Seagrass leaves in 3-D: Using computed tomography and low-frequency acoustics to investigate the material properties of seagrass tissue

As a preliminary step toward development of a predictive model of sound speed in water containing seagrass, the acoustic and material properties of leaf tissue for Thalassia testudinum, Syringodium filiforme and Halodule wrightii were assessed using a one-dimensional acoustic resonator and computed tomography imagery. The high-resolution three-dimensional imagery provided valuable information about leaf morphology and produced precise estimates for plant tissue volume and the volume of air contained within the aerenchyma of the plant tissue, which in turn yielded measurements of the plant tissue density (Thalassia = 1032.34 ± 115.42 kg·m− 3; Syringodium = 664.17 ± 10.38 kg·m− 3; Halodule = 964.96 ± 21.72 kg·m− 3). The acoustic resonator technique was used to investigate the species-specific and biomass-dependent differences in the propagation of sound through seawater containing seagrass leaves in the 0.2 kHz to 10 kHz frequency band. The three species each caused a unique decrease in sound speed with an increase in biomass. The values for leaf air volume, tissue volume and tissue density were then combined with the results of the acoustic experiments and an effective medium acoustic model to determine the acoustic compressibility of the leaves. Further implementation of two effective medium models failed to yield measurements of leaf tissue compressibility. Failure of these models indicates that tissue elasticity and structure must be considered for a complete understanding of acoustic propagation in seagrass meadows.