Dynamic benthic oxygen fluxes lessen hypoxia effects on open continental shelves

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 m2 box 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.