Hypoxia off the Oregon Coast

The coast of Oregon is known for its highly productive surface waters fueled by nutrients upwelled to the surface. This high productivity is important to support local fisheries, particularly Dungeness crabs. However, these cycles of upwelling and production can also lead to harmful conditions of hypoxia (low oxygen) in bottom waters that can have devast­ing impacts on local fisheries.

Hypoxia typically occurs near the seafloor in productive coastal areas when high summer productivity leads to the deposition of a large amount of organic matter on the seafloor. As this organic matter decomposes, oxygen is removed from the water via respiration. Under a stratified water column the oxygen-rich surface water cannot mix down to replenish the oxygen poor bottom water and hypoxic conditions set in

In the case of the Oregon coast, hypoxic events are linked to shifts in upwelling dynamics, in addition to the decay of organic matter that sinks out of the productive surface waters. Looking at OOI Coastal Endurance Array data from the seafloor on the Oregon shelf at 25m depth during the of summer 2014, there is a clear period of downwelling captured during the end of June (gray bar). During this period, bottom temperatures and oxygen concentrations are high and salinities are low, indicating that warmer, fresher, oxygen-rich surface waters were mixed throughout the water column. There is then an abrupt shift to a period of upwelling in which cold, salty, low-oxygen water is advected from deeper offshore water onto the shelf.

Hypoxic episodes during summer 2014 on the Oregon Shelf. In the first two episodes, bottom oxygen levels hovered around the hypoxic threshold of 62 μMol kg–1 (red dashed line) and one prolonged event of severe hypoxia in July. Prior to the prolonged event in July there is a clear shift to a period of downwelling (gray bar) before the upwelling resumes. Bottom temperature and salinity data at the same location are also provided to highlight associated changes in water column structure due to shifts between upwelling and downwelling.

Access the Data

Disclaimer: data used in this example and provided in the .csv file were downloaded from the OOI on Aug 9, 2019. The file format and/or contents could have changed if downloaded directly from OOI Net after this date.

Access from OOI Net:

Pull Data Using Python Code. Code demonstrates how to download dissolved oxygen data from the OOI system using the Machine-to-Machine (M2M) interface and export the data as a .csv file.

Data Review Pages
Dissolved Oxygen Sensor

Endurance Array Oregon Inshore
Surface Mooring

Location: Oregon Continental Shelf, NE Pacific
Lat/Lon: 44.6598°N, 124.095°W
Water Column Depth: 25m
Platform: Seafloor Multi-Function Node
Dissolved Oxygen (DOSTA-D)

Essentials of Oceanography Textbook Sections

5.4 Why does seawater salinity vary?
7.2 What creates ocean surface currents and how are they organized?
7.3 What causes upwelling and downwelling?
12.4 How are marine organisms adapted to the physical conditions of the ocean?
13.1 What is primary productivity?
13.2 What kinds of photosynthetic marine organisms exist?
16.4 What changes are occurring in the oceans as a result of global warming?

For more details, check out the full “Textbook Crosswalk”

Next Gen Science Standard Connections

HS-LS1-7. Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed, resulting in a net transfer of energy. This dataset illustrates oxygen reduction in the surrounding water column as a result of cellular respiration.

Additional Resources

National Ocean Service Podcast – Dealing with Dead Zones: Hypoxia in the Ocean
NPR Weekend Edition – Coastal Pacific Oxygen Levels Now Plummet Once A Year

OOI Science Theme

Coastal Ocean Dynamics and Ecosystems (sub theme Hypoxia on Continental Shelves)

Related Publications

Chan, F., et al. 2019. The dynamics and impact of ocean acidification and hypoxia: Insights from sustained investigations in the Northern California Current Large Marine Ecosystem. Oceanography 32(3):62–71. https://doi.org/10.5670/oceanog.2019.312.

Barth, J.A., et al. 2018. Warm blobs, low-oxygen events, and an eclipse: The Ocean Observatories Initiative Endurance Array captures them all. Oceanography 31(1):90–97. https://doi.org/10.5670/oceanog.2018.114.

Adams, K.A., et al. 2016. Intraseasonal Cross-Shelf Variability of Hypoxia along the Newport, Oregon, Hydrographic Line. Journal of Physical Oceanography 46:2219–2238. https://doi.org/10.1175/JPO-D-15-0119.1.

McCann-Grosvenor, K., et al. 2014. Dynamics of the benthic boundary layer and seafloor contributions to oxygen depletion on the Oregon inner shelf. Continental Shelf Research 84:93-106. https://doi.org/10.1016/j.csr.2014.05.010.