Application of geoacoustic inference to assess the diurnal effects of photosynthetic activity in a seagrass meadow

Megan S Ballard1, Kevin Lee2, Jason D Sagers2, Gabriel Venegas2, Andrew R McNeese2, Abdullah Faizur Rahman3 and Preston S Wilson4, (1)Applied Research Laboratories at the University of Texas at Austin, Austin, United States, (2)Applied Research Laboratories at the University of Texas at Austin, Austin, TX, United States, (3)University of Texas Rio Grande Valley, Coastal Studies Lab, Edinburg, TX, United States, (4)University of Texas at Austin, Walker Department of Mechanical Engineering and Applied Research Laboratories, Austin, TX, United States
Abstract:
Seagrasses provide a multitude of ecosystem services: they alter water flow, cycle nutrients, stabilize sediments, support the food web structure, and provide a critical habitat for many animals. However, due to threats to seagrass meadows and their associated ecosystems, these habitats are declining globally. Acoustical methods can be a powerful remote sensing tool to efficiently monitor seagrass meadows, alleviating the problem of space and time aliasing associated with traditional spot measurements. The acoustic field is highly sensitive to the presence of gas bubbles in liquids, which result in acoustic dispersion, absorption, and scattering. The biological processes and physical characteristics associated with seagrass result in trapped gases, which are present in the aerenchyma of seagrass leaves, roots, and rhizomes as well as in freely floating bubbles in the water. To investigate the use of acoustical methods to monitor ecosystem health, an experiment was conducted in the Lower Laguna Madre where the seabed was covered by a dense growth of Thalassia testudinum. During the experiment, a combustive sound source was used to produce broadband signals at ranges of 25 m to 400 m from an array of receivers. The data were collected over a period of one day to assess the diurnal effects of photosynthetic activity on the acoustic measurements. An additional set of measurements were collected in a nearby sparsely vegetated site to provide a baseline data set for comparison. Supporting measurements of irradiance, dissolved oxygen, and water temperature were acquired at the experiment site. The acoustic data were analyzed for the purposes of inferring environmental parameters in the seagrass meadow, including the gas volume present in the environment. An effective medium model for gas bubbles in liquids was used to characterize the frequency-dependency of sound speed and attenuation in the waveguide. [Work sponsored by ARL:UT IR&D and ONR.]
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