Distribution of soft-bottom polychaetes assemblages at different scales in shallow waters of the northern mediterranean Spanish coast

Resumen

Soft-bottom polychaete assemblages inhabiting shallow waters of the northern Mediterranean Spanish coast were studied with the objective to elucidate its present structure, distribution and diversity at different scales and with different degree of anthropogenic pressure. Samples were obtained at different depths along the coast between the border of the Spanish-French border to the vicinity of Valencia. Four different studies are presented In the Barcelona case study, we revisited an station 20 years after cessation of impact due to sewage discharges. By 2008, we found benthic recovery and clear increase in complexity and diversity. It changed from an assemblage 98 % dominated by a C. capitata complex to an assemblage dominated by M. fragilis, C. capitata, and O. hartmanni. We found more species and more diversity in trophic groups, whereas the abundance and biomass values were clearly reduced by almost two and one order of magnitude, respectively. Although, as silt-clay content in sediments improved, diminishing from 95 to 25 %, as well as PCBs and PAHs during the period, their metal content was still very high. This situation may inhibit their full recovery to normal conditions. In the Alt Empordà case study, we revisited 42 sampling sites, previously sampled by French scientists in 1970-72. Using the same methodologies we found the Bay of Roses polychaete assemblages belonging to the LFS and LSM communities described by Labrune et al. (2007), a decrease in the number of species and individuals at the shallowest assemblages and similar numbers for the deepest one. D. arietina dominate fine sand sediments while L. latreilli did teh same in sandy mud sediments. In the northeastern part of the Cap de Creus, the number of species and individuals were similar to those described by Desbruyères et al. (1972-73) and other notable differences in species composition were registered. In the regional scale case study, 59 sampling sites were considered in 22 inshore-offshore transects located at depth gradient (10, 20 and 30m) of the LFS community (Labrune et al., 2007) were studied along the North-western Mediterranean portion of coast between the French border and the Valencia City. The spatial distribution, diversity of polychaetes shows maximal and minimal densities and diversities related to other areas of the Mediterranean Sea. Polychaete assemblages found in it study were similar to establish in the vicinity of Gulf of Lions and other areas of the Mediterranean Sea, although in this case, our results do not reveals linked to granulometric parameters, it was related to depth. Finally, a mesoscales survey have been carried out analizing samples from the mouth of the Rhone River (France) to the vicinity of Valencia (Spain), obtained from the LFS community. Polychaete assemblages found at the Littoral sands of the North-western Mediterranean are clearly described by the large presence of D. arietina and O. fusiformis both in abundance and biomass, being this more acute in the stations sample at the Gulf of Lions. In sites where coarse sand sediments were found, near rocky shores or highly dynamic deltas, the LFS community was replaced by the Littoral Coarse Sands community (LCS). The Polychaete fauna was well represented along the Mediterranean Spanish coast. We found an important number of the species that constitute the pool of polychaete species inhabiting shallowest sites of the Western mediterranean. Their heterogeneity, diversity, and species richness could range between low to high, depend on size and composition of sediment, depth, submarine topography and human pressure at localities which it inhabit. The structure and distribution of polychaetes assemblages could be related to other factors besides the factors mentioned above, as nutrient availability closed to river discharges or other top-down and bottom-up biotic factors.