skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: The role of seasonal hypoxia and benthic boundary layer exchange on iron redox cycling on the Oregon shelf
Abstract Widespread hypoxia occurs seasonally across the Oregon continental shelf, and the duration, intensity, and frequency of hypoxic events have increased in recent years. In hypoxic regions, iron reduction can liberate dissolved Fe(II) from continental shelf sediments. Fe(II) was measured in the water column across the continental shelf and slope on the Oregon coast during summer 2022 using both a trace metal clean rosette and a high‐resolution benthic gradient sampler. In the summer, Fe(II) concentrations were exceptionally high (40–60 nM) within bottom waters and ubiquitous across the Oregon shelf, reflecting the low oxygen condition (40–70 μM) at that time. The observed inverse correlation between Fe(II) and bottom water oxygen concentrations is in agreement with expectations based on previous work that demonstrates oxygen is a major determinant of benthic Fe fluxes. Rapid attenuation of Fe(II) from the benthic boundary layer (within 1 m of the seafloor) probably reflects efficient cross‐shelf advection. One region, centered around Heceta Bank (~ 44°N) acts a hotspot for Fe release on the Oregon continental shelf, likely due to its semi‐retentive nature and high percent mud content in sediment. The results suggest that hypoxia is an important determinant of the inventory of iron is Oregon shelf waters and thus ultimately controls the importance of continental margin‐derived iron to the interior of the North Pacific Basin.  more » « less
Award ID(s):
2023708
PAR ID:
10482787
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Limnology and Oceanography
ISSN:
0024-3590
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; ; ; ; et al (, Frontiers in Marine Science)
    null (Ed.)
    Hypoxia and associated acidification are growing concerns for ecosystems and biogeochemical cycles in the coastal zone. The northern Gulf of Mexico (nGoM) has experienced large seasonal hypoxia for decades linked to the eutrophication of the continental shelf fueled by the Mississippi River nutrient discharge. Sediments play a key role in maintaining hypoxic and acidified bottom waters, but this role is still not completely understood. In the summer 2017, when the surface area of the hypoxic zone in the nGoM was the largest ever recorded, we investigated four stations on the continental shelf differentially influenced by river inputs of the Mississippi-Atchafalaya River System and seasonal hypoxia. We investigated diagenetic processes under normoxic, hypoxic, and nearly anoxic bottom waters by coupling amperometric, potentiometric, and voltammetric microprofiling with high-resolution diffusive equilibrium in thin-films (DET) profiles and porewater analyses. In addition, we used a time-series of bottom-water dissolved oxygen from May to November 2017, which indicated intense O 2 consumption in bottom waters related to organic carbon recycling. At the sediment-water interface (SWI), we found that oxygen consumption linked to organic matter recycling was large with diffusive oxygen uptake (DOU) of 8 and 14 mmol m –2 d –1 , except when the oxygen concentration was near anoxia (5 mmol m –2 d –1 ). Except at the station located near the Mississippi river outlet, the downcore pore water sulfate concentration decrease was limited, with little increase in alkalinity, dissolved inorganic carbon (DIC), ammonium, and phosphate suggesting that low oxygen conditions did not promote anoxic diagenesis as anticipated. We attributed the low anoxic diagenesis intensity to a limitation in organic substrate supply, possibly linked to the reduction of bioturbation during the hypoxic spring and summer. 
    more » « less
  2. (, US NSF Ocean Observatories Initiative)
    The Oregon Slope Base Deep Profiler Mooring is located adjacent to the continental slope off the coast of Oregon at a water depth of ~2,900 meters. Here, ocean water properties are profoundly impacted by the California Current and internal waves. The coastal region of the Pacific Northwest is a classic wind-driven upwelling system where nutrient-rich deep waters rise to replace warmer surface waters, resulting in high marine productivity that attracts zooplankton, fish, and marine mammals. Near-bottom fauna are periodically negatively impacted by the flow of deep waters with very low oxygen concentrations (hypoxic events), and upwelling of corrosive, acidified waters onto the continental shelf. This mooring contains a Wire-Following Profiler that hosts six scientific instruments and moves through the water column along the mooring riser, continuously sampling ocean characteristics over a specified depth interval (150 meters below sea surface to near bottom) and is connected to an electro-optical cable that provides a large supply of power and bandwidth. When coupled with other Cabled Array and Endurance Array installations off the central Oregon coast, the Slope Base Deep Profiler Mooring allows for measurements of a variety of coastal phenomena, including cross-shelf and along-shelf variability. 
