Harmonization is more important than experience—results of the first Nordic–Baltic diatom intercalibration exercise 2007 (stream monitoring)

Abstract

The goal of this study was a harmonization of diatom identification and counting among diatomists from the Scandinavian and Baltic countries to improve the comparison of diatom studies in this geographical area. An analysis of the results of 25 diatomists following the European standard EN 14407 during an intercalibration exercise showed that a high similarity was achieved by harmonization and not because of a long experience with diatoms. Sources of error were wrong calibration scales, overlooking of small taxa, especially small Navicula s.l., misidentifications (Eunotia rhomboidea was mistaken for Eunotia incisa) and unclear separation between certain taxa in the identification literature. The latter was discussed during a workshop with focus on the Achnanthes minutis-M. Kahlert (*) : sima group, the separation of Fragilaria capucina var. gracilis from F. capucina var. rumpens, and Nitzschia palea var. palea from N. palea var. debilis. The exercise showed also that the Swedish standard diatom method tested here worked fine with acceptable error for the indices IPS (Indice de Polluo-sensibilité Spécifique) and ACID (ACidity Index for Diatoms) when diatomists with a low similarity (Bray-Curtis <60%) with the auditor in at least one of the samples are excluded.

Quality assurance of diatom counts in Europe: towards harmonized datasets

“…However, taxon identification uncertainties often contribute much more to overall inconsistency between datasets than other error sources (Kelly, 1997 ;Prygiel et al, 2002 ;Alverson et al, 2003 ;Van der Molen & Verdonschot, 2004 ;Lavoie et al, 2005 ;Besse-Lototskaya et al, 2006 ;Kahlert et al, 2009 ). Even though all uncertainty sources should be minimized as much as possible, we think that agreement on consistent diatom identification will be the greatest challenge in the dataset harmonization.…”

“…An identification exercise might only consist of a number of samples to be counted and the comparison of the results of the participating laboratories in a report. Still, when the initial comparison is followed by a workshop and agreements on how to handle difficult taxa are attained, between analyst variation can be reduced (Kahlert et al, 2009 ). Evaluations of NorBAF (Nordic-Baltic Network for Benthic Algae in Freshwater, including Sweden, Finland and the Baltic countries) also showed that the workshop was the most appreciated part of the intercalibration exercise, where results were eagerly discussed and clarifications could be made.…”

“…For example, the identification of single taxa from the Achnanthidium minutissimum complex was problematic in most identification exercises (Table 3 ). Sweden uses ecomorphotypes based on valve width thereby avoiding the need to differentiate on a species or variety level (Kahlert et al, 2007(Kahlert et al, , 2009, whereas other countries try to clarify existing taxa concepts of this complex (e.g. Dreßler et al, 2014 ).…”

“…The reason for this is the lack of harmonization of diatom identification and counting techniques among regions or countries, which leads to inconsistent diatom datasets if not a form of taxa harmonization is used before analysis (Vyverman et al, 2007 ;Stevenson et al, 2010 ;Kelly et al, 2014 ). Thus, to prevent dataset inconsistencies, some form of standardized quality assurance (QA) is imperative (Kahlert et al, 2009 ;Dreßler et al, 2014 ).…”

“…The term harmonization exercise emphasizes the need of taxonomic consensus between laboratories and countries (e.g. Kahlert et al, 2009 ). Intercalibration is the process that ensures that several laboratories produce compatible data (Taylor, 1987 ).…”