-lichen crustose-lichen
Флорист, CC BY 4.0, via Wikimedia Commons Trio3D, CC BY-SA 4.0, via Wikimedia Commons

Bushy lichens (left) indicate clean air while crustose lichens (right) indicate polluted air

Analysing data to determine air pollution levels

  • Analysis of lichen distribution data can provide information about levels of air pollution
  • E.g. the maps below show the distribution of a bushy lichen species (Usnea subfloridana, also known as 'old man's beard') and a crustose lichen species (Xanthoria parietina) in England and Southern Scotland between 1960 and the present day

Lichen distribution map

UK map showing the distribution of a bushy lichen and a crustose lichen species

Lichen monitoring over time provides data on species distribution that can provide information about air pollution

  • These data suggest the following possible conclusions:
    • Air pollution levels are higher around large cities
      • Crustose lichen levels are higher in built-up areas such as London and Sheffield, while bushy lichen growth is very low in these areas
    • Air pollution was spread over a wider area between 1960-1999 and then became more concentrated in specific regions between 2000-2019
      • For crustose lichen the blue dots are quite spread out, but the areas of red dots are highly concentrated
    • Air pollution levels in central Wales, the South-West and the North-East are lower
      • Bushy lichens are most common in these areas

Problems with indicator species

  • While the above points are possible conclusions from the data provided, it is worth noting that:
    • Lichen growth may be affected by factors other than air pollution; this means that the patterns in the data don't always fit what we might expect, e.g.
      • There are many crustose lichens in the Welsh borders between 200-2019, but there are no large cities here that could explain this in terms of air pollution
        • This could perhaps be connected to nitrogen released from agriculture rather than city air pollution
      • There also appears to be some growth of bushy lichens around Sheffield, which doesn't fit with the above conclusion about high levels of air pollution here
    • Indicator species do not provide quantitative measurements for pollution levels
    • This data is only taken from two lichen species; multiple species would need to be analysed before being confident about any patterns

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Naomi H

Author: Naomi H

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.