The oceans

Hypoxic Zone

Center for Ocean Solutions,

22 July, 2013

Photosynthesis by phytoplankton in surface waters of the ocean constitutes the base of most marine food chains. As organic material derived from this productivity (dead plankton, feces, etc.) sinks, microbes digest this material and remove oxygen from the water (and add carbon dioxide). In areas with a deep, stable water column, this leads to a midwater oxygen minimum zone (OMZ) where the oxygen concentration is <10% of that at the ocean’s surface, and pH is lower as well. The world’s largest OMZ covers most of the eastern Pacific Ocean and is associated with the highly productive California Current, Humboldt (Peruvian-Chilean) Current and equatorial upwelling systems. The depth of the upper boundary for the OMZs depends on latitude – ranging from <200 m to >600 m.

Definitely not a ‘dead zone,’ the OMZ is actually critical to marine ecological processes. Its upper boundary is the daytime home of vast numbers of zooplankton, small lantern fish and krill that avoid visual predators by hiding in this dark region. At night these organisms migrate to the surface to feed on phytoplankton. Although the cold, hypoxic depths of the upper OMZ are hostile to most pelagic predators, a variety of deep divers take advantage of this rich daytime foraging ground. Such diving predators include tunas, sharks and marine mammals. But these predators are limited by either low oxygen, cold temperature or both and can remain in the upper OMZ for only short periods (tens of minutes to several hours). The Humboldt squid (Dosidicus gigas) may be the only major predator that can tolerate the hostile conditions of the OMZ all day long.1

Midwater Climate Change Impacts

During 2008 several studies indicated that the OMZ in the eastern Pacific has expanded vertically since 1960, meaning that the 10% oxygen boundary has moved closer to the surface, in some cases by as much as 100 m. This was documented in the Gulf of Alaska,2 in the Monterey submarine canyon,3 off southern California4 and in the equatorial region.5

Two main explanations have been advanced for this global-scale change. First, increasing sea-surface temperatures due to global warming have increased stratification, which hinders mixing of atmospheric oxygen into deeper waters. Changes in global wind patterns may have also altered oceanic currents, potentially moving water with low-oxygen into new areas. Second, primary productivity has increased in some oceanic regions, increasing the amount of sinking organic material feeding the midwater microbes responsible for the OMZ. One factor driving increased productivity could be the rapidly increasing deposition of atmospheric nitrogenous gasses into the world’s oceans from agricultural fertilization and fossil-fuel emissions.6 Excess nitrogen can stimulate phytoplankton growth just as it fertilizes crops on land.

Ecological impacts

Expansion of the OMZ will have many complex ecological effects. Hypoxic waters will stress bottom-dwelling organisms that are unable to move. A shallower OMZ will also bring the organisms that inhabit the upper boundary of the OMZ closer to the surface during daytime, which might make them more susceptible to aerobic, visual predators. Vertical compression of the oxygen-rich habitat for pelagic predators may increase their susceptibility to fishing pressure.7 There may be big winners that can utilize the OMZ, particularly Humboldt squid. During the last decade this species has expanded its range northward from Baja California to southeast Alaska in parallel with OMZ expansion.8-10

Current research: Shoaling onto the shelf

As stated in Moss Landing Marine Laboratories graduate Ashley Booth’s thesis:

“Monitoring of oxygen at the seawater intakes of the Monterey Bay Aquarium (17 m depth) shows that pulses of cold, low-oxygen water often reach the nearby inshore environment that includes kelp forest, rocky reefs, and sandy spawning grounds for market squid (Booth 2011). Oxygen concentrations can fall as low as 20% of that at the surface, a level likely to be stressful to many resident organisms. The nearest source of cold, low-oxygen water like this is more than 100 m deep in the Monterey canyon. During the upwelling season (Spring and Summer) this deep water is shallower and is brought inshore by tidally driven internal waves (waves that travel on the boundary between water layers of different temperatures). What we do not know is how nearshore organisms might cope with such hypoxic events or how the frequency or severity of hypoxic events might be changing in the face of a shoaling OMZ in the canyon. These are all important questions that have become the focus of new research in Monterey Bay.”

A Stanford undergraduate research course started preliminary studies directed at these issues in Spring 2009, and local researchers from Stanford, MBARI, NOAA and Moss Landing Marine Labs have followed up with a five-year proposal to the NOAA Coastal Hypoxia Research Program. This team has also partnered with a group at Scripps Institute of Oceanography, because they realized that similar hypoxic events occur off La Jolla as well. Thus, intrusions of deep OMZ water into nearshore environments may be an important phenomenon over much of the California coast