10 pages, 8 figures, 1 table.We demonstrate the utility of using the equivalent bottom roughness for calculating the friction factor and the drag coefficient of a seagrass meadow for conditions in which the meadow height is small compared to the water depth. Wave attenuation induced by the seagrass Posidonia oceanica is evaluated using field data from bottom-mounted acoustic doppler velocimeters (ADVs). Using the data from one storm event, the equivalent bottom roughness is calculated for the meadow as ks ~ 0.40 m. This equivalent roughness is used to predict the wave friction factor ƒw, the drag coefficient on the plant, CD, and ultimately the wave attenuation for other storms. Root mean squared wave height (Hrms) is reduced by around 50% for incident waves of 1.1 m propagating over ~1000 m of a meadow of P. oceanica with shoot density of ~600 shoots m−2.E. Infantes acknowledges the financial support received from the Spanish Ministerio de Educacion y Ciencia, FPI scholarship program (BES-2006-12850). G. Simarro is supported by the Spanish government through\ud the Ramón y Cajal program. A. Orfila and G. Simarro are grateful for financial support from Spanish MICINN through project CTM2010-12072. M. Luhar was supported on grant number 0751358 from the U.S. National Science Foundation Ocean Sciences Division.Peer reviewe

We studied the role of hydrodynamics in the establishment of sea grass seedlings for two Mediterranean sea grass species, Posidonia oceanica and Cymodocea nodosa, by combining flume and field studies. Flume measurements under both unidirectional and oscillatory flow showed that P. oceanica seedlings experienced higher drag forces than C. nodosa, which could be related to the larger total leaf area. Drag coefficients were between 0.01 and 0.1 for Reynolds numbers of 10³ and 10⁵. As a result, P. oceanica seedlings required 40‐50% of root length anchored to the sediment before being dislodged, whereas C. nodosa required ≈ 20%. To validate the flume results, seedling survival in sandy beds was evaluated for two depths (12 and 18 m) at two field locations. To calculate near‐bottom orbital velocities at the planting sites, deep‐water waves were propagated to shallow water using a numerical model. Our results showed that P. oceanica seedlings experienced high losses after the first autumn storms when near‐bottom orbital velocities exceeded 18 cm s^‐1. The loss of C. nodosa seedlings was much lower and some seedlings survived velocities as high as 39 cm s^‐1. Thus, flume and field results are consistent in explaining relative higher losses of P. oceanica seedlings than for C. nodosa.