Insights from the 3rd Italian Interlaboratory Ring Test on Benthic Diatoms

Auteurs

Falasco E., Arnaud E., Della Bella V., Wetzel C.E., Zorza R., Van de Vijver B.

Référence

Nova Hedwigia, vol. 121, n° 3-4, pp. 323-351, 2025

Description

Starting from 2007, ring tests on benthic diatom taxonomy and related indices has been spreading throughout Europe. Indeed, in the framework of the WFD application and the use of diatoms as indicators for the ecological status assessment of rivers, interlaboratory harmonization has been recognised as an important tool to assure the reliability and comparability of results. In the present work we show the results of the third Italian ring test, carried out in 2023–2024, involving 39 diatomists from different Italian laboratories and 2 international auditors. Two sampling sites were chosen, both classified as belonging to the Central (lowland) river type and having their origin in lakes. The two sampling localities were chosen to represent different ecological status, respectively good and moderate. The Ticino sampling site is located just downstream the Lago Maggiore (Lombardy, Northern Italy), the second largest lake in Italy; in the chosen stretch, riverbed is 50– 80m large and the main human pressure is represented by wastewater treatment plants. The Chiese sampling site is located downstream of Lake Idro (Lombardy, Northern Italy) and is subject to various human pressures, including the presence of residential and industrial areas, as well as water extraction for agricultural and hydroelectric purposes. The participants’ results, encompassing taxonomic composition and the diatom index calculated using the Italian ICMi, were assessed using the Bray-Curtis similarity index (with a 60% threshold) and z-scores. Important differences could be observed among participants. Most of these differences arose from taxonomic misidentifications, mainly concerning the genera Achnanthidium (in particular the A. minutissimum complex and A. delmontii) and Fragilaria (in particular the F. rinoi and F. vaucheriae complex) and/or to the overlooking of small taxa, such as Fistulifera saprophila and Mayamaea permitis. Even though long experience in diatom identification or the number of samples counted per year are important, these do not guarantee the similarity of results between participants and auditors. In this context, the main objectives of ring tests, training courses, or workshops – problem sharing and knowledge transfer – are the best tools for reducing differences in results.

Lien

doi:10.1127/nova_hedwigia/2025/1133