An experimental approach to estimate uncertainty of diatom community analysis in the accreditation process

Diatoms, belonging to the class Bacillariophyceae, are one of the biological elements required for ecological quality status (EQS) assessment according to the Water Frame Directive. Italy has adopted the Intercalibration Common Metric index (ICMi) to evaluate the EQS using diatoms. Developed through the InterCalibration exercise and validated at the European level, the ICMi still presents uncertainty elements related to ecological, spatial and temporal heterogeneities. Quality control assurance has become necessary for ecological tests due to their importance in data acquisition during monitoring activities. The aim of this work was to describe the approach used for the measurement of sampling and/or laboratory phase uncertainty associated with ICMI according to ISO/IEC 17025. Following ISO recommendations, we adopted the “top-down” approach to evaluate uncertainty from repeatability using experimental data. The study was designed to determine the variability of the method in relation to both the sampling and laboratory phases. The approach consisted of analyzing three samples, each with Bad, Moderate or High ecological quality status, ten times, plus ten replicates of sampling from the Moderate status site. Sampling, treatment, analysis, identification and counting were performed according to standard procedures. The ICMi was calculated for all samples. The experiment was performed under the following repeatability conditions: same operator, microscope and iconographic guides. We obtained a standard deviation of repeatability in each series. This study is an attempt to quantify, for the first time, the variability of the diatom analysis process through the value of the ICMi, trying to reduce at least the main sources of errors (identification and counting). It represents an approach to be followed for the accreditation process of a diatom-based laboratory test method according to ISO/IEC 17025. •Diatoms, are one of the biological elements required for ecological quality status (EQS) assessment according to the Water Frame Directive 2000/60/EC•The Intercalibration Common metrics ICMi is the method adopted by Italy for the EQS assessment using diatoms•This work described the approach used for the measurement uncertainty associated with ICMI according to ISO/IEC 17025•This study attempted to quantify, the variability of diatom analysis trying to reduce at least the main sources of errors