Do environmental stress-induced changes in faunal composition and bioturbation affect sediment geoacoustic properties? A test case in the Gulf of Mexico dead zone

Abstract

Four sites on the continental shelf off Louisiana, each subjected to different historical exposures to low concentrations of bottom-water dissolved oxygen, are investigated in terms of their macrobenthos species composition, sediment physical properties, and sediment geoacoustic properties (sound speed, attenuation, and impedance). From macrobenthos species composition, feeding type is identified, which allows categorization of some bioturbation activity as either dilation or compaction of sediment. Dilation and compaction should affect sediment properties of bulk density and porosity, which are significant predictor variables of geoacoustic properties. Different levels of oxygen stress correspond with statistically separable macrobenthos assemblages, abundance and diversity of biogenic structures (burrows and voids), and ratios of dilators to compactors. Sediment sound speed and attenuation values measured in subcores from box cores are compared at each of the four sites with different faunal composition and average bottom-water dissolved oxygen concentrations to test the long-term (historical) effects of different bioturbation regimes on geoacoustic properties. Sediment acoustic properties are regressed on sediment physical properties and compared with established empirical fits from a worldwide database to identify possible anomalies. The impedance of surficial sediment is measured at each site with the Acoustic Sediment Classification Profiler to examine possible large-scale (side-wide) effects on geoacoustic properties.