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

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:

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

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

Awaiting image: Water quality index

Image caption: 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