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.

⚠️ MIR class boundaries depend on the type of watercourse (bed material, catchment size) - the regulation gives them separately for each type. 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, and in mountain and upland streams the thresholds are markedly higher. For a large lowland river MIR is not calculated at all. The same MIR = 40 therefore gives class II in one river and class III in another. Without 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 three elements:

• evenness J (Pielou: H′/H′max) - where H′ is the Shannon index computed not over species but over plant communities, with their areas as the quantitative measure;
• 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;
• typological term N/P - the ratio of the phytolittoral area to the area of the whole lake, which makes it possible to compare shallow and deep lakes.

ESMI = 1 − exp[−J · Z · exp(N/P)]

The colonisation index 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 work differently: if their communities cover more than 25% of the phytolittoral, the lake is assigned class I.

ESMI requires area measurement (of the phytolittoral and of the lake), not only a list of communities. Without a bathymetric plan and marked transects it cannot be computed - which is why the lake index takes far more work than the river one.

What is needed for the calculation

MIR (rivers):

• a reliable species list (identification!) on a fixed 100 m reach,
• the cover of each species on the nine-degree scale.

ESMI (lakes):

• a list of plant communities on fixed transects and the share of each on the Braun-Blanquet scale,
• the depth range of the vegetation,
• areas: of the phytolittoral, of the lake and of the zone down to the 2.5 m isobath.

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

The biodiversity indices calculator (the link under the article) will compute H′ and Pielou’s evenness J - a component of ESMI. For ESMI, paste in the communities and their mean cover in percent, converted from the Braun-Blanquet scale, instead of taxa with abundances. It will not compute the colonisation index Z, the whole ESMI or MIR.

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; the values given above come from Journal of Laws 2021 item 1475.

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.