A biological source of bubbles in sandy marine sediments

Abstract

Gas in sediments, even in small quantities, will modify the propagation and scattering of sound. Shallow water littoral environments are often sufficiently well lit by sunlight to support healthy populations of benthic and epibenthic marine microalgae. Photosynthesis in marine algae produces oxygen. Oxygen saturation levels as high as 600% have been measured in the pore water of a sandy sediment at 1 mm depth, decreasing to 100% saturation at ca. 4.5 mm. Bioturbation and physical processes routinely mix materials at the surface of the seabed, including algae, to depths of at least a few cm. Mixing times and depths vary with the sediment type and the species and abundance of organisms present, but time scales of minutes to hours are common. While light is rapidly attenuated with depth in sand, measurements show that it can penetrate to depths of a few mm. Physical and biological mechanisms are suggested which could produce gas bubbles in oxygen saturated pore water. Laboratory measurements of sound scattering from marine algae on a sand surface suggest a possible method for in situ bubble detection in shallow marine environments. [Work supported by ONR.]