Inversion of acoustic waveguide propagation features to measure oxygen synthesis by Posidonia oceanica

As part of Yellow Shark experiments in the Giglio basin, off the west coast of Italy, measurements of acoustic waveguide impulse response were conducted in February and May 1995 over an extensive bed of Posidonia oceanica (L.) Delile around Scoglio d'Africa, a minor island of the Tuscan Archipelago. The purpose of the measurements was to determine the applicability of inversion techniques developed for bottom geo-acoustic properties to the monitoring of oxygen synthesis by P. oceanica. A broadband sound source and a 4-element vertical receiving array were bottom moored at 1541 m distance in an isobath (25 m water depth), dense and homogeneous part of the P. oceanian prairie. The waveguide impulse response was measured during one day by 1 min repeated transmissions of broadband signals in the frequency band 0.1-1.6 kHz. The water sound speed profiles calculated from repeated CTD measurements were slightly downward refracting and exhibited little temporal variability except for a mild surface heating in the afternoon. A set of CTD and oxygen profiles as a function of daytime and season was obtained in 1997 to support the present study. In this paper acoustic, solar radiation, oxygen and CTD data are analyzed and discussed. The analysis shows a strong correlation between photosynthesis and the impulse response of the acoustic waveguide. The most evident feature is a marked change of attenuation and time dispersion characteristics when photosynthesis begins. Frequency- and depth-dependent rapid variations of received energy (2-5 dB) and time spread (3-10 ms) are observed. The time of occurrence and rate of change of these variations are consistent with solar time and oxygen concentration measured in situ. The phenomena is attributed to bubbles of photosynthetic oxygen formed on the P. oceanica leaf blades.