Zostera marina (Linnaeus, 1753) is a flowering marine plant that occurs from temperate to subantarctic regions (Green and Short, 2003), forming meadows that are recognized as being among the most important ecosystems on the planet (Costanza et al., 1997; Duffy, 2006; Duarte et al., 2008; Dewsbury et al., 2016). Eelgrass is a foundation species, providing essential functions and services including coastal protection, erosion control, nutrient cycling, water purification, carbon sequestration, as well as food and habitat for a variety of species (Duarte 2002; Heck et al. 2003; Healey & Hovel 2004, Orth et al. 2006; Barbier et al., 2011; Fourqurean et al. 2012; Cullen-Unsworth & Unsworth 2013; Schmidt et al. 2011, 2016). Eelgrass can have a strong influence on the spatial distribution of associated fauna by altering the hydrodynamics of the marine environment (Fonseca and Fisher 1986), stabilizing sediments (Orth et al. 2006), providing abundant resources, available surface area, and increased ecological niches. Meadows also provide protection from predation by providing greater habitat complexity both above and below ground (Heck and Wetstone 1977; Orth et al. 1984; Gartner et al. 2013, Reynolds et al., 2018). Local patterns and regional differences in the taxonomic and functional diversity of assemblages associated with five Zostera marina meadows occurring over a distance of 800 km along the coast of France were investigated with the objective of determining which factors control community composition within this habitat. To this end, we examined 𝛂- and 𝜷-diversity of species- and trait-based descriptors, focused on polychaetes; bivalves and gastropods, three diverse groups exhibiting a wide range of ecological strategies (Jumars, Dorgan, & Lindsay, 2015) and having central roles in ecosystem functioning through activities such as bioturbation or trophic regime (Queirós et al., 2013, Duffy et al., 2015). Here we present the abundance (Table 1) and the functional trait database (Table 2) used for the benthic macrofauna found to live in association with eelgrass meadows in Chausey, Dinard, Sainte-Marguerite, Ile d’Yeu and Arcachon, sampled in the fall of 2019. Eight biological traits (divided into 32 modalities, Table S1) were selected, providing information linked to the ecological functions performed by the associated macrofauna. The selected traits provide information on: (i) resource use and availability (by the trophic group of species, e.g. Thrush et al. 2006); (ii) secondary production and the amount of energy and organic matter (OM) produced based on the life cycle of the organisms (including longevity, maximum size and mode of reproduction, e.g. (Cusson and Bourget, 2005; Thrush et al., 2006) and; (iii) the behavior of the species in general [i.e. how these species occupy the environment and contribute to biogeochemical fluxes through habitat, movement, and bioturbation activity, e.g. (Solan et al., 2004; Thrush et al., 2006; Queirós et al., 2013). Species were scored for each trait modality based on their affinity using a fuzzy coding approach (Chevenet et al., 1994), where multiple modalities can be attributed to a species if appropriate, and allowed for the incorporation of intraspecific variability in trait expression. Information for polychaetes was primarily extracted from Fauchald et al (1979), Jumars et al (2015), and Boyé et al (2019). Information for mollusks was obtained either from biological trait databases (www.marlin.ac.uk/biotic, www.univie.ac.at/arctictraits, Bacouillard et al 2020) or from publications (e.g. Queiros et al. 2013; Thrush et al, 2006; Caine, 1977). Information was collected at the lowest possible taxonomic level and when missing was based on data available in other species of the genus, or in some cases, in the same family (only for traits with low variability for these families).

Figure 1. Map indicating the locations of the 5 study sites of Zostera marina meadows in France: three in the the English Channel, and two in the Bay of Biscay (all sites were sampled in 6 different stations).