The macrophyte index (MIR, ESMI) - how aquatic vegetation is assessed
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 river reach on the basis of a list of species and their cover. Each species is assigned:
- an indicator value (how far the species points to fertile/poor waters),
- a weight (how “strong” an indicator it is).
The result is a weighted average of the species’ indicator values, taking their cover into account - converted into a status class. To simplify: the more species typical of clean, poor waters, the better the result.
ESMI - for lakes
For lakes the ESMI is used (the macrophyte ecological status index), which combines, among others, the diversity of submerged vegetation and the depth range of its occurrence (which depends on water transparency). A shallow range and poor vegetation indicate deterioration.
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).
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.
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