Ecosystem monitoring of a seagrass meadow using wideband acoustic measurements

Seagrasses are sentinel species whose sensitivity to changing water conditions makes it an indicator for sea level rise and climate change. The biological processes and physical characteristics associated with seagrass are known to affect acoustic propagation due to gas bodies contained within the seagrass tissue as well as photosynthesis-driven bubble production that results in free gas in the water. In this work, acoustical methods are applied to assess the health of seagrass meadows using a ray-based model that includes losses due to the dispersion, absorption and scattering of sound related to the gas bodies in the seagrass tissue and free bubbles in the water. The approach is applied to data collected in the Lower Laguna Madre where the seabed was covered by a dense growth of Thalassia testudinum. During the experiment, a piezoelectric source transmitted frequency-modulated chirps (0.1 to 100 kHz) over a four-day period. At the peak of photosynthesis-driven bubble production in the late afternoon, an additional decrease in the received level of more than 10 dB was observed. The volume fraction of gas present in the environment is estimated through acoustic modeling and related to seagrass biomass and photosynthesis. [Work supported by ARL IR&D and ONR.]