Gómez-Pujol, L., Orfila, A., Álvarez-Ellacuría, A., Terrados, J. and Tintoré, J., 2013. Posidonia oceanica beach-caster litter in Mediterranean beaches: a coastal videomonitoring study.

Mediterranean nearshore sandy and rocky bottoms are colonized by the endemic reef-building seagrass Posidonia oceanica. This species loses leaves in autumn that form large litter patches in the surf zone and huge litter banks on adjacent beaches, resulting in wedge and layered structures of few centimetres to several meters in thick (banquettes). Some authors pointed the importance of those banquettes for the protection of sandy beaches because they dissipate wave energy. By contrast other authors state that this effect is almost negligible. This work deals with the role of Posidonia oceanica accumulations in Mediterranean beach morphodynamics. By means of coastal video-monitoring and wave records we assess the marine conditions related to the formation and destruction of banquettes and evaluate their role in the protection of two sandy beaches (Cala Millor and Son Bou, Balearic Islands). Results indicate that banquettes are common beach features at the study sites although they are not persistent and experience complex construction and destruction dynamics throughout the year. Two different types of banquette construction can be differentiated over the year: one related to the reworking of older seagrass beach cast by alongshore currents and a second as a response to the incorporation of new volumes of dead leaves after energetic winter storms (Hs ~2 to 3 m). In both cases, seagrass cast accumulations are continuously built up and destroyed and rarely persist before the arrival of new sea storms. Therefore, at least for semi-enclosed sandy beaches, the protection role of seagrass banquettes should be reconsidered.