Successful management and restoration of coastal vegetation requires a quantitative process-based understanding of thresholds hampering (re-)establishment of pioneer vegetation. We expect scouring to be important in explaining the disappearance of seedlings and/or small propagules of intertidal plant species, and therefore quantify the dependence of scouring depend on plant traits (flexibility, size) and physical forcing by current velocity. Flume studies with unidirectional flow revealed that scouring around seedlings increased exponentially with current velocity and according to a power relationship with plant size. Basal stem diameter rather than shoot length was found to be the factor controlling scouring volume. Flexible shoots caused far less scouring than stiff shoots, provided that the bending occurred near the sediment surface as was the case for Zostera, and not on top of a solid tussock base as we observed for Puccinellia. Therefore, shoot stiffness is likely to strongly affect the chances for initial establishment in hydrodynamically exposed areas. Plant traits such as shoot stiffness are subject to a trade-off between advantages and disadvantages, the outcome of which depends on the physical settings

In an 8‐week aquarium experiment, we investigated the interactive effects of waves (present vs. absent) and water‐column nutrient level (high vs. low) on the survival, growth, morphology, and biomechanics of the seagrass, Zostera noltii. Survival was reduced when plants were exposed to both waves and high nutrient levels. Wave and nutrient interaction significantly reduced aboveground biomass and leaf lengths, whereas waves independently reduced growth rate, internode abundance, elongation, and appearance rates. Nutrient supply significantly reduced the strength of the leaves. Wave and nutrient interaction was the main driving force affecting survival and morphological properties of seagrass, whereas dynamical characteristics were independently affected by waves, and nutrient supply affected mainly biomechanical properties. In conclusion, this experiment revealed that the combination of exposure to waves and high nutrient levels was detrimental for Z. noltii, which indicates that this could be an important unexplored force involved in seagrass declines.

Hydrodynamics affect and are affected by the presence of seagrasses. Previous studies have suggested that such effects could modify the capacity of these marine plants to cope with adverse environmental conditions. However, the direct impact of hydrodynamics on seagrass performance remains undemonstrated. In this study, the direct effects of current velocity on seagrasses were examined by cultivating single plants of Zostera noltii in 12 flume tanks with contrasting current velocities (0.01, 0.10 and 0.35 m s–1), while avoiding differences in other abiotic factors. Our results demonstrated that current velocity directly affects (1) dynamic plant parameters such as growth, elongation, recruitment and plastochrone index (PI), (2) the morphometry of leaves, rhizome and roots and (3) the architecture of Z. noltii plants. Growth, elongation and recruitment rates increased significantly with increasing velocities, whereas the PI was significantly reduced. The resulting morphotypes appeared to represent acclimation to hydrodynamic stresses: they had (1) an improved anchoring system and (2) a reduced risk of shoot breakage. That is, at high velocity, the root system was enlarged, the aboveground/belowground biomass ratio (AG/BG ratio) was reduced and the cross sections of leaves and rhizomes increased. [KEYWORDS: Seagrass ; Hydrodynamics ; Acclimation ; Plant morphometry ; Growth Flume tank