Expansion of the oxygen minimum zone

November 8, 2013

In yesterday’s blog, I talked about the importance of studying midwater communities and the oxygen minimum zone (OMZ). I also mentioned that as the ocean warms due to climate change, OMZs are expanding. Why is that, you might ask? There are a few key factors in this expansion:

1. Warmer water holds less dissolved oxygen than colder water. While the warming of the ocean begins at the sea surface, the warming does penetrate deeper in the water column as well. This means that waters that may not have been considered by scientists as “within the OMZ” a few years ago, now are, because the increasingly warmer temperatures lead to less oxygen available for animals to respire.

2. Although the warming penetrates down past the surface waters, it is decidedly more severe near the surface. This causes stratification of the water column, where warmer water above does not mix well with cooler water below because of differences in density. Stratification restricts the mixing of this oxygen-rich surface layer with the waters below, leading to further depletion of the oxygen levels in the deep midwater.

3. Increased carbon dioxide in the atmosphere mixes with surface water and increases the amount of carbon dioxide available to phytoplankton. This increases production by phytoplankton which means that more dead and dying material will sink to deeper waters where bacteria will break it down, using up oxygen.

All of these factors lead to an OMZ that can cover a wider depth range or even cover a larger geographical area. So, how will this expansion of the OMZ affect animals in the midwater?

The midwater lab has been studying the respiration of common midwater animals for over a decade. Deep-sea animals in the lab surely behave, feed, and respire differently than they do in their natural habitat. The midwater lab developed the Midwater Respirometry System (MRS) to measure respiration of deep-sea animals in their habitat. The MRS is made up of modified detritus samplers (see photo below) that measure the oxygen levels and temperature in the sampler.

ROV pilot Randy Prickett looks on as the ROV Doc Ricketts is pulled up through the Western Flyer’s moonpool. You can see the MRS at the bottom of the vehicle.

ROV pilot Randy Prickett looks on as the ROV Doc Ricketts is pulled up through the Western Flyer’s moonpool. You can see the MRS at the bottom of the vehicle.

Researchers collect animals in these chambers and hang the entire system on a mooring, leaving it there for one, two, or even more days. These data provide a more reliable measurement of respiration of the sampled animals than could be obtained in the lab. Using the MRS, researchers don’t have to account for physiological changes that might occur as the animal is brought to the surface, where all of the environmental conditions are drastically different from their deep-sea home.

Knowing how animals respire in the midwater under “normal” conditions will lead to insights about how they might be able to adapt (or be stressed) by changes in the OMZ.

Yesterday, we deployed the MRS after collecting a common member of the midwater community. This siphonophore, Nanomia bijuga, mainly eats krill and is very abundant at depths between 300 and 500 meters. Today, after the MRS hung on the mooring for 24 hours at around 300 meters, we recovered it and Kim Reisenbichler will analyze the data to see if the experiment was a success.

The primary animal that was collected and respired by the MRS was a very abundant siphonophore, Nanomia bijuga.

The primary animal that was collected and respired by the MRS was a very abundant siphonophore, Nanomia bijuga.


When the ROV is secured on board, the MRS drawer is deployed so the science team can access the samples. Bruce Robison, Rob Sherlock, and Kim recover the Nanomia bijuga from the MRS specimens for further analysis in the lab.

When the ROV is secured on board, the MRS drawer is deployed so the science team can access the samples. Bruce Robison, Rob Sherlock, and Kim recover the Nanomia bijuga from the MRS specimens for further analysis in the lab.


Today, we observed this very rare jelly, Stellamedusa ventana. It was discovered by the midwater lab in the 1990s and was named after MBARI’s ROV Ventana.

Today, we observed this very rare jelly, Stellamedusa ventana. It was discovered by the midwater lab in the 1990s and was named after MBARI’s ROV Ventana.

—Susan von Thun