Dependence of acoustic propagation in a seagrass meadow on diurnal and seasonal time scales

Acoustic propagation in seagrass meadows is sensitive to gas produced by photosynthesis and respiration. Gas volumes, both encapsulated within the seagrass tissue and released as free bubbles into the water column, result in the dispersion, adsorption, and scattering of sound. Because the oxygen production cycle is largely driven by sunlight, these acoustical effects have a diurnal dependence. Previous work has examined the use of acoustics as a remote sensing tool for monitoring seagrass photosynthetic activity (Hermand, 2004, Felisberto, 2015). To investigate the temporal dependence of the acoustical effects of seagrass photosynthesis, an acoustic propagation experiment was conducted in a Thalassia testudinum meadow in the Lower Laguna Madre, a shallow bay on the Texas Gulf of Mexico coast. A piezoelectric sound source transmitted frequency-modulated chirps (0.1 kHz to 100 kHz) every 10 minutes over several diurnal cycles, during which oceanographic probes measured water temperature, salinity, dissolved oxygen, and changes in water depth due to the tides. The received acoustic signals were match-filtered to obtain band-limited impulse responses, enabling identification of various propagation paths within the waveguide. The measurements were conducted both in winter and summer to examine the seasonal dependence of seagrass photosynthesis and its effect on the acoustic propagation environment. Dependence of the received acoustic amplitude and frequency content on time-of-day, season, dissolved oxygen, and other environmental parameters will be discussed with the goal of using acoustics to study seagrass photosynthesis and productivity. [Work supported by ARL:UT IR&D and ONR.]