HL only: Topic 4.4 Water pollution. A biotic index is a numerical, indirect measure of water quality based on the pollution tolerance, relative abundance and diversity of organisms in an aquatic community. It measures the biological effects of pollution rather than pollutant concentrations directly.
How the mechanism works
- Aquatic macroinvertebrates are collected using kick sampling.
- Organisms are identified to a specified taxonomic level.
- Each taxon is assigned a score reflecting tolerance to organic pollution and low dissolved oxygen.
- Presence, and sometimes abundance, is converted into scores and combined into an index.
- The index is compared with water-quality categories; the scale depends on the index used.
| Indicator response | Interpretation |
|---|---|
| Many pollution-sensitive taxa, such as mayfly and stonefly nymphs | Usually indicates high dissolved oxygen and relatively good water quality |
| Mainly pollution-tolerant taxa, such as some worms and midge larvae | May indicate organic pollution, high biochemical oxygen demand (BOD) and low dissolved oxygen |
| High diversity with sensitive taxa present | Generally suggests a less-stressed aquatic ecosystem |
For example, the UK Biological Monitoring Working Party (BMWP) system assigns scores to macroinvertebrate families by pollution sensitivity. A site with several sensitive families receives a higher total score than one dominated by tolerant organisms.
Strengths and limitations
A biotic index integrates effects over time because organisms experience conditions continuously. However, temperature, flow, substrate, season, sampling effort and identification errors can alter results. It should be interpreted alongside chemical and physical measurements.
The common misconception is that one indicator species proves pollution. In reality, the index assesses the wider community, and non-pollution variables must also be considered.
Exam technique
For an explain question, link organic pollution to increased BOD, reduced dissolved oxygen, loss of sensitive taxa and dominance of tolerant taxa. For an evaluate question, balance time-integrated biological evidence against seasonal variation, sampling bias and non-pollution factors.