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-PL is the version adapted to Polish fauna: it covers 88 families (the Kownacki and Soszka methodology, 2004), each scored from 1 to 10 according to sensitivity to oxygen deficits and to the toxic products of organic matter decomposition. An extract from the scale, with examples from each level:

The scored families with their points are in the BMWP-PL / ASPT calculator - you tick the families you found, and the calculator sums the points and computes ASPT.

⚠️ The list there holds 118 entries, not 88. In the GIOŚ handbook the list exists in two versions: the river one (App. IV, 88 entries - the one that feeds ASPT_PL inside MMI_PL) and a broader one, for lakes and dam reservoirs (App. II/III). The latter adds Osmylidae, Spongillidae, Lestidae, Nereidae, Bryozoa and Araneae among others and splits Heptageniidae into two genus-level entries: Epeorus and Rhithrogena at 8 points, the remaining genera at 6 points. The scores in both layers come from the same source. Entries off the river list are marked in the calculator; you can hide them in the list, but ticked ones still count. If you are working up a river sample, you stay at family level: the river list (App. IV) scores the whole Heptageniidae family at 8 and never goes lower.

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.

Example. You found four families in the sample: Leptoceridae (10), Baetidae (6), Gammaridae (6) and Chironomidae (3).

BMWP = 10 + 6 + 6 + 3 = 25 ASPT = 25 ÷ 4 = 6.25

What counts is the presence of families (not the abundance of individuals): ten freshwater shrimps and one count the same.

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 an oxygen deficit).

ASPT is more stable than BMWP alone, and that is arithmetic rather than taste: BMWP grows with every family found, so it rewards greater effort - longer sampling gives a higher sum at the same site. ASPT divides by the number of families, so it measures the quality of the assemblage, not the size of the sample.

What BMWP-PL does not do: it is not the official classification

Five quality classes were historically defined for BMWP-PL itself, but the ecological status of rivers in the Polish state monitoring is not assessed with this index today. The index in force is MMI_PL (the Polish Multi-Metric Index), in which ASPT is one of six components - the one with the greatest weight. You do not put raw values into the formula but normalised values (WJE, the ecological quality ratio):

MMI_PL = 0.334 × WJE(ASPT_PL) + 0.266 × WJE(log₁₀(Sel_EPTD + 1)) + 0.167 × WJE(S) + 0.083 × WJE(EPT) + 0.083 × WJE(H′) + 0.067 × WJE(1 − GOLD)

Each metric is first normalised to an ecological quality ratio: WJE = (metric − DPZ) / (GPZ − DPZ), where DPZ and GPZ (the lower and upper anchor points) are the 5th and 95th percentile of that metric’s distribution in the biocenotic river type (six types; values in table 3 of the GIOŚ handbook, pp. 148-149). A result below 0 is recorded as 0, above 1 as 1. S is the number of families, EPT the number of mayfly, stonefly and caddisfly families, H′ the Shannon-Wiener index computed from family abundances (natural logarithm), and GOLD the share of snails, oligochaetes and dipterans. The class boundaries depend on the biocenotic type of the river - different for a mountain stream and a lowland river (table 4, pp. 150-151).

In practice: BMWP-PL and ASPT serve well for your own monitoring and comparisons over time at the same site. Do not present the result as a “WFD ecological status class” - it is a different measure, not a different name for the same one.

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.

Sources

• BMWP-PL family scores (river list: 88 entries, 1-10 points) - we cite the score table from the GIOŚ edition: A. Kolada (red.) (2020), Podręcznik do monitoringu elementów biologicznych i klasyfikacji stanu ekologicznego wód powierzchniowych. Aktualizacja metod, Biblioteka Monitoringu Środowiska, GIOŚ, Warszawa, ISBN 978-83-950881-2-4 - PDF, App. IV (rivers), p. 160 and App. II/III (lakes and dam reservoirs), pp. 173-174 and 293-294.
• Source methodology: A. Kownacki, H. Soszka (2004), Wytyczne do oceny stanu rzek na podstawie makrobezkręgowców oraz do pobierania prób makrobezkręgowców w jeziorach, Zakład Ochrony Przyrody PAN, Warsaw.
• Scientific basis of the method: T. Fleituch, H. Soszka, D. Kudelska, A. Kownacki (2002), Macroinvertebrates as indicators of water quality in rivers: a scientific basis for Polish standard method, Archiv für Hydrobiologie Supplement 141: 225-239.
• List for the calculation: our BMWP-PL / ASPT calculator.
• MMI_PL - formula, WJE normalisation, anchor points and class boundaries by biocenotic river type: GIOŚ (2020), Handbook (as above), chapter Makrozoobentos w rzekach, §3.7 and §4, pp. 148-151; additionally methodological chapter (PDF).
• Methodology for lakes (the LMI index, separate from the river one): GIOŚ - method development (PDF).

🧮 Tool: BMWP-PL / ASPT calculator - mark the families found, and we will calculate the biotic index and ASPT for you. (Polish BMWP-PL variant.)

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.