    more » « less
  3. ; ; ; ; ; ; ; ; ; (, Journal of geophysical research Oceans)
    St Helena Bay (SHB), a retentive zone in the productive southern Benguela Upwelling System off western South Africa, experiences seasonal hypoxia and episodic anoxic events that threaten local fisheries. To understand the drivers of oxygen variability in SHB, we queried 25 years of dissolved oxygen (DO) observations alongside high‐resolution wind and hydrographic data, and dynamical data from a high‐resolution model. At 70 m in SHB (mid‐bay), upwelling‐favorable winds in spring drove replenishment of cold, oxygenated water. Hypoxia developed in summer, becoming most severe in autumn. Bottom waters in autumn were replenished with warmer, less oxygenated water than in spring—suggesting a seasonal change in source waters upwelled into the bay. Downwelling and deep mixing in winter ventilated mid‐bay bottom waters, which reverted to hypoxic conditions during wind relaxations and reversals. In the nearshore (20 m), hypoxia occurred specifically during periods of upwelling‐favorable wind stress and was most severe in autumn. Using a statistical model, we extended basic hydrographic observations to nitrate and DO concentrations and developed metrics to identify the accumulation of excess nutrients on the shelf and nitrogen‐loss to denitrification, both of which were most prominent in autumn. A correspondence of the biogeochemical properties of hypoxic waters at 20 m to those at 70 m implicates the latter as the source waters upwelled inshore in autumn. We conclude that wind‐driven upwelling drives the replenishment of respired bottom waters in SHB with oxygenated waters, noting that less‐oxygenated water is imported later in the upwelling season, which exacerbates hypoxia. 
    more » « less
  4. ; ; ; ; ; ; ; ; (, Journal of Geophysical Research: Oceans)
    Abstract The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient‐enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf‐wide carbon production contributing to hypoxia formation. This study applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations reproduced spatial and temporal patterns of carbon production, respiration, and bottom‐water oxygen gradients compared to field observations. We used multiyear simulations to quantify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial displacement of carbon production and respiration in our simulations was created by westward and offshore POC flux via phytoplankton carbon flux in the surface layer and POC flux in the bottom layer, supporting heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results support existing studies suggesting the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are most variable. 
    more » « less
  5. (, US NSF Ocean Observatories Initiative)
    The Oregon Slope Base Shallow Profiler Mooring is situated adjacent to the continental slope off the coast of Oregon at ~2,900 meters water depth. Here, ocean water properties are profoundly impacted by the California Current and internal waves. The coastal region of the Pacific Northwest is a classic wind-driven upwelling system where nutrient-rich deep waters rise to replace warmer surface waters, resulting in high marine productivity that attracts zooplankton, fish, and marine mammals. Near-bottom fauna are periodically negatively impacted by the flow of deep waters with very low oxygen concentrations (hypoxic events), and upwelling of corrosive, acidified waters onto the continental shelf. This two-legged mooring is attached to an electro-optical cable that supplies power and bandwidth and hosts a Shallow Profiler (SF01B), a 200m Platform (PC01B), and a Winch Controller (SC01B). The 200 m platform and Shallow Profiler both house scientific instrumentation, and the profiler is tethered to a mooring-mounted winch that allows it to travel across a fixed depth in the water column (20 m to 200 m below sea surface), determined by currents and wave conditions at the surface. The mooring is co-located with a Low-Power junction box that collects complementary data near the seafloor. When coupled with other Cabled Array and Endurance Array installations off the central Oregon coast, the Slope Base Shallow Profiler Mooring allows for measurements of a variety of coastal phenomena, including cross-shelf and along-shelf variability. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email