Effects of hypoxia on sediment properties in the northern Gulf of Mexico

The effects of hypoxia on benthic community structure and processes are well documented but the direct and indirect effects of hypoxia on seafloor properties are unknown. Using bottom oxygen data compiled by Louisiana Universities Marine Consortium, we chose four provinces with sea floors having different number of annual exposures to hypoxia from 1985 to 2006. The provinces were arrayed east to west along the 30-m depth contour from the Mississippi River bird's foot (FH), to south of Terrebonne Bay (BH), and to south of Atchafalaya Bay (HO and NO). Several sediment properties can be affected as a consequence of the vertical zonation and diversity differences between benthos living in hypoxic and normoxic areas. Sediment physical and acoustic properties were measured on cores collected from the March/April 2009 cruise to attain baseline values for comparison among provinces and between pre-hypoxic and persistently hypoxic provinces to be sampled in early September 2009, August 2010, and July 2011. Average values for sediment compressional wave velocity ratio were 1.063, 1.003, 0.979, and 0.995 for provinces NO, BH, FH, and HO, respectively. Average values for sediment compressional wave attenuation were 0.93, 0.30, 0.11, and 0.32 dB/m?kHz for provinces NO, BH, FH, and HO, respectively. Average values for sediment shear strength in the uppermost 5 cm of subcores were 1.3, 1.8, 0.6, and 0.8 kPa for provinces NO, BH, FH, and HO, respectively. The gradients of sediment shear strength in the top 5 cm of sediment were 0.40, 0.42, 0.17, and 0.28 kPa/cm for provinces NO, BH, FH, and HO, respectively. Sediment grain size distribution and bulk density were measured at 1-cm intervals in the top 2 cm of the core and at 2-cm intervals below those increments on 22 cores. The sediments at provinces NO, BH, and HO were very poorly sorted to extremely poorly sorted sand-silt-clays (mean: 4.9, 8.3, and 8.1 phi, respectively); the sediment at the FH province was a very poorly sorted silty clay (mean: 11.1). Average values for sediment density were 1.819, 1.597, 1.289, and 1.519 g/cm
³
for provinces NO, BH, FH, and HO, respectively. Sediment permeability was measured from each of the 24 box cores on 13-cm-long cores with a falling head permeameter.