Oxygen and the Ocean

Home , Oxygen cycle, Oxygen minimum zone

Aurélien Paulmier

Oxygen (O) is the most The role of the ocean The ocean is oxygenated in the common atom on our planet, par- sunlit layer that is in contact with the ticularly in the air, in the oceans The oxygen cycle is based on atmosphere, and is mixed by wind and in the form of water (H2O), and exchanges between compartments, waves. This is the layer where most in the molecules of all living orga- primarily the atmosphere and the marine photosynthesis takes place, nisms. For the last 2.8 billion years, oceans, since at least 50% of the but below this layer, the concentration cyanobacteria and algae in the oxygen we breathe comes from the of O2 tends to decrease with depth. oceans, and plants on the conti- ocean. Atmospheric O2 penetrates Near the poles, salty cold waters nents, have been using solar energy the ocean at the poles and is released resulting from the formation of sea for their photosynthesis, thereby at the equator. Seasonally, the ocean ice, sink to the bottom (> 4,000 m), allowing the formation of oxygen absorbs O2 in autumn and winter, enriched in O2. These deep waters

(O2), of which the Earth was largely and degasses it to the atmosphere in circulate from the northern and sou- deprived at its origin. The concen- spring and summer. These transfers thern Atlantic Ocean to the Indian tration of O2 in the atmosphere are explained on the one hand by phy- Ocean, and then to the North Pacific, is high (20.8%), and is directly sical-chemical mechanisms, since O2 where they upwell after a journey las- linked to life, which is a chemical is more soluble in cold conditions, and ting around1,000 years, representing aberration compared to O2 rate on on the other hand, by biologically- the major oxygenation mechanism in other planets. This gaseous mole- related episodes of phytoplankton the ocean (cf. II.11). Along the way, cule, in dissolved form in aquatic blooms and their degradation. driven by current modulation, these environments, is also a powerful

ATMOSPHERIC O oxidant, able to form compounds 2 with almost all other chemical elements. The appearance of O2, a waste product of photosynthesis, Destruction of ozone

DMS CO , N O, CH was a disaster for primitive living 2 2 4 SOUTH POLE aerosols greenhouse gases organisms, and was the cause of wind Organic matter nutrients mass mortality until the emergence photosynthesis of breathing or respiratory func- eddies plankton EQUATOR level of 0 eutrophication disturbed ecosystem bloom tions. Breathing O then became OXYGENATED LAYER 2 -10m respiration -50m enclosed basin coastal current vital for all aerobic species: bacteria, nitrogen loss -100m OMZ plants, and animals. Paradoxically, toxic H2S -150m HYPOXIA acidification Upwelling O also produces free radicals that of nutrient- SUBOXIA remineralization 2 rich water -200m ANOXIA -200m damage biological molecules and ANOXIC SEDIMENT deep water -500m formation cells, causing mutations and ultima- ANOXIC SEDIMENT tely, death. Consequently, the level continental slope -1 000m

up to 10,000 km of oxygenation, which is relatively sediment -4 000m stable in the atmosphere, plays a COAST 5 à 100 km up to 3,000 km OFF SHORE key role in the regulation of life. In DISTRIBUTION OF OXYGEN Mechanisms of oxygen minimum zone(OMZ) formation return, living organisms control the Impacts on climate and ecosystems rate of O2 through mechanisms that Fig. 1 – Distribution of oxygen (O2) and OMZs in the ocean (blue) with their produce oxygen, photosynthesis, main associated mechanisms (white) and areas of impact (orange), at the and consume oxygen, respiration large offshore scale (on the right) and at a smaller scale near the coast (on / remineralization. the left). © M. P. CHARRIA and A. PAULMIER. n

64 | What is the ocean? 80°N waters gradually lose their O2. They are exposed to a rain of particles