Comparison of the performances of two biotic indices based on the MacroBen database

The pan-European MacroBen database was used to compare the AZTI Marine Biotic Index (AMBI) and the Benthic Quality Index (BQI_ES), 2 biotic indices which rely on 2 distinct assessments of species sensitivity/tolerance (i.e. AMBI EG and BQI E[S<sub<50</sub>]_0.05) and which up to now have only been compared on restricted data sets. A total of 12 409 stations were selected from the database. This subset (indicator database) was later divided into 4 marine and 1 estuarine subareas. We computed E(S₅₀)_0.0 in 643 taxa, which accounted for 91.8% of the total abundances in the whole marine indicator database. AMBI EG and E(S₅₀)_0.05 correlated poorly. Marked heterogeneities in E(S₅₀)_0.0 between the marine and estuarine North Sea and between the 4 marine subareas suggest that sensitivity/tolerance levels vary among geographical areas. High values of AMBI were always associated with low values of BQI_ES, which underlines the coherence of these 2 indices in identifying stations with a bad ecological status (ES). Conversely, low values of AMBI were sometimes associated with low values of BQI_ES resulting in the attribution of a good ES by AMBI and a bad ES by BQI_ES. This was caused by the dominance of species classified as sensitive by AMBI and tolerant by BQI_ES. Some of these species are known to be sensitive to natural disturbance, which highlights the tendency of BQI_ES to automatically classify dominant species as tolerant. Both indices thus present weaknesses in their way of assessing sensitivity/tolerance levels (i.e. existence of a single sensitivity/tolerance list for AMBI and the tight relationship between dominance and tolerance for BQI_ES). Futurestudies should focus on the (1) clarification of the sensitivity/tolerance levels of the species identifiedas problematic, and (2) assessment of the relationships between AMBI EG and E(S₅₀)_0.05 within and between combinations of geographical areas and habitats.