The macrobenthic fauna community of a 70-km 2 Zostera noltii seagrass bed (Arcachon bay, France) was studied by sampling 49 stations systematically. A total of 126 taxa were identified. Cluster Analysis based on χ 2 distance showed that in this apparently homogeneous habitat, four distinct macrobenthic communities could be identified. Multiple Discriminant Analysis highlighted the major contribution of the overlying water mass as a forcing variable, and, to a lesser extent, of tidal level and Z. noltii's below-ground parts. Seven stations did not constitute any conspicuous group, and were characterized by a low biomass of leaf (<28gDWm - 2 ), considered as the lowest value to constitute a Z. noltii community. Less than 24% of the seagrass bed was situated in more oceanic waters and at a quite low tidal level. In this relatively stable environment, the macrofauna community was characterized by a high species richness (mean=39) and a moderate density and high biomass (12 638individualsm - 2 and 25gAFDWm - 2 , respectively). Annelids dominated, particularly the oligochaetes. When physical constraints increased (emersion or brackish water conditions), diversity decreased, abundance and biomass increased. The seagrass bed (55%) was flooded with highly fluctuating waters in term of temperature and salinity, here species richness was low (mean=27) but abundance and biomass were high (24 384individualsm - 2 and 28gAFDWm - 2 , respectively), with a dominance of molluscs. The meadow (7%) was in external waters but at a higher tidal level (2.4m vs 1.8m above medium low tide level). This community was characterized by the particularly high density (41 826individualsm - 2 ) and dominance of oligochaetes (79% of total abundance). Species richness was high (mean=37) here. A fourth community, extending over 12% of the meadow was dominated by the gastropod Hydrobia ulvae but could not be linked to a specific forcing variable. This study confirmed the almost complete replacement of the native clam Ruditapes decussatus by the Manila clam Ruditapes philippinarum since its introduction in the 1980s.