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Organic carbon mineralization and nutrient cycling in benthic environments are critically important for their biogeochemical functioning, but are poorly understood in coastal upwelling systems. The main objective of this study was to determine benthic oxygen fluxes in a muddy sediment in the Ria de Vigo (NW Iberian coastal upwelling), by applying the aquatic eddy covariance (AEC) technique during 2 campaigns in different seasons (June and October 2017). The main drivers of benthic fluxes were studied and compared among days in each season and between seasons. The 2 campaigns were characterized by an upwelling-relaxation period followed by a downwelling event, the last of which was due to the extratropical cyclone Ophelia in October. The mean (±SD) seasonal benthic oxygen fluxes were not significantly different for the 2 campaigns despite differences in hydrodynamic and biogeochemical conditions (June: -20.9 ± 7.1 mmol m -2 d -1 vs. October: -26.5 ± 3.1 mmol m -2 d -1 ). Benthic fluxes were controlled by different drivers depending on the season. June was characterized by sinking labile organic material, which enhanced benthic fluxes in the downwelling event, whereas October had a significantly higher bottom velocity that stimulated the benthic fluxes. Finally, a comparison with a large benthic chamber (0.50 m 2 ) was made during October. Despite methodological differences between AEC and chamber measurements, concurrent fluxes agreed within an acceptable margin (AEC:benthic chamber ratio = 0.78 ± 0.13; mean ± SD). Bottle incubations of water sampled from the chamber interior indicated that mineralization could explain this difference. These results show the importance of using non-invasive techniques such as AEC to resolve benthic flux dynamics.
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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 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.
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- Award ID(s):
- 2126112
- PAR ID:
- 10643152
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 70
- Issue:
- 10
- ISSN:
- 0024-3590
- Format(s):
- Medium: X Size: p. 2998-3015
- Size(s):
- p. 2998-3015
- Sponsoring Org:
- National Science Foundation
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