The method is based on the principle that different
aquaticinvertebrates have different tolerances to
pollutants. In the case of BMWP, this is based on the sensitivity/tolerance to
organic pollution (i.e. nutrient enrichment that can affect the availability of
dissolved oxygen). It is important to recognise that the ranking of sensitivity/tolerance will vary for different kinds of pollution. In the case of BMWP/Organic pollution rankings, the presence of
mayflies or
stoneflies for instance indicate the cleanest waterways and are given a tolerance score of 10.[2] The lowest scoring invertebrates are worms (Oligochaeta) which score 1. The number of different macroinvertebrates is also an important factor, because a better quality water is assumed to contain fewer pollutants that would exclude "sensitive" species - resulting in a higher diversity.
Kick sampling, where a net is placed downstream from the sampler and the river bed is agitated with the foot for a given period of time (the standard is 3 minutes), is employed. Any macroinvertebrates caught in the net are stored and preserved with an alcohol solution, and identified to the
family level, this can be done with the live organisms as well.
The BMWP score equals the sum of the tolerance scores of all macroinvertebrate families in the sample. A higher BMWP score is considered to reflect a better water quality. Alternatively, also the Average Score Per Taxon (ASPT) score is calculated. The ASPT equals the average of the tolerance scores of all macroinvertebrate families found, and ranges from 0 to 10. The main difference between both indices is that ASPT does not depend on the family richness. Once BMWP and ASPT are calculated, the Lincoln Quality Index (LQI) is used to assess the water quality in the Anglian Water Authority area.
Other indices that can be used to assess water quality are the
Chandler Score, the
Trent Biotic Index and the Rapid Bioassessment Protocols.[3]