Subtidal sandy bedforms are under the particular protection of the NATURA 2000 directive because they constitute important habitats where 30% of the species are endemic. The ecosystems they support as well as their dynamic are however little investigated. Taking into account that bedforms areas are currently targeted for the commissioning of offshore wind farms and that a substantial need of knowledge will be required to implement environmental impact assessments, the present study aims to describe how the benthic ecosystem functioning vary in relation with (i) the type of bedform, (ii) the season as well as (iii) within bedforms (from the troughs to the crests).

We present here the biological traits database used to address these issues. Macrobenthic organisms were collected on an 80 km² area typified by various kind of bedforms and located offshore the Dunkirk harbor. Sampling were performed in the autumn of 2019 (October) and the spring of 2020 (May) at 23 stations distributed according to a stratified sampling design. A total of 9 traits were selected in order to reflect (i) the response of benthic organisms to the physical disturbance induced by sand dunes migrations and (ii) the dynamics of macrobenthic communities in sublittoral sediments. Traits were the mobility, fragility, position on substrata, average size, feeding mode, longevity, egg development, larval development and life stage. Traits were split in 31 modalities and a score was attributed to each species in order to reflect their affinity to each modality (fuzzy-coding principle; Chevene et al., 1994). A score of 0 means no affinity whereas a score of 3 stands for full affinity of a species to a given modality. Scores were attributed from data available on the online BIOTIC database (MarLIN, 2006) and completed thanks to published (Foveau et al., 2020) or unpublished databases (Garcia unpublished data; Sturbois et al., unpublished data; Rigolet et al., unpublished data). In case of missing data for certain species, scores of the higher taxonomic level (mostly genus level) were attributed. For the trait "life stage", the score corresponded to the ratio between the abundance of juvenile and the abundance of adult in the community.

The present dataset includes 107 taxa.