Monterey Bay Aquarium Research Institute
CANON Initiative
Case study:
Low oxygen

Presented by Ricardo Letelier and Francisco Chavez

The key question is how to characterize, monitor, and quantify the role of low oxygen regions in the elemental balance of the oceans. Answering this question requires researchers to understand the dynamics of oxygen content in the ocean, characterize the response of microbial populations to oxygen gradients, and to analyze how sensitive metabolic processes are to various levels of oxygen.

Important large-scale changes may be occurring such as oxygen minimium zones (OMZs) expanding and increasing in intensity. Large-scale circulation of oxygen may also affect the maintenance of the East Boundary hypoxic and anoxic zones. On the local scale, important issues include how sensitive biological processes are to very low oxygen conditions. Another issue is how chemical and physical gradients affect microbial communities and rate processes. In coastal margins important issues include the characterization of the bentho-pelagic boundary.

world map of the global distribution of oxygen at the oxygen minimum

World map of the global distribution of oxygen at the oxygen minimum. Keeling et al.(2010) Annual Review of Earth and Planetary Sciences, 199

Oxygen minimum zone (OMZ)

The oxygen minimum zone (OMZ) constitutes approximately 0.1% of the global ocean volume. It is in contact with two to three percent of the ocean floor. The OMZ is primarily in contact with the continental shelf and slope. The OMZ helps to contribute to microbial diversity as well as 30-50% of the total nitrogen lost. The nitrogen is lost due to a process called denitrification, which is the reduction of nitrate to nitrogen by bacteria. It is also a significant source of carbon dioxide (CO2) as well as nitrous oxide (N2O). Variations in the total amount of the OMZ has been offered as an explanation in past changes in atmosphere N2O and CO2.

Series of images showing the concentration of the OMZ in the Eastern South Pacific bordering South America.
Series of images showing the concentration of the OMZ in the Eastern South Pacific bordering South America. Source: Ulloa & De Pol (2004).

Low oxygen research questions

  • Are microbial diversity (and abundance) and oxygen correlated (in OMZs)?

  • How does oxygen modify microbial processes/rates (in OMZs)?

  • How do we measure/quantify the role of changing OMZs on air-sea greenhouse gas fluxes?

  • How do we measure/quantify the role of OMZs on the biological pump?

Current autonomous sampling approaches

Picture of a Glider being deployed at sea
Glider being deployed at sea. Source: Cepeda-Morales et al. 2009

Researchers focus on repeating measurements of the physical characteristics and the depth of the oceans and the surrounding land such as continental shelves.

They use tracers to sample the water chemistry and use time series monitoring to assess the concentration of OMZs. One problem with sampling is that it is very easy to contaminate samples with additional oxygen.

Gradients of the concentration of oxygen can also be very steep such as supersaturation to almost zero over depth change of 30 meters

Sampling approaches

Current sampling approaches involve sampling one liter of water in less than one hour for determining microbe abundance and diversity. Only sampling 50 milliliters of water in less than a minute is required in order to determine the abundance and contextual chemistry (nutrients). In order to effectively catalog large scale processes of nitrate deficits, large fleets of autonomous systems (gliders and autonomous underwater vehicles) with sensor information for the chemistry of the ocean water is needed. Future approaches to sampling involve using a long-range autonomous underwater vehicle sampling tracers along isopycnals. Another approach is to use argo floats with oxygen sensors.

Unanswered questions

  • Are these microbial assemblages stable and cosmopolitan?
  • What happens at the boundaries O2 gradients in space and time)?
  • What is the sensitivity of some processes (e.g. annamox and denitrification) to small changes in O2
  • What is the role of the bentho-pelagic boundary over the continental shelf?

Image showing the mesocale variability of the OMZ using data from a glider

Mesocale variability of the OMZ using data from a glider. Source: Cepeda-Morales et al. 2009

Going forward

In order to look forward researchers first need to figure out the relevant scales. Researchers need to sample biological diversity and processes at the same scale of physical parameters. Perturbation experiments are necessary to determine if low oxygen zones have an effect on microbial populations.

Last updated: Dec. 04, 2013