The macrophyte index (MIR, ESMI) - how aquatic vegetation is assessed

Knowledge base · July 28, 2026

Mapping macrophytes gives a list of species and their cover - and macrophyte indices turn this data into a status assessment. In Poland you will most often meet two: MIR for rivers and ESMI for lakes.

The idea: species carry information

Different aquatic plants have different requirements and different tolerance of fertility and alteration. That is why the composition and abundance of the vegetation tell you about the state of the water - and the index formalises this relationship.

MIR - the Macrophyte River Index

Calculated for a 100 m river reach from the species list and their cover, with the formula:

MIR = ( Σ Lᵢ · Wᵢ · Pᵢ ) / ( Σ Wᵢ · Pᵢ ) × 10

The cover scale has nine degrees and is non-linear - successive degrees correspond to widening percentage ranges:

The result falls between 10 and 100 (10 = the most degraded waters, 100 = the least). The × 10 at the end of the formula is exactly what produces that range.

⚠️ The class boundaries are not one number for a whole country - they depend on the type of watercourse (bed material, catchment size). The Polish regulation sets them separately for each type, and the spread is wide: for a lowland sandy-loam river class I starts at 46.8 or 44.7 (depending on catchment area), for a stream in a peat-forming area at 44.5, in mountain and upland streams the thresholds are markedly higher, and for a large lowland river MIR is not calculated at all. The same MIR = 40 therefore means class II in one river and class III in another - without knowing the type of watercourse the index cannot be translated into a class.

ESMI - for lakes

For lakes, ESMI (Ecological State Macrophyte Index) is used. It is built from two independent things, and that is the most interesting part of it:

• phytocenotic diversity (H) - computed with the Shannon formula, but not over species, over plant communities, with their areas as the quantitative measure;
• the colonisation index (Z) - the ratio of the phytolittoral area (the part of the bed actually vegetated) to the area bounded by the 2.5 m isobath.

The second component is in practice coupled to transparency: the murkier the water, the shallower the submerged vegetation reaches and the smaller the phytolittoral. ESMI therefore responds to eutrophication along two paths at once - impoverished communities and a shrinking colonised zone.

The result falls between 0 and 1 (0 = maximally degraded vegetation, 1 = closest to reference). Class boundaries are assigned directly to ESMI values, without conversion into EQR - and unlike MIR they are common to every lake type in which the index is used: class I from 0.680, II from 0.410, III from 0.205, IV from 0.070 (below 0.070 - class V). Two exceptions are worth knowing: in lobelia lakes ESMI is not computed at all, and in coastal lakes a separate index, ESMI_JP, applies, with thresholds exactly half as high (0.340 / 0.205 / 0.103 / 0.035).

⚠️ “Charophyte lakes” are not a water type in the regulation - the typology divides lakes by calcium content, the Schindler coefficient and mixing regime. Charophytes carry a different force there: if their communities cover more than 25% of the phytolittoral, the lake is assigned class I.

A practical consequence easy to forget: ESMI requires area measurement (of the phytolittoral and of the lake), not only a species list. Without a bathymetric plan and marked transects it cannot be computed - and that is the main reason why the lake index is far more expensive to produce than the river one.

What is needed for the calculation

• a reliable species list (identification!) on fixed transects/reaches,
• an assessment of cover according to a uniform scale,
• (in lakes) the depth range of the vegetation.

This is why a consistent method and repeatability are so important (see mapping macrophytes).

Sources

• MIR - formula, species indicator values, weights and the cover scale: methodology developed in the team of K. Szoszkiewicz; description of the method for state monitoring - GIOŚ, macrophytes in rivers (PDF).
• ESMI - construction of the index and field survey: the method of H. Ciecierska and A. Kolada; GIOŚ - guidelines for field work (PDF).
• The class boundaries for both indices follow from the Polish Regulation of 25 June 2021 on the classification of surface water bodies (Journal of Laws 2021 item 1475) - text in ELI; MIR in Annex 7 (rivers), ESMI in Annexes 8 and 23 (lakes). We do not copy the tables here - they change between cycles. ⚠️ MIR thresholds depend on the type of watercourse (the values above belong to the types named), whereas ESMI thresholds are common to every lake type in which the index is used.

In practice

The index is only as good as the input data - the lists of species and cover from successive seasons. In LimnoLog you record them as custom indicators (species, cover, index value) and track the change over time. Macrophytes are one of the biological elements of status assessment.