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

Abstract

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.

Clonal building, simple growth rules and phylloclimate as key steps to develop functional–structural seagrass models

“…Leaf No. 5 (L-5) is unique in that it is the first leaf of the bud primordium, growing beneath the sheath of the oldest leaf (L-1), and developing into L-1 in the new non-apical shoot generated from the apical shoot (Brun et al 2006). Therefore, L-5 is not considered part of the apical shoot, and develops later than L-3.…”

Waves and high nutrient loads jointly decrease survival and separately affect morphological and biomechanical properties in the seagrass Zostera noltii

“…We conducted morphometric measurements on all of them, which allowed us to estimate the dynamical properties of the plants following Peralta et al (2005; Table 1). Furthermore, living EPUs (tagged at the beginning) were individually weighed (fresh wt) to calculate the net growth rate (GR; mg fresh wt EPU 21 d 21 ) using a linear growth model (Table 1).…”

“…For example, leaf size of Zostera marina was increased in response to nutrient enrichment (Short 1983), whereas Zostera noltii reduced leaf size under high flow velocities (Peralta et al 2006). Hence, the aim of the present study is to assess in an aquarium experiment the interactive effects of wave forces and nutrient enrichment on the survival, morphodynamical, and biomechanical properties of seagrass Zostera noltii, a fast-growing species with an extensive morphological plasticity .…”

“…Hydrodynamic conditions from waves and currents are also known to have direct and indirect effects on seagrass ecosystems. For example, increased wave energy can directly reduce survival and number of seagrass shoots (van Katwijk and Hermus 2000), while increased flow velocity can directly affect plant growth and morphometry (Schanz and Asmus 2003;Peralta et al 2006;de los Santos et al 2010). In addition, increased flow velocity and/or waves can have indirect effects on the photosynthetic rates and nutrient uptake by reducing boundary layers (Koch 1994).…”

“…Hence, the aim of the present study is to assess in an aquarium experiment the interactive effects of wave forces and nutrient enrichment on the survival, morphodynamical, and biomechanical properties of seagrass Zostera noltii, a fast-growing species with an extensive morphological plasticity . In most seagrass studies, morphodynamical responses are typically included by measuring variables such as length, appearance, and elongation rates of the different modules of the plant (i.e., shoots, rhizomes, and roots [Short et al 1995;Lee and Dunton 2000;Peralta et al 2005]). Biomechanical properties such as leaf strength and stiffness are, however, poorly studied on seagrasses (but see Patterson et al 2001), even though they have been identified as important acclimation traits to the mechanical drag that imposes stress in several marine organisms (Koehl 2000) as well as in terrestrial plants (Anten et al 2005).…”