Assessment of soft-bottom polychaete assemblages in the Gulf of Lions (NW Mediterranean) based on a mesoscale survey

Ninety-two stations (10, 20, 30, 40 and 50 m depth) located on 21 inshoreeoffshore transects between the FrencheSpanish border and the mouth of the Rhoˆne River were sampled during the 1998 Fall. Their polychaete fauna was analyzed to: (1) describe the distribution pattern of polychaete assemblages at the scale of the whole Gulf of Lions, (2) identify the relationships between these assemblages and the main environmental parameters, (3) establish a link between these assemblages and the benthic communities already described for Northwestern Mediterranean, and (4) assess recent historical changes in trends in densities and a diversity between assemblages. Three polychaete assemblages were identified using cluster analysis and associated procedures. These assemblages were tightly associated with depth and sediment granulometry as indicated by the concordance between their spatial distributions and the sedimentary map of the Gulf of Lions. Assemblage I contained most of the 10 and 20 m deep stations and was associated with littoral fine sands. Assemblage I was found all over the Gulf of Lions and was characterized by high abundance and high biomass due to the presence of large numbers of the serpulid Ditrupa arietina. Assemblage II was mostly composed of 30 m deep stations and was associated with littoral sandy mud. Assemblage II was dominated by the lumbrinerid Lumbrineris latreilli. It was composed of two sub-assemblages (IIa and IIb) in relation with sediment granulometry. Sub-assemblage IIb was associated with finer sediment than sub-assemblage IIa. Sub-assemblage IIa was only found in the Southern part of the Gulf of Lions, whereas sub-assemblage IIb was mostly found in the Northern part of the Gulf of Lions. This pattern is probably indicative of the sedimentation of fine particles originating from the Rhoˆne River at intermediate depth as suggested by: (1) the sedimentary map of the Gulf of Lions, and (2) the outputs of recent models of sediment transport in the Gulf of Lions. Assemblage III was composed of 40 and 50 mdeep stations; it was associated with terrigeneous coastal mud and found all over Gulf of Lions. Assemblage III was dominated by the sternaspid Sternaspis scutata. These three assemblages were tightly related with the benthic communities already described independently by Picard [Picard, J.,1965. Recherches qualitatives sur les bioce´noses marines des substrats meubles dragables de la re´gion marseillaise. Recueil des Travaux de la Station Marine d’Endoume 52, 1e160] and Guille [Guille, A., 1971b. Bionomie benthique du plateau continental de la coˆte catalane franc¸aise. II e Les communaute´s de la macrofaune. Vie et Milieu 21, 149e280] based on samples collected along the Provenc¸al and the French Catalan rocky coasts, respectively. Unfortunately, no quantitative data were available to compare the a-diversity of the three newly defined assemblages with historical data collected by these two authors. We therefore compared our diversity measurements with the data collected by Desbruye`res et al. [Desbruye`res, D., Guille, A., Ramos, J.M., 1972/73. Bionomie du plateau continental de la coˆte catalane espagnole. Vie et Milieu 23, 335e363] in the Bay of Rosas (Spanish Catalan coast). Due to differences in the sampling effort between the two studies, this comparison was restricted to trends between assemblages. Our results show maximal densities and minimal a-diversity in the Littoral Fine Sand assemblage, which was not the case in Desbruye`res et al. [Desbruye`res, D., Guille, A., Ramos, J. M., 1972/73. Bionomie du plateau continental de la coˆte catalane espagnole. Vie et Milieu 23, 335e363]. Major temporal changes obviously reflect the increases of Ditrupa arietina (in the Littoral Fine Sands assemblage), and to a lesser extent Lumbrineris latreilli (in the Littoral Sandy Mud assemblage).