Biotic indices convert information on macroinvertebrate community composition — taxon presence, abundance and pollution tolerance — into a single numerical value or ecological quality class. In Poland, the choice of index is governed by the requirements of the Water Framework Directive (WFD, 2000/60/EC) and the national classification system maintained by GIOŚ.
The BMWP Score System
The Biological Monitoring Working Party (BMWP) score was developed in the United Kingdom and has been adapted for continental European rivers. Each family-level taxon present in a sample is assigned a score from 1 (pollution-tolerant) to 10 (pollution-sensitive). The family scores are summed to give the total BMWP score.
In Polish practice, the BMWP-PL version uses adapted tolerance scores calibrated against the reference conditions of Central European rivers, where the dominant substrate, elevation and catchment geology differ from the British rivers on which the original index was based. Stonefly families such as Perlidae and Chloroperlidae carry scores of 10; Chironomidae and Oligochaeta carry scores of 2 and 1 respectively.
ASPT — Average Score Per Taxon
The Average Score Per Taxon (ASPT) divides the BMWP score by the number of scoring taxa:
ASPT = BMWP total score ÷ number of scoring taxa
ASPT reduces the influence of sampling effort on the index value and is less sensitive to the number of taxa collected. Values above 6.0 are generally associated with good ecological status in Polish river types; values below 4.0 indicate significant organic enrichment. ASPT is used alongside total BMWP in Polish national assessment because neither metric alone fully captures community change across the full pollution gradient.
EPT Richness
EPT richness — the count of Ephemeroptera, Plecoptera and Trichoptera taxa (usually at genus or species level) in a sample — is a straightforward measure of pollution sensitivity. These three orders collectively require well-oxygenated, structurally complex habitats and are among the first to disappear when organic loading, acidification or fine sediment input increases.
In Polish lowland rivers, EPT richness is naturally lower than in upland rivers due to substrate and hydrological characteristics; reference values therefore differ by river type. A lowland sand-bed river type may have a reference EPT richness of 8–12 genera, while a Carpathian gravel-bed type may reach 20–25.
WFD ecological status classes for biological quality elements in Poland: High — Ecological Quality Ratio (EQR) ≥ 0.80; Good — 0.60–0.79; Moderate — 0.40–0.59; Poor — 0.20–0.39; Bad — < 0.20. EQR = observed index value ÷ reference condition value for the river type.
The Polish MMI (Multimetric Index)
The Polish Multimetric Index (MMI) for macroinvertebrates integrates multiple biological metrics to satisfy WFD requirements for assessing all ecologically relevant aspects of community change. The index used in routine WFD surveys in Poland combines metrics covering:
- Taxonomic richness (total number of taxa)
- EPT richness (order-level)
- BMWP-PL score
- Shannon diversity index
- Relative abundance of sensitive taxa (EPT as % of total)
- Relative abundance of tolerant taxa (Oligochaeta + Chironomidae as % of total)
Each metric is standardised against type-specific reference values, and the mean of the standardised metric scores gives the final MMI value expressed as an EQR between 0 and 1.
Reference Conditions in Poland
Reference conditions represent the biological state expected in the absence of, or with only minimal, human pressure. For Polish rivers, reference communities are described using a combination of historical data, near-reference sites and predictive modelling. GIOŚ maintains type-specific reference values for each recognised Polish river type, covering lowland calcareous rivers, lowland siliceous rivers, upland rivers and mountain streams.
Comparison of Index Properties
| Index | Taxonomic Level | Sensitive to | Limitation |
|---|---|---|---|
| BMWP-PL | Family | Organic pollution, oxygen depletion | Increases with sampling effort |
| ASPT | Family | Organic pollution | Less responsive to richness changes |
| EPT richness | Genus/species | Organic pollution, acidification, sediment | Requires type-specific reference |
| MMI (Polish) | Mixed | Multiple pressures (WFD compliant) | Requires calibration per river type |
Limitations and Interpretation
No single index captures all ecologically relevant changes. Hydromorphological degradation — channelisation, bank reinforcement, removal of riparian vegetation — can reduce macroinvertebrate quality without substantially changing BMWP scores if organic water quality remains good. The Polish MMI incorporates structural metrics that partially address this limitation, but direct hydromorphological assessment using tools such as the River Hydromorphology Assessment (RHS-PL) remains complementary to biological monitoring.
References
- Armitage, P.D. et al. (1983). The performance of a new biological water quality score system based on macroinvertebrates over a wide range of unpolluted running-water sites. Holarctic Ecology 6: 380–392.
- Chief Inspectorate for Environmental Protection [GIOŚ] (2021). Classification methodology for biological quality elements — rivers. Warsaw.
- Birk, S. et al. (2012). Three hundred ways to assess Europe's surface waters: An almost complete overview of biological methods to implement the Water Framework Directive. Ecological Indicators 18: 31–41.
- European Environment Agency — Water quality data.