Date of Award

Spring 2024

Project Type

Thesis

Program or Major

Ocean Engineering

Degree Name

Master of Science

First Advisor

Gabriel R Venegas

Second Advisor

Tracy Mandel

Third Advisor

Anthony Lyons

Abstract

Seagrass meadows are an essential part of littoral ecosystems, which provide many ecosystems service. They sequester carbon, filter seawater, protect coastlines from erosion, and serve as important nursery habitats. When seagrasses photosynthesize, they remove carbon dioxide from the environment and supersaturate the surrounding water with oxygen, at which time, bubbles form on the seagrass leaves and eventually rise to the atmosphere. Acoustic waves are highly sensitive to the presence of these bubbles, impacting naval sonar applications, such as mine hunting and underwater navigation, and therefore are also of importance to the Navy. Recently, seagrass have experienced a consistent decline in population worldwide due to anthropogenic and natural causes, which have disastrous consequences for our climate. Therefore, better quantitative methods at higher temporal resolution are to further monitor seagrass photosynthetic productivity and their response to surrounding hydrodynamics to understand and thereby prevent further seagrass decline. Acoustics has been shown to be a powerful tool to quantify photosynthetic bubble production. Several field studies have been published, whereby an acoustics model of a bubbly-liquid was fit to acoustic data collected in a seagrass meadow. These bubbly-liquid acoustics models assume that the bubbles are freely rising and spherical in shape, while many of the bubbles in the seagrass meadow are adhered to the seagrass leaves and non-spherical. In addition, the seagrass leaves are assumed to be uniformly distributed in the acoustics measurement, while currents and waves have been shown to cluster the bubbles. Therefore, more laboratory investigation is needed to further understand these complexities before metrics such as photosynthetic productivity can be derived in the field with confidence. To the author’s knowledge, there has not yet been a controlled laboratory investigation of the applicability of this bubble acoustics model to bubbles that are adhered to seagrass leaves. This thesis will test the applicability of a simple adaptation to an existing model, the Effective Medium Mixture (EMM) model, to predict the sound speed and attenuation of the synthetic bubbly seagrass. All inputs to the EMM were measured directly, such as the size distribution and volume fraction of the bubbles, and the volume fraction and mechanical properties of the synthetic seagrass. Using a Kongsberg 200 kHz source and a Reson TC-4034 hydrophone, sound speed and attenuation were measured directly in the synthetic bubbly seagrass. To within 95% confidence interval of the measurement and model outputs, the EMM model was found to be appropriate for bubbles adhered to seagrass in stationary conditions. It was found that when bubbles adhere to the synthetic seagrass, the bubbles are oblong. If bubbles are imaged in a direction normal to the grass surface, a correction must be applied to account for an overestimation of bubble volume fraction. Next, the EMM was used to estimate time-varying bubble volume fraction of seagrass subjected to water waves. The period of the time-varying bubble volume fraction was consistent with the period of the water waves, however, mean bubble volume fraction was overestimated when the seagrass was subjected to water waves. This bias was attributed an increase in multiple scattering effects due to clustering of the bubbles within the control volume, which are not accounted for in the EMM. This work demonstrates a proof-of-concept for acoustics to monitor seagrass bubble production and hydrodynamics in a controlled laboratory environment.

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