Nutrients in the Open Ocean – Water Column Profiles

All plants, whether they are tomatoes in your garden, trees in the forest, or phytoplankton in the ocean require three things to grow – water, sunlight, and nutrients. In the ocean, as there is no shortage of water, the dominant factors impacting phytoplankton growth are sunlight and nutrients. Often in the open ocean, however, nutrients are at their highest concentrations deep below the sunlit surface layer of the ocean. In these areas, phytoplankton growth tends to be limited to the thin layer at the bottom of the surface mixed layer that has enough light to facilitate growth and abuts the nutrient-rich deep waters. If the surface mixed layer is too deep, by the time the phytoplankton cells are deep enough to reach the nutrients they need for growth, phytoplankton production becomes light limited rather than nutrient limited. The same processes that limit phytoplankton growth in the open ocean can lead to highly productive coastal margins as upwelling brings these nutrient rich deep waters to the surface – check out the Coastal Upwelling nugget.

This nugget explores water column profiles of two OOI Regional Cabled Array sites in the NE Pacific – Oregon Slope Base at the base of the continental slope and Axial Base to the east, 500 km offshore. At both sites, there is a clear surface mixed later with limited nutrients, though this layer is more pronounced at Axial Base. The highest nutrient concentrations are in the deeper waters. An interesting feature in the Axial Base profiles is the peak in oxygen concentration at the base of the mixed layer due to a thin layer of primary production where nutrients are accessible but the phytoplankton are still in an area with enough light to reproduce.

Upper water column profiles from the Axial Base Cabled Shallow Mooring Profiler. Water temperature profiles show a clear surface mixed layer throughout the year with a deepening of the layer during the winter. Note that the surface mixed layer is more driven by temperature than salinity which is more uniform. Nutrients, though depleted a the surface, are high at depth. During the summer months when the surface layer is shallowest, note the thin layer of high oxygen focused at the base of the mixed layer. This is due to increased primary production as phytoplankton have access to both light and nutrients in this thin layer. After this period of high production, the newly generated biomass sinks out of the area leading to a decrease in oxygen due to decompositon of these dead cells in the fall. Note also the increase in Colored Dissolved Organic Matter (CDOM) during this time and the increase in nitrate as nutrients are remineralized and mixed back into the water column.

Access Axial Base Data

Disclaimer: data used in this example and provided in the .csv file were downloaded from the OOI on Feb 20, 2020. The file format and/or contents could have changed if downloaded directly from OOI Net after this date.

Access from OOI Net:
RS03AXPS-SF03A-4A-NUTNRA301
RS03AXPS-SF03A-2A-CTDPFA302
RS03AXPS-SF03A-3A-FLORTD301

Pull Data Using Python Code. Code demonstrates how to download data from multiple instruments on the cabled Shallow Profilers from the OOI system using the Machine-to-Machine (M2M) interface, downsample the dataset, and export as a .csv file.

Plot Wire-Following Profiler Data Using Matlab Code. Code demonstrates how to graph wire-following profiler data from a .csv file.

Axial Base Shallow Profiler Mooring Data Review Pages:
Nitrate
CTD
3-Wavelength Fluorometer

Access Slope Base Data

Disclaimer: data from the Slope Base Profiler Mooring were downloaded from the OOI with the Axial Base Profiler Mooring on Feb 20, 2020. Data can be pulled and exported as a .csv file using the same Python code.

Access from OOI Net:
RS01SBPS-SF01A-4A-NUTNRA101
RS01SBPS-SF01A-2A-CTDPFA102
RS01SBPS-SF01A-3A-FLORTD101

Plot Wire-Following Profiler Data Using Matlab Code. Code demonstrates how to graph wire-following profiler data from a .csv file.

Oregon Slope Base Shallow Profiler Data Review Pages:
Nitrate
CTD
3-Wavelength Fluorometer

Upper water column profiles from the Slope Base Cabled Shallow Mooring Profiler. These profiles show similar dynamics to the Axial Base profiler. Note, however, that the water column has less of a clear surface mixed layer and as such parameters are more uniform in the water column. Basic features still exist wherein deeper waters are colder, saltier, and have more nitrate. Note the increase in CDOM in surface waters during the fall, this may be due to runoff from from land and the Columbia River Plume.

Oregon Slope Base Shallow Profiler Mooring (RS01SBPS)

Location: At the base of the Continental Slope off the coast of Oregon in the NE Pacific
Lat/Lon: 44.529°N, 125.3893°W
Water Column Depth: 2,906 m
Platform: Shallow Profiler (SF01A)
Instruments: All instruments located on the Shallow Profiler
CTD (CTDPF-A)
3-Wavelength Fluorometer (FLORT-D)
Nitrate (NUTNR-A)

Axial Base Shallow Profiler Mooring (RS03AXPS)

Location: Near the base of the Axial Seamount at the far western edge of the Juan de Fuca tectonic plate
Lat/Lon: 45.8305°N, 129.7535°W
Water Column Depth: 2,607 m
Platform: Shallow Profiler (SF03A)
Instruments: All instruments located on the Shallow Profiler
CTD (CTDPF-A)
3-Wavelength Fluorometer (FLORT-D)
Nitrate (NUTNR-A)

Essentials of Oceanography Textbook Sections

5.6 How does seawater salinity vary at the surface and with depth?
13.1 What is primary productivity?

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

Next Gen Science Standard Connections

HS-LS2-4. Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem. These profiles show a cycle of primary production followed by decomposition that releases nutrients back into the water column. These nutrients are then sequestered below the mixed layer until conditions are favorable for the next period of high production.

HS-LS2-5. Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere. These data provide one component of this broader model by highlighting productivity generated in the open ocean due to changes in the depth of the surface mixed layer.

OOI Science Theme

Ocean Circulation, Mixing, and Ecosystems

Related Publications

Henderikx Freitas, F., et al. 2018. Temporal and spatial dynamics of physical and biological properties along the Endurance Array of the California Current ecosystem. Oceanography 31(1):80–89. https://doi.org/10.5670/oceanog.2018.113.