Direct effects of current velocity on the growth, morphometry and architecture of seagrasses: A case study on Zostera noltii

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.