Acoustic characterization of Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa




Johnson, Jay Richard, Ph. D.

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Seagrasses are a vital part of the coastal ecosystem; they serve as a habitat for fish, stabilize the seabed, are significant primary producers, and act as efficient carbon sinks. Increased anthropogenic pressures have put stress on seagrass ecosystems, and thus, there is a growing need to map and monitor seagrass meadow growth and decline. Acoustic mapping and monitoring techniques show a potential to be both cost-effective and robust, but there is still a lack of a physics-based model of acoustic propagation in seagrass meadows. This dissertation expands the knowledge necessary for such a predictive forward model of sound propagation in seagrass meadows by conducting various acoustic experiments on the two endemic Mediterranean seagrass species Posidonia oceanica and Cymodocea nodosa. The mean tissue density was measured for both species using direct mass and indirect volume measurements, and the bulk modulus of the leaf tissue of both species was measured using a one-dimensional acoustic resonator technique and finely divided tissue pieces via an effective medium model. The mean tissue density and adiabatic bulk modulus of P. oceanica were measured as 1115 kg m⁻¹ and 1.9 GPa, respectively, and measured for C. nodosa as 930 kg m⁻¹ and 1.5 GPa, respectively. Confidence intervals on the bulk moduli measurements were calculated using a Monte Carlo uncertainty simulation. The effect of leaf morphology and epiphytic coverage on the low-frequency (1 to 5 kHz) acoustic response of seagrass leaf-blades was also explored using a similar one-dimensional acoustic resonator. Variability in response was measured across three different length scales: between species, within species, and within individual leaves. These acoustic measurements are compared with microscopic images of transverse leaf cross-sections, measurements of leaf size, and estimates of aerenchyma void-fraction. A high-frequency (0.5 to 5 MHz) acoustic pulse transmission apparatus was developed to measure the variability in acoustic response along individual seagrass leaves. The system was modeled with a three-layer medium model to get estimates of transmission loss (TL) and phase speed c [subscript ph] in the leaves. In P. oceanica, TL abruptly decreased by up to 25 dB/mm and c [subscript ph] increased from as low as 0.4 to 0.95 times the sound speed in the surrounding water at 14 cm from the leaf base. In C. nodosa, while TL decreased and c [subscript ph] increased towards the apex of the blade, the changes were at a more consistent rate, unlike in P. oceanica.


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