Oxygen depletion in the Ocean: a major problem

© Vincent Hilaire / Tara Expeditions Foundation

Warming and acidification are no longer enough to describe the upheavals in the marine world. We now have to add a third component: deoxygenation, i.e., the depletion of oxygen in certain zones of the Ocean.

The ocean is often called “the blue lung of the planet” – comparable to a huge forest. This is correct: billions of micro-algae inhabiting the ocean’s surface waters inject as much oxygen into the atmosphere via photosynthesis as all the world’s forests. But we shouldn’t forget that in certain oceanic regions, mostly located at depths between 100 and 500 meters, oxygen is critically lacking. Like temperature, salinity and nutrients, the concentration of dissolved oxygen is very heterogeneous in the ocean.

6gkjyhgh (1) Oxygen concentration at a depth of 300 meters © World Ocean Atlas 2009 nodc.noaa.gov

Present in all the oceans

In general, oxygen concentration decreases with depth. In some ocean regions, a convergence of natural phenomena sometimes gives rise to what scientists call OMZs — “oxygen minimum zones”. Cold waters rising up from the depths are low in oxygen but rich in nutrients and CO2. They cause phytoplankton blooms on the surface. At greater depths the degradation of all this organic matter consumes a large amount of oxygen and sometimes completely depletes the water. Paradoxically, although life develops with difficulty in the OMZ, just a few hundred meters above we find the most productive areas of the ocean that correspond to great fishing zones. The problem is that ocean circulation does not properly resupply these OMZs, and oxygen eventually becomes exhausted. The largest deoxygenated zones are found along the eastern coasts of the Atlantic and Pacific Oceans. Two other famous OMZs are located in the Bay of Bengal and in the Arabian Sea. (pink-red areas in the map below)

The realm of unicellular microorganisms and bacteria

What are the impacts of this lack of oxygen on the distribution and diversity of life? If you could dive into an OMZ, what would you find there? These more acidic and oxygen-free waters host mostly microorganisms, especially Archaea, which are single-celled prokaryotes, and bacteria with particular metabolisms that can do without oxygen. Organisms that need more oxygen (fish for example) cannot survive here, while others with less intense metabolisms or capable of capturing oxygen from the water, can withstand relatively low concentrations.

Des copépodes sous le microscopeMicro-organisms be observed through a microscope © A. Deniaud / Fondation Tara Expéditions

Another problem related to these OMZs is that molecules such as methane (CH4), nitrous oxide (N2O) or hydrogen sulphide (H2S) are the metabolic products of the bacteria and Archaea living here. CH4 and N2O are major greenhouse gases (respectively 25 and 300 times more detrimental than CO2). As for H2S, it is toxic to certain marine organisms.

While scientists have been working intensively for 30 years on this worrisome phenomenon that threatens fisheries and therefore the food security of some countries, the problem is gradually becoming known to the general public.

Margaux Gaubert

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