Determination of the biological diatom index (IBD NF T 90–354): results of an intercomparison exercise

A diatom intercomparison exercise on sampling, slide preparation, counts and identification has been carried out in September 1999 with 24 French-speaking diatomists on the River Loup (Alpes Maritimes, France). Few operators strictly respected the standard, especially for the number of individuals to be counted. It has however been demonstrated this had a very low impact on the index value and that old counts can be used for index calculation, if there are at least 300 frustules or valves. Counts higher than 400 did not lead to a better precision. The total variability largely depended on the operator and was distributed unevenly between sampling, which appears to be a critical stage, slide preparation and counting. For those who strictly respected the standard procedure, the natural variability can be estimated to about one unit value of the Biological Diatom Index. This intra-operator variability can be considered as very low, as the IBD scale is from 0 to 20, and not significant in assessing water quality. Despite this, the inter-operator variability is greater. When comparing operators' results, after excluding those who did not respect the standard requirements, the main source of variability is due to misidentifications, especially for small Achnanthes (A. minutissima and A. biasolettiana) and Cocconeis (C. placentula var. placentula, var. euglypta and var. lineata). Intercomparison exercises, internet exchanges for checking and reference material collections should be encouraged both to satisfy the internal and external quality controls and to acquaint biologists with the importance of field and laboratory protocols, as well as to allow progress in diatom identification.