G. De Falco á S. Ferrari á G. Cancemi á M. Baroli Relationship between sediment distribution and Posidonia oceanica seagrass Received: 27 May 1999 / Revision accepted: 2 February 2000 Abstract Relationships between sedimentary features and the distribution and structure of Posidonia oceanica meadows were investigated in the Gulf of Oristano (Sardinia) in a study site stretching from the mouth of the Tirso River in a more sheltered, inner sector of the gulf to an outer, more exposed sector near the rocky Cape San Marco. The results demonstrate that gradients in sediment texture and composition can be related to meadow distribution and areal coverage. We suggest that this involves the mechanical trapping of ®ne parti- cles by the plants, enhanced mud deposition due to dampening of wave action in the meadows, and the admixture of bioclastic sediments linked to increased production by epiphytes and invertebrates. Independent evidence of seagrass erosion was recorded in an unveg- etated breach in the meadow. The mass of ®ne sediment which would be resuspended in the wake of meadow destruction was estimated at 30±90 ´ 10 3 tonnes km )2 , an amount which could substantially in¯uence water quality and ecosystem stability in the gulf. Introduction Relationships between sedimentation patterns and sub- merged vegetation occurrence can provide a useful tool to understand coastal sedimentary processes and their ecological signi®cance. Several studies have stressed the role of marine plants in modifying bottom hydrody- namics (e.g., Fonseca et al. 1982; Komatsu 1996). In a Zostera marina (eelgrass) bed, Gambi et al. (1990) demonstrated the presence of two micro-environments which diered in hydrodynamic features: the canopy- water interface, characterized by high shear stress and high turbulence, and the inside of the canopy which had low shear stress and reduced turbulence. In this way, seagrass meadows ®lter and anchor sediments, in¯u- encing sediment dynamics in littoral zones (e.g., Moriarty and Boon 1989), and contributing to water clarity and shoreline maintenance. Indeed, the loss of seagrass beds has often been related to modi®cations of sedimentary regimes as well as to a general deterioration in the quality of coastal waters (e.g., Short and Short 1984). The seagrass Posidonia oceanica is considered to play an important role in the coastal geomorphology of the Mediterranean Sea. In addition, P. oceanica meadows form a key ecosystem component in the shallow waters of the basin, and are an important resource for ®shery (e.g., Mazzella et al. 1993). Thus, their role is compa- rable to that of other plants in temperate and tropical seas supporting high biodiversity (e.g., Larkum et al. 1989). Several studies have shown the in¯uence and depen- dence of these meadows on the nature and dynamics of coastal sediments in the Mediterranean region (e.g., Blanc 1975; Jeudy de Grissac 1984; Blanc and Jeudy de Grissac 1984; Jeudy de Grissac and Boudouresque 1985). For example, the plant is capable of adapting the direction of its growth (horizontally or vertically) to the rate of sediment deposition. In this way, P. oceanica creates a terraced structure (matte) which consists of intertwined roots and rhizomes as well as trapped sedi- ment (Jeudy de Grissac and Boudouresque 1985). The structure eectively dampens wave energy, thereby af- fecting the composition of the bottom sediments which are enriched in pelitic components, and also in biogenic debris produced by resident organisms. In turn, rapid changes in the dynamics of the coastal sediments may cause the meadows to silt up, and thus result in impacts on the ecosystem as a whole, leading to subsequent deterioration of environmental conditions in the coastal zone (e.g., Manzanera et al. 1995; Gongora Geo-Marine Letters (2000) 20: 50±57 Ó Springer-Verlag 2000 G. De Falco (&) á S. Ferrari á G. Cancemi á M. Baroli IMC, International Marine Centre, LocalitaÁ Sa Mardini, 09072 Torregrande-Oristano, Italy e-mail: defalco@barolo.icb.ge.cnr.it G. Cancemi Equipe EcosysteÁmes Littoraux, University of Corsica, BP 52, 20250 Corte, France