Biological Monitoring Working Party (BMWP)

You can use aquatic invertebrates (insects which live in the beds of streams and rivers) to tell you something about the quality of the water. Some aquatic invertebrates need very clean water to thrive others can tolerate fairly polluted water. The Biological Monitoring Working Party (BMWP) have devised a scoring system which you can use according to the types of invertebrate you find in your stream or river bed.

A high BMWP Score -or Biotic Index -as it is sometimes called, means the invertebrate is very sensitive to pollution, therefore, for the invertebrate to be present - the water must be clean. An example of an animal that has a very high Biotic Index is a Mayfly nymph (its biological name is Ephemeroptera).

Aquatic invertebrates that have a low Biotic Index -suggesting they can tolerate fairly high pollution levels are; - Non-biting midge larvae (Diptera) and true worms (Oligochaeta). You are likely to find a lot of these and they can range in colour from bright red to brown.

Table to allocate the Biotic Score to the Invertebrates

Method:

1. Identify and then count all of the different invertebrates you have collected at each site

2. Give a 'score' to each invertebrate using the table above – so at Site One you may have 12 True worm (12 x 1 = 12), 6 Leeches (6 x 3 = 18) and 2 Mayfly nymphs (2 x 10 = 20).

3. Add up the total BMWP score at each site e.g. (12 + 18 + 20 = 40)

4. Divide your total BMWP score (40) by the total number of invertebrates at each site e.g. (12 + 6 + 2 = 20) – so 40/20 = 10.

5. The Average Biotic Index for Site One is 10.

6. Repeat this for your remaining sites.