Hydrodynamic constraints to the seaward development of Posidonia oceanica meadows

Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, forms large meadows from the sea surface down to 40 m. The depth of the lower limit of the meadows marks the boundary between the infralittoral and the circalittoral zone, and is said to be normally set by light attenuation underwater, while the role of water movement has been little explored. In this paper, the position (i.e. distance from the shoreline and depth) of P. oceanica meadow lower limits along the whole Ligurian coastline (about 350 km) was related to the annual storm wave base. This depth represents the limit of interaction between waves and seafloor and corresponds to L0/2, where L0 is the annual offshore wave length. In all meadows, the lower limit has never been found deeper than the annual storm wave base, and its depth (Zc) showed related to L0 according to the equation Zc = 0.32∙L0 + 5.62. In the coastal tracts affected by the least intense waves, the reduction of water movement with depth represents the most important constraint to the seaward development of the meadow, whereas light availability plays a major role in meadows affected by the most intense waves. The present study represents the first attempt at understanding the role of hydrodynamic factors in setting the depth limit of seagrass meadows. If corroborated by future research at other sites, this will have important implications for both basic and applied science, as it would imply rethinking about the relative importance of water movement and light in seagrass depth distribution, and could allow for a better estimate of the extent of meadow regression in anthropized areas.