Seasonal variations in the diversity and structure of decapod communities in the changing hydrological scenario of the northwest African upwelling

Multidisciplinary surveys conducted on-board RV Dr Fridtjof Nansen in the northwest Africa region during November 2011 and June 2012 allowed exploration for the first time of seasonal variations in diversity and structure of decapod communities inhabiting the changing hydrological scenario of the shelf and upper slope off Mauritania (20–750 m depth). Decapods were gathered with a commercial bottom trawl at 95 stations, 39 of which were visited in both surveys. In addition, 249 oceanographic profiles and 95 sediment samples were collected. Shelf and slope assemblages, respectively characterized by the dominance of benthic (63 species) and swimming decapods (37 species), were segregated at 100–150 m depth, coinciding with the shelf-break and the hydrological frontier separating the surface mixed layer from the deeper water masses. The assemblages showed a certain stability in their structure but significant seasonal differences in their composition. Abundance and biomass, and in particular specific richness, were strongly correlated with depth in the case of swimming decapods, which were clearly dominant during the warm season, especially in the southern zone. The most significant seasonal variations were mainly due to the Penaeidae family and its associated species,  Parapenaeus longirostris (Lucas, 1846). The latitudinal decreasing trend in upwelling permanence and productivity toward the south, responding to the strength and displacement of trade winds along the Mauritanian coast, which are observable at the surface and sea-bottom, were the main reason explaining the seasonal variations observed in the distribution patterns and assemblages structure of Mauritania decapods.