Syllabus Edition
First teaching 2024
First exams 2026
Indicators of Water Quality (HL) (DP IB Environmental Systems & Societies (ESS))
Revision Note
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:
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
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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