Abstract
Among a suite of abiotic and biotic factors, the hydrodynamic regime strongly influences the success of seagrass recruitment through sexual propagules. Uprooting of propagules by drag forces exerted by currents and waves is one of the main causes for the failed establishment and the consequent recruitment. Substrate type and stability play a key role in determining the success of colonization through sexual propagules, as seedling establishment probabilities proved to be significantly higher on rocky bottoms than on unstable unconsolidated substrates. In this research, the current and wave flow intensity thatPosidonia oceanicaseedlings anchored to rocky substrates can withstand before uprooting were evaluated and the influence of substrate complexity on seedling anchorage success and anchorage strength was investigated.P. oceanicaseedlings withstood the current velocity of 70 cm s–1and increased orbital flow velocities up to 25 cm s–1. Seedling adhesion strength ranged from 3.92 to 29.42 N. Results of the present study corroborate the hypothesis that substrate complexity at scales relevant to the size of propagules is a crucial feature forP. oceanicaseedling establishment. The intensity of unidirectional and oscillatory flow that seedlings can withstand without being dislodged assessed in this study support the hypothesis thatP. oceanicasexual propagules, once adhered to a consolidated substrate, are able to tolerate high hydrodynamic stress. The results of the present study contribute to re-evaluation of the habitat requirements ofP. oceanica, assessing the range of hydrodynamic conditions that this species can tolerate during the early stages of its life history.
Highlights
Spatial distribution and abundance of seagrasses are controlled by a suite of abiotic and biotic factors (Roca et al, 2016)
To analyze the effects of substrate complexity on seedling anchorage, seedlings were grown for 4 months on 10 × 10 cm calcareous tiles characterized by three different levels of complexity: (i) Low complexity (LC), tiles provided with complexity only at the μm scale, i.e., the natural roughness of the stone; (ii) Medium complexity (MC), tiles provided with complexity at the μm and mm scale, having five parallel linear crevices (0.4 × 0.4 cm), and (iii) High complexity (HC), tiles provided with complexity at the μm, mm, and cm scale, having a central slot (2 × 1 cm) and four parallel crevices (0.4 × 0.4 cm) (Figure 1)
Number of standing leaves (Nle), maximum leaf length (Mll), maximum leaf width (Mlw), total leaf area per seedling (Tla, i.e., the sum of the area of all standing leaves), number of root (Nr) and of root branches (Nb), total root length (Trl, i.e., the sum of the length of all the roots), average root width (Rw), number of adhered root (N.ar), and root adhesion length (Ral, i.e., the sum of the length of all the portions of the seedling roots attached to the substrate) were measured
Summary
Spatial distribution and abundance of seagrasses are controlled by a suite of abiotic and biotic factors (Roca et al, 2016). Seagrass ecophysiology has focused on identifying light requirements for meadow persistence, and only recently, the hydrodynamic regime that seagrasses can tolerate has received attention (Koch, 2001; Infantes et al, 2009; Vacchi et al, 2010, 2014). Hydrodynamic stress induced by water movements on submersed aquatic vegetation is thought to set the upper depth limit of meadow distribution (Koch et al, 2006; Infantes et al, 2009; Vacchi et al, 2010), but it can determine the morphology of P. oceanica beds (Buia et al, 2004; Lasagna et al, 2011)
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