Impacts of infaunal activity and physical disturbance on acoustic properties of muddy coastal sediments

Storm disturbance of shallow coastal sediments can resuspend subtidal sediment and transport and deposit sand from beach erosion, creating sorted layers of differing grain sizes. These sediments provide habitat for diverse and abundant infaunal organisms that mix these layers (bioturbation), but how the reestablishment of disturbance-tolerant infauna affects recently disturbed sediment structure, including acoustic properties, is poorly understood. In this laboratory study, we compared the effects of physical disturbance (resuspension of surface muddy sediment and addition of sand, simulating a storm), infaunal activities (burrowing by brittle stars), and the combination of both on acoustic properties of sediments. We hypothesized that acoustic properties would reflect faster reconsolidation and more mixing of the initial mud layer with the deposited sand in sediment with infauna compared to defaunated sediment. At several timepoints following disturbance (1–14 days), we measured sound speed and attenuation at 400 kHz at multiple depths within and below the resuspended layer. After 14 days, we measured porosity, grain size, and bioturbation with tracer particles to relate to acoustic properties. Infaunal effects on post-storm sediment structure may be important to consider for acoustic mapping of reconsolidating sediment in shallow coastal areas.