Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Monitoring water pollution with invertebrate indicator species

Class practical

This is a fieldwork exercise that involves sampling water from streams (ideally) or ponds, in two different situations. Sample data are provided which could be used to discuss the process if you cannot repeat the practical work.

Lesson organisation

This will depend on the access you have to a safe enough area for collecting water samples. If you do not have a suitable area for students to assess, or do not have time for students to collect their own samples, you could collect samples yourself and keep the invertebrates in a tray of water for a day or so. A video or digital camera would be useful to make a record of the animals found – especially to keep information for later identification of unfamiliar invertebrates. Accompanying adults with photography skills could take on this task. An example set of data from two streams is provided for analysis and discussion. You could work through this before or after students collect invertebrates from their own water samples.

Apparatus and Chemicals

For each group of students:

Copies of the student sheet

Copies of the indicator animals sheet

Copies of other keys (if available)

Plastic trough or enamel dish (deep enough for water at a depth of 3-5 cm without spilling)


Disposable gloves, optional (Note 2)

Digital still or video camera, optional

Health & Safety and Technical notes

Take hygiene precautions to minimise risk of infections from pond water (Note 1).
Before working outside, prepare a risk assessment of the area and put any necessary risk control measures in place (Note 2).

1 Pond water could contain disease-causing microbes. Take hygiene precautions, and ensure that students cover cuts or abrasions on their hands or lower arms with waterproof plasters or gloves. Ensure students do not eat, scratch their noses, or rub their eyes during the fieldwork. Students will need to wash their hands with appropriate cleansers after being in contact with the stream water and before eating or drinking. Refer to CLEAPSS Supplementary Risk Assessment SRA 09 09/06 School ponds for more details. See also the guidance leaflet 'Group safety at water margins'  downloadable from

2 Refer to CLEAPSS handbook section 17 for more information on planning safe outdoor activities, and to CLEAPSS supplementary risk assessment Practical activities in the school grounds (SRA 08, October 2006). Some elements of these ideas are listed in the safety notes for Biodiversity in your backyard.

You should also refer to your Local Authority or employer’s guidelines for working outside the classroom in planning these activities.

Ethical issues

Teachers should be careful to introduce the invertebrate indicator species in a way that promotes a good ethical attitude towards them, and not a simply instrumental one. Although they are simple organisms that may not 'suffer' in the same way as higher animals, they still deserve respect. Animals should be returned promptly to their natural environment or a suitable holding tank after being identified. Ethical approaches in field work require all animals to be returned to their habitat after observations have been made.


SAFETY: Observe appropriate hygiene precautions after working with pond or stream water. Carry out a risk assessment of the area where the students will be collecting freshwater organisms. Consider trip and slip hazards, particularly those associated with working in or near water. Consider also specific hygiene issues, for example, excreta left by dogs or other animals, the location of any sewage outfall, and any hazards from sharp materials in the area. Consider whether extra adult supervision is needed to reduce risk. Beware that some invertebrates such as dragonfly nymphs may bite.


a Collect all the equipment needed to collect invertebrates, and copies of keys for identifying invertebrates.


b Collect some water in a large container – about 2 cm deep.

c Collect samples of invertebrates using the net and transfer them to the tray. Try to use the same technique each time you collect a sample – holding the net in the same way for the same length of time. Scoop up some of the material from the bottom of the stream, or stir up the bottom material and place the net downstream to catch it.

d Study the catch in the tray. Try to identify the animals against the chart Invertebrate indicators of pollution (596 KB).

e After identifying and counting the animals, pour the water gently back into the pond or stream. If you cannot identify an animal, count how many there are and make a drawing or take a photograph to identify later.

f Present the results in a table. Prepare a graph showing how polluted the water samples are.
See Invertebrate indicators in two streams XLS (15 KB).

Teaching notes

Discourage students from trying to catch fish. Encourage them to treat all living things with the respect they deserve.

The pictures given on the chart you can download from this page are typical of the invertebrates you are likely find. Be aware that there are other species in each group, so the organisms you find may look different. The Field Studies Council key to freshwater invertebrates (link below) provides a more comprehensive list with colour illustrations.

These indicator animals are usually quite easy to find if the students have a bit of patience. They are all delicate and so students must handle them with care.

Identification clues:

  • Mayfly nymphs have three tails, whereas stonefly nymphs have two.
  • Caddis fly larvae collect bits of twig and stone around themselves to make a protective case. If any pieces of twig or stone seem unusually mobile, they are probably caddis fly larvae.

As well as an exercise in pollution monitoring, students can study ways in which living things are adapted to survive in their natural habitat. This follows on from considering why some organisms live only in unpolluted streams, while others survive despite pollution.

For example, bloodworms and sludge worms are red because they contain haemoglobin. They take in oxygen by diffusion over the whole of their body surface. This allows them to survive in water with lower oxygen concentrations than other similar organisms. Mayfly and stonefly nymphs breathe using external gills. They have no haemoglobin, so the oxygen circulates in their bodies in simple solution. This means they need a higher concentration of oxygen to survive than bloodworms and sludge worms. In unpolluted water, with a high concentration of oxygen, nymphs are more mobile than worms and compete more successfully for food than the worms.

Low oxygen level is not an immediate result of pollution with waste high in nitrogen. First there is an expansion in the population of plant material (a bloom). While growing, these plants will release oxygen into the water. After the initial bloom, there is not enough nitrate to maintain the population growth. When the plants die off and rot, the decomposing microorganisms use up the oxygen in the water and reduce the oxygen level.

Health & Safety checked, September 2009


Download the student sheet Invertebrate indicators of pollution (596 KB) with questions and answers.
Download the student sheet Monitoring water pollution with invertebrate indicator species (371 KB) with questions and answers.

Web links
Details of the Field Studies Council fold out guide The freshwater name trail. This could be helpful in identifying the main groups of invertebrate.

(Website accessedOctober 2011)