Relationship between sediment distribution and Posidonia oceanica seagrass

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 fine particles 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 unvegetated breach in the meadow. The mass of fine sediment which would be resuspended in the wake of meadow destruction was estimated at 30–90 × 10³ tonnes km⁻², an amount which could substantially influence water quality and ecosystem stability in the gulf. Received: 27 May 1999 / Revision accepted: 2 February 2000