Low-frequency acoustic behavior of photosynthetically active seagrasses

Acoustic remote sensing techniques are an important tool for mapping seagrass coverage. Three different photosynthesis-related processes can occur in seagrasses that affect the acoustic behavior. First, the air channels within the leaf pressurize with produced gas. Next, bubbles form on the leaf blades. Finally, some of these bubbles break off and enter the water column. A one-dimensional acoustic resonator technique was adapted to monitor the photosynthetic activity of two Mediterranean seagrasses, Posidonia oceanica and Cymodocea nodosa. Measurements of the low-frequency (1–8 kHz) effective sound speed of a mixture of seagrass leaf blades and artificial seawater were taken at regular intervals during periods of no direct light and exposure to photosynthetically active radiation. The acoustic response is compared to independent dissolved oxygen measurements and visual observation of bubble formation.