Identifying benthic macrofaunal assemblages and indicator taxa of intertidal boulder fields in the south of the Bay of Biscay (northern Basque coast). A framework for future monitoring

Abstract The French southern marine subregion of the Bay of Biscay presents local environmental features explaining the presence of specific communities in remarkable habitats. The aim of the study is, for the first time, to assess benthic macrofaunal assemblages and to identify indicator species of each assemblage within intertidal boulder field habitats in this marine subregion. Mobile boulders were sampled with a stratified random sampling design in upper and lower midlittoral zones in March–June 2015. Sessile (in percentage cover) and mobile macrofauna (in abundance) communities were identified and counted within 0.1 m 2 quadrats. Ecological function and precision between the two groups were found dissimilar thus species richness, abundance, macrofauna distribution and indicator species/taxa of sessile and mobile macrofauna were analyzed separately. 78 species/taxa of macrofauna were recorded. A restricted list of 12 singletons (8 for mobile and 4 for sessile macrofauna) and 33 combinations of species/taxa were identified as significant indicators of each assemblage. Species ecological features (food webs, signs of disturbance, alien species, biogeographical range limit) were also considered as additional selection criteria. Therefore, species with a high ecological interest but not considered statistically as valid indicators were also highlighted. Thus, this work constitutes a framework for future monitoring of the Basque intertidal rocky shore and could be used as an alternative to sampling the entire biodiversity. It also meets the MSFD requirements (e.g. take into account marine subregion specificities; integrate the metric “macrofauna” and supports several descriptors as the D1 “Biodiversity” and D6 “Seafloor integrity”).