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Continuous acoustic monitoring of physiological and environmental processes in seagrass prairies with focus on photosynthesis
Hermand, J.-P. (2006). Continuous acoustic monitoring of physiological and environmental processes in seagrass prairies with focus on photosynthesis, in: Caiti, A. et al. Acoustic sensing techniques for the shallow water environment: inversion methods and experiments. pp. 183-196. dx.doi.org/10.1007/978-1-4020-4386-4_14
In: Caiti, A. et al. (2006). Acoustic sensing techniques for the shallow water environment: inversion methods and experiments. Springer: Dordrecht. ISBN 978-1-4020-4372-7. 332 pp.

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  • Hermand, J.-P.

Abstract
    The development of methods for the reliable and efficient assessment of the health status of submerged aquatic vegetation is of considerable interest nowadays. The present paper explores the use of acoustics for the remote sensing of physiological processes of aquatic plants in relation to the environment, dealing specifically with the leaf photosynthesis of seagrasses at the scale of a prairie. An exploratory study started in spring 1995 on the basis of long-range transmission measurements carried out over a prairie of Posidonia oceanica, an endemic phanerogam of the Mediterranean Sea. Results of a shorter-range experiment carried out at the end of the summer 1999 in the Ustica Island marine reserve (Sicily) are hereby presented. Acoustic signal propagation and ambient noise due to biological processes were measured during four days under controlled experimental conditions. Chirp signals were emitted repeatedly from a broadband sound source and received on a two-hydrophone vertical array; the range was 53 m and the frequency band was 0.216 kHz, broader than during the first experiment. Ground truth data of dissolved oxygen and temperature in the acoustic section were obtained contemporaneously. Detailed statistical analyses of the medium impulse responses, and in particular of their energy time distribution, allowed detecting cyclic variations of the sound propagation characteristics. Some of the latter are strongly correlated with the release of photosynthetic oxygen measured above the foliage by a dissolved oxygen sensor. Photosynthesis is shown to produce excess attenuation of certain acoustic paths and a faster decay of reverberation. The main diurnal variations are ascribed to non dissolved gases that are present in the air channels running inside and along the length of the leaf blades, and to the production of oxygen microbubbles sticking to the blade surface. Other variations can be attributed to gas movements in the rhyzome and root systems. The Posidonia prairies form a thick layer where gas void fraction varies with the phase of photosynthesis cycle. During the active phases sound speeds lower than in bubble-free seawater, together with absorption and scattering effects, modify the multiple reflections from the rocky substratum. In our experimental setup, modeling results explain why the multipaths with intermediate grazing angles are the most sensitive to photosynthesis. The results confirm the ones obtained in the first experiment of 1995, even if conducted under quite different environmental conditions and with a different measuring arrangement. They indicate that in situ measurements of photosynthesis by acoustic methods can provide new insights into the physiology of seagrasses and in particular their response to environmental forcing, at the scale of a prairie. The study is not intended to be species-specific, being now extended to other temperate seagrass species like Cymodocea sp. and Zostera sp. acoustic measurements with in-situ hydrographic data and comprehensive modeling of the seagrass scattering and absorption mechanisms in the frequency band of interest. Furthermore, our research confirmed that the behavior and the movements both of fish and benthic organisms can be contemporaneously monitored, improving greatly the possibility of an indirect, non invasive study with the possibility of collecting data inaccessible so far as further discussed in [19). The proposed method is not specific to Posidonia seagrass; other temperate seagrass species includinig Cymodocea sp. and Zostera sp. are currently being studied. Extension and application of the method for a much longer observation period would allow studying not only the species-specific response to any environmental forcing but also the relationships between leaf photosynthesis and seasonal plant growth and productivity as well as the differences in the seasonal pattern among species, at the scale of a prairie.

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