Comparing the acoustic response of bubbles adhered to artificial seagrass blades with models for freely oscillating bubbles

Seagrass beds are threatened coastal ecosystems that play a significant role in carbon sequestration despite covering a small fraction of Earth’s oceans by area. During photosynthesis, oxygen bubbles fill channels within the seagrass blades, nucleate onto the blades’ surface, and rise freely through the water column. Since bubbles interact strongly with acoustic fields, acoustics have been used to monitor seagrass photosynthesis. However, many acoustics models for bubbly liquids only consider freely oscillating bubbles. To determine the extent of these models’ applicability to seagrass monitoring, we conducted a controlled laboratory experiment isolating the effect of bubbles adhered to seagrass blades on the propagation and backscatter of acoustic signals. After oxygenating a tank containing a bed of artificial seagrass, we used an ES200-7CD transducer, RS-4034 hydrophone, and EK80 transceiver to measure acoustic backscatter, phase speed, and attenuation at several points in time after oxygenation. A calibrated Olympus Tough TG-6 digital camera and custom softbox were used to concurrently measure the radius distributions of bubbles adhered to the blades. We used these distributions to predict the acoustic response using models for freely oscillating bubbles. A comparison between these predictions and the measured acoustic response will be discussed. [Work supported by ONR and NSF.]