Influence of the structure of Posidonia oceanica meadows modified by bottom trawling on crustacean assemblages: comparison of amphipods and decapods

The seagrass Posidonia oceanica plays an important role as habitat for inve rtebrates. A correlative study was done in El Campello (SE Spain) to unders tand the relationships between assemblage structure of crustaceans and modi fication of P. oceanica structure caused by bottom trawling. We compared th e changes of community structure of the most important groups of crustacean s, amphipods and decapods with changes of cover, density, detritus and rhiz ome fragments. Sampling was carried out in four control and four impact loc ations, along a gradient of degradation (17 m depth, August 1992). Multivar iate analysis was used for community data (nm-MDS) and P, oceanica structur e (PCA). Both groups showed a similar number of species. The amphipod commu nity showed a clear pattern of community changes with significant differenc es between impact and control locations (global R = 0.298, p<0.001) and wit h significant correlation with P. oceanica structure (global rho = 0.21, p< 0.05). Detritus (fine and coarse) and rhizome fragments had the highest inf luence on amphipod distribution. Spatial variability inside of locations wa s very high. Few species (Dexamine spiniventris, Lysianassa longicornis or Ampelisca spp.) were responsible for disimilarity among locations. Decapods did not show a correlation with the structure of P. oceanica. The amphipod community seems to be a strong indicator of the ecological implications of seagrass degradation.