Lakes and Watercourses

Periphyton – diatoms
in watercourses
Introduction
Periphyton algae play an important part as a primary producer, particularly
in running waters. These algae are also a good indicator of water
quality. Analysis of periphyton algae may therefore serve to supplement
or replace chemical or other biological studies. Periphyton algae may be
the only feasible biological indicator in slow-flowing waters.
The assessment is confined to watercourses and is based on two
indices, both designed to reflect the degree of eutrophication and general
pollution impact.
Assessment of current conditions
TABLE 38.
CURRENT CONDITIONS: index, diatoms
Class Description IPS index or IDG index
1 Very high index figure ≥ 17.5
2 High index figure 14 – 17.5
3 Moderate index figure 10.5 – 14
4 Low index figure 7 – 10.5
5 Very low index figure < 7
The IPS (Indice de polluo-sensibilité) and IDG (Indice diatomique
génerique) indices are calculated as:
∑ A i I i V i /∑ A i V i
where Ai is the relative abundance of taxon i, expressed as a percentage,
V i is the indicator value of taxon i (1 – 3) and I i is the sensitivity to
pollution of taxon i (1 – 5). The result obtained using the above formula is
converted into a scale of 1 – 20 as follows:
4.75 · original index value –3.75.
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