Dispersal of seagrass propagules: interaction between hydrodynamics and substratum type

Waves and currents influence not only the spatial distribution of seagrass meadows but also the transport, establishment and survivorship of seagrass propagules and hence the success of seagrass recruitment from sexual reproduction. We quantified the dispersal of propagules of 3 seagrass species (Posidonia oceanica, Cymodocea nodosa and Zostera marina) over substrata of different complexities (sand, coarse gravel and P. oceanica mattes of different shoot abundances) under unidirectional flow in a hydraulic flume. Threshold velocities indicate that Z. marina seeds start to move earlier over a flat sandy bottom (14 cm s(-1)) than seeds of P. oceanica (20 cm s(-1)) and C. nodosa (21 cm s(-1)). Propagule trapping increased with bottom complexity, which was related to the flow reduction that each substratum generated and the boundary layer thickness. Trapping rates were higher in coarse gravel and in mattes with higher abundances of dead shoots, where flow was reduced more than 50%. Over sand, flow reduction was minimal and propagules were not trapped. Furthermore, notable differences between P. oceanica early life stages were observed, with seeds trapped first, followed by seedlings of increasing ages. This result may be related to the smaller frontal area (area exposed to water flow) and higher settling velocity of the younger life stages. Together, our results provide important insights into the drivers of seagrass recruitment, which are of interest for restoration purposes and numerical modelling.