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First teaching 2024

First exams 2026

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Indicators of Water Quality (HL) (DP IB Environmental Systems & Societies (ESS))

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

Indicator Species

  • Indicator species are organisms that are used to:

    • Assess the quality of an environment

    • Indicate the presence of specific environmental conditions, including pollution

  • Different groups of organisms, including invertebrates, plants, and algae, can act as indicator species in polluted or unpolluted aquatic habitats:

Chart showing pollution levels and indicator species, from low to high: Stonefly nymph, Mayfly nymph, Freshwater shrimp, Bloodworm, Tubifex worm, no life.
Fresh water pollution invertebrate indicator species

Indicators of polluted water  

1. Invertebrates

  • Bloodworms (the aquatic larvae of midges) are commonly found in polluted waters, especially those contaminated with organic matter

    • Their presence indicates low dissolved oxygen levels and high organic pollution

  • Tubifex worms also thrive in polluted waters with high organic content and low oxygen levels

2. Plants

  • The Common Reed is tolerant of high nutrient levels, such as nitrates and phosphates, often found in eutrophic or polluted waters

  • Duckweed is a small floating plant that thrives in nutrient-rich waters, including those polluted with agricultural runoff or sewage effluents

3. Algae

  • Excessive nutrients, particularly nitrogen and phosphorus, can trigger harmful algal blooms dominated by blue-green algae (cyanobacteria)

    • These blooms are often associated with nutrient pollution and indicate degraded water quality

Indicators of unpolluted water

1. Invertebrates

  • Stonefly nymphs are sensitive to water pollution and are often found in well-oxygenated, clean streams and rivers

    • Their presence indicates good water quality

  • Mayfly nymphs are also highly sensitive to pollution and require clean, well-oxygenated water

2. Plants

  • Water Crowfoot is a plant species commonly found in clean, well-oxygenated freshwater streams and rivers

    • Its presence indicates good water quality and suitable habitat conditions for other aquatic organisms, e.g. fish

3. Algae

  • Diatoms are a diverse group of algae, and their presence in high diversity and abundance is often associated with clean, well-oxygenated waters

Using indicator species

  • These organisms are just a few examples of indicator species commonly used in water quality assessments

  • By studying the presence, abundance, and diversity of these organisms, scientists can better understand the pollution levels and overall health of aquatic ecosystems

    • It is important to note that the selection of indicator species varies depending on the specific region, ecosystem, and type of pollution being assessed

  • Monitoring the presence or absence of these organisms helps to assess the condition of water bodies

    • This aids in conservation and management efforts to improve water quality and protect clean, unpolluted waters

Close-up of a brown stonefly nymph with segmented body, six legs, antennae, and two tail-like cerci on a smooth, reflective surface.
Indicator species such as the stonefly nymph can tell you that water is clean, but not how clean (Dave Huth from Allegany County, NY, USA, CC BY 2.0, via Wikimedia Commons)

Limitations of using indicator species

  • Although using indicator species is a fairly simple and cost-effective method of determining whether a habitat is polluted or not, it has some drawbacks:

    • It can't give accurate numerical (quantitative) figures for exactly how much pollution is present

    • Other factors, such as the presence of predators or disease, can also affect the presence or absence of indicator species

  • If more detailed information on pollution levels is required, non-living indicators can be used instead

    • For example, dissolved oxygen meters and chemical tests can be used to:

      • Very accurately determine the concentration of dissolved oxygen in the water

      • Show changes in levels of water pollution over time

Biotic Indices

  • Biotic indices are tools used to assess the overall health and pollution levels of an aquatic ecosystem

    • Based on the presence, abundance, and diversity of indicator species within a community

  • A biotic index provides an indirect measure of water pollution by evaluating the impact on different species according to their

    • Pollution tolerance

    • Diversity

    • Relative abundance

Step 1 = selection of indicator species

  • Indicator species are selected based on their known sensitivity or tolerance to water pollution

  • These species are representative of different ecological niches

Step 2 = sampling and data collection

  • Sampling is conducted at different sites within the water body

  • The presence, abundance, and diversity of indicator species are recorded

Step 3 = calculation of biotic index

  • The collected data is used to calculate a biotic index value

  • This is a numerical score or rating that reflects the overall quality of the aquatic habitat

  • The index is based on factors such as species diversity, tolerance values, and relative abundance

Step 4 = interpretation of biotic index

  • The biotic index is then interpreted to determine the pollution level of the ecosystem

    • Higher biotic index values indicate cleaner or less polluted waters, whilst lower values indicate higher pollution levels

  • For example, the Trent Biotic Index is a widely used biotic index for assessing freshwater pollution

    • It focuses on macroinvertebrates (insects, crustaceans, molluscs) as indicator species

    • The index assigns tolerance values to different species based on their known sensitivity to pollution

    • If the Trent Biotic Index score for a particular section of the river is high, it suggests a healthy and less polluted ecosystem

      • This is because it indicates the presence of a diverse community of pollution-sensitive macroinvertebrates

    • Conversely, a low Trent Biotic Index score indicates poor water quality and higher pollution levels

      • In this case, pollution-tolerant species dominate the community

Water Quality Index (WQI)

  • A Water Quality Index (WQI) is a single number representing the overall quality of water in a particular area

    • The WQI simplifies complex water quality data into one clear score

    • This makes it easier to assess and compare water quality across different locations

  • WQI is calculated by combining results from multiple water quality tests

    • Each of these measures a specific factor or pollutant in the water

How water quality index is calculated

Individual water quality parameters

Diagram showing factors affecting Water Quality Index (WQI): TDS, BOD, DO, pH, nitrates, faecal coliform, temperature, turbidity, suspended solids.
Commonly used water quality parameters
  • Common parameters include:

    • pH

    • Dissolved oxygen

    • Biochemical oxygen demand

    • Turbidity

    • Temperature

    • Nitrate levels

    • Total dissolved solids

  • Each parameter has its own rating scale, which indicates how contaminated the water sample is

Weighted averaging

  • Each parameter is given a weighting based on its importance to water quality

    • E.g. dissolved oxygen is often given a high weighting because it is crucial to the survival of fish and certain other aquatic organisms

  • The weighted scores for each parameter are combined to produce a single WQI score

    • E.g. the WQI score for a sample may range from 0 (poor quality) to 100 (excellent quality)

Vernier’s WQI example

  • Vernier, a scientific equipment company, provides tools and calculators for measuring WQI using several key water quality parameters

    • Vernier’s WQI incorporates parameters like temperature change, faecal coliform, BOD, and nitrate levels

    • E.g. a Vernier WQI of 75 might indicate “good” water quality, suitable for most uses, whereas a score of 40 would show poor quality, requiring treatment before use

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.