Tank and field experiments of short-range acoustic propagation through a seagrass canopy

Aiming at monitoring the bubbles released during seagrass photosynthesis in different conditions, three experiments were made: two in an outdoor tank and one in the field. The outdoor tank experiments were conducted in winter and summer conditions with Cymodocea nodosa plants transplanted from a nearby meadow. The field experiment took place in September 2018 on a Posidonia oceanica meadow, where bubbles formation due to plants photosynthesis has been reported. In all the experiments continuous wave pulses in the band 1020 kHz were used to estimate the diel change of effective sound speed and the attenuation over the plants. Additionally, high frequency backscatter images at 0.5, 1, 2 and 4 Mhz were obtained to figure out bubbles signature. The results suggest that even in winter, the tank conditions, without water movement, allows for the formation of a small amount bubbles of small radii. In summer tank conditions, the bubbles formation due to photosynthesis of seagrasses significantly increases, but also algae may contribute for the bubbles formation in the environment. The field experiment was carried out after the peak of a bad weather event. There was no evidence of bubbles formation, what can be related to water movement and leaves stirring, that prevents bubbles formation.