Search for: All records

Abstract Under the supposition that organisms inhabiting physically dynamic marine environments are better able to survive hypoxic conditions than those experiencing little turbulent or advective augmentation of oxygen fluxes, we evaluated summertime benthic macrofauna communities, in situ aquatic eddy covariance measurements, and ex situ sediment core incubations from 5 latitudinally distinct mid‐shelf locations off Oregon–Washington, USA. Despite bottom water dissolved oxygen (DO) concentrations averaging from 17 to 75 μmol L⁻¹, invertebrate faunal collections contained mixtures of 11 to 28 taxa per 0.1 m²box core and increased in richness and abundance at sites with greater velocity variation. Eddy covariance velocity records of 18‐30 hours regularly showed the arrivals of internal waves. Oxygen fluxes, derived in 15‐min intervals, correlated with multiple flow parameters assessed from velocity components. Daily averages of the oxygen fluxes to the sediment were determined to range from −3.5 to −23 mmol m⁻² d⁻¹, and these fluxes, assumed to fully represent seabed respiration, were 2 to 5 times greater than rates of DO uptake by sediment cores from the same locations. Velocity profiles measured from 0.3 to 2.5 m above the seafloor at a subset of sites were consistent with a wave‐current boundary layer modulated by ocean swell. These findings illustrate how natural physical processes can relieve the stress of hypoxia exposure on the benthos. Physical dynamics play critical roles in supplying DO and determining sediment grain size, permeability, and the activities of benthic organisms. Thus, these factors need consideration when predicting the impacts of low DO concentrations in coastal regions. 

Abstract A striking feature of Oxygen Deficient Zones (ODZs) on the eastern boundary of the Pacific Ocean are large subsurface plumes of iodide. Throughout the oceans, iodate is the predominant and thermodynamically favored species of dissolved iodine, but iodate is depleted within these plumes. The origin of iodide plumes and mechanism of reduction of iodate to iodide remains unclear but is thought to arise from a combination of in situ reduction and inputs from reducing shelf sediments. To distinguish between these sources, we investigated iodine redox speciation along the Oregon continental shelf. This upwelling system resembles ODZs but exhibits episodic hypoxia, rather than a persistently denitrifying water column. We observed elevated iodide in the benthic boundary layer overlying shelf sediments, but to a much smaller extent than within ODZs. There was no evidence of offshore plumes of iodide or increases in total dissolved iodine. Results suggest that an anaerobic water column dominated by denitrification, such as in ODZs, is required for iodate reduction. However, re‐analysis of iodine redox data from previous ODZ work suggests that most iodate reduction occurs in sediments, not the water column, and is also decoupled from denitrification. The underlying differences between these regimes have yet to be resolved, but could indicate a role for reduced sulfur in iodate reduction if the sulfate reduction zone is closer to the sediment‐water interface in ODZ shelf sediments than in Oregon sediments. Iodate reduction is not a simple function of oxygen depletion, which has important implications for its application as a paleoredox tracer.