Dataset of macrobenthic species, organic matter content and grain-size distribution in surficial seafloor sediments in outer continental shelf, pockmark fields and Capbreton Canyon tributaries in the southeastern Bay of Biscay

This dataset presents the macrobenthic species abundance and biomass in soft bottom areas located at southeastern Bay of Biscay. Data on organic matter content and grain-size distribution in surficial seafloor sediments is also provided. Samples were obtained with Shipek and Smith-McIntyre type grabs in several surveys carried out between 2010 and 2020, covering a range of depths of 32–2241 m. Abundance and biomass of macrobenthic species are given. This database is useful for (i) research on spatial variability of macrobenthic communities, and (ii) baseline knowledge of species in the area. The research article on these data [1] was published in the journal Regional Studies in Marine Science. Title: Macrofaunal variability in the continental shelf and canyons in the southeastern Bay of Biscay.


Value of the Data
• The dataset is useful for studies on benthic communities' ecology in the Basque continental shelf and Capbreton Canyon tributaries' area, as information on benthic communities in the studied area is scarce. • The results obtained in this research provide valuable information to be compared with other regions of the world. • The dataset allows to give a baseline of macrobenthic diversity within the area being useful for studies in marine macroecology or studies in presence of allochthonous species. • The dataset is useful for researchers in marine benthic ecology, as well as for management agencies and governments.

Data Description
Here we provide a dataset on macrobenthic species density and biomass in soft bottom seafloor located in the south-eastern Bay of Biscay (east Atlantic Ocean), together with sedimentological data on organic matter content and grain-size distribution of surficial sediments.
Densities and biomass (dry weight) were calculated per square meter.
Regarding the sediment data, mean grain size, percentage of gravel, sand and mud contents and organic matter content are provided. These data are provided in the file "Sediment Characteristics".    Sediment samples were taken with a 0.04 m 2 Shipek ( Fig. 2 ) and 0.1 m 2 Smith-McIntyre grabs ( Fig. 3 ). For each sample, a subsample of sediment of ca. 140 cm 3 was extracted and frozen for sediment analysis in laboratory ( Fig. 4 ). The remaining sample was sieved through a 1 mm mesh size ( Fig. 5 ) to extract macrofauna. The sample was preserved in 10% neutralized formalin for laboratory analysis. Although this leads to an effective reduction of the sample, the abundances of macroinfauna were not corrected, and therefore, the densities and biomass were calculated as if the total area sampled was 0.04 m 2 or 0.1 m 2 (for Shipek and Smith-McIntyre samples, respectively).

Experimental Design, Materials and Methods
Macroinfauna was identified to the lowest taxonomic level possible and counted by the same group of taxonomists (Sociedad Cultural INSUB, Donostia, Spain) for all the campaigns. Biomass was measured after drying at 90 °C for 24 h.
The particle size distribution of sediments was estimated by (a) dry sieving (only samples with low mud content) or (b) laser diffraction combined with wet sieving of the > 2 mm fraction. The method used for each one was as follows: a) Dry sieving was carried out using an analytical sieve shaker for 15 min with a set of stainlesssteel sieves (mesh sizes: 63, 125, 250, 500, 1000, 2000, and 4000 μm). b) The laser diffraction particle size analyser (LDPSA) used was a Beckman-Coulter LS 13 320, with a 750 nm laser beam (software version: 4.19). The measurement time adopted was 60 s, with 9-11% of obscuration. Sonication was applied (with a power of 73 W) for 30 s before and during measurement. The Fraunhofer diffraction model was used in the analysis. LDPSA  was combined with determination of gravel fraction (i.e., > 2 mm) by wet sieving. The results of the measurements done with laser diffraction were transformed into dry sieving equivalent according to Rodríguez and Uriarte [3] as indicated in Table 1 .
Mean grain size was calculated using the graphical method by Folk and Ward [4] with the software GRADISTAT [5] . Percentage of gravel corresponds to the percentage of particles of sediment ≥ 2 mm (in % of weight); the percentage of sand corresponds to the percentage of particles of sediment < 2 mm and ≥ 63 μm (in % of weight or volume); and percentage of mud corresponds to the percentage of particles of sediment < 63 μm (in % of weight or volume).
Organic matter content was calculated by means of weight loss on ignition, at 450 °C during 6 h [6] . Table 1 Transformations from the results obtained by the laser diffraction particle size analyser to dry sieving equivalent.