Photosynthesis of seagrasses observed in situ from acoustic measurements

The possibility of using acoustic methods to monitor in situ the response of seagrasses to environmental factors is investigated. The effects of photosynthesis on sound propagation have been observed in prairies of Posidonia oceanica (L.) Delile. This paper reports results of one of the experiments conducted in the marine reserve of the island of Ustica, off Sicily, in September 1999. The long-range propagation of an acoustic signal and the ambient noise were measured during four days under well controlled experimental conditions. Low-frequency, broad-band, frequency-modulated signals were transmitted repeatedly between a sound source and a distant two-hydrophone receiver 53 m apart. The frequency range was 0.2-16 kHz. Ground truth data of dissolved oxygen and temperature were obtained along the transect with an oceanographic probe. Detailed statistical analyses of the medium impulse responses, and in particular of their energy time distribution, allow extracting 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 cause excess attenuation of multipaths and faster decay of reverberation. The main variations are ascribed to nondissolved gases that are present in the air channels running inside the length of the leaf blades and to oxygen microbubbles that stick to the blade surface. Other variations can be attributed to gas movement in the rhyzome and root systems. The Posidonia grasses 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, and absorption and scattering effects modify the multiple reflections from the rock substratum. Modeling results explained why the multipaths with intermediate grazing angles are the most sensitive to photosynthesis in our experimental setup. The results confirm the ones obtained in a first test in May 1995, even if conducted under different environmental conditions and with different measurement geometry. They indicate that in situ measurements of photosynthesis by acoustic methods can provide new insights into the mechanisms and adaptive responses of seagrasses to environmental factors at the scale of a prairie