BMWP-PL - how to calculate the macroinvertebrate biotic index

Knowledge base · July 26, 2026

A study of benthic macroinvertebrates ends with a list of organisms - and the BMWP-PL index turns it into a single, comparable number describing water quality. It is one of the most popular biotic indicators; it is worth understanding how it arises.

The idea: points for each family

BMWP (Biological Monitoring Working Party) rests on a simple notion: each family of macroinvertebrates is assigned a score reflecting its sensitivity to pollution.

  • sensitive families (e.g. many mayflies, stoneflies, caddisflies) → high scores,
  • tolerant families (e.g. some chironomids, oligochaetes) → low scores.

BMWP-PL is the version adapted to the Polish fauna (with the family scores chosen accordingly).

How to calculate it

  1. Identify the organisms to family level.
  2. For each family present, read off its score from the BMWP-PL table.
  3. Sum the scores of all families → this is the BMWP.
  4. Calculate the ASPT (Average Score Per Taxon) = BMWP ÷ the number of scored families.

What counts is the presence of families (not the abundance of individuals).

How to read the result

  • high BMWP and ASPT → clean, well-oxygenated water, rich in sensitive families,
  • low values → loading (most often with organic matter and oxygen deficiency).

ASPT tends to be more stable than BMWP alone, because it depends less on the size of the sample. The greatest value lies in comparison over time of the same site and method.

The most common pitfalls

  • imprecise identification (to family) - it changes the result,
  • a different sample size/method - compare only what was collected in the same way,
  • treating a single result as a verdict - what counts is the trend.

In practice

The raw data for the index is a list of families from a site and a date. In LimnoLog you record them as custom indicators (families / BMWP value / ASPT) and track the result over time alongside the water parameters. On what the index itself is against the background of other elements, we write in elements of status assessment.

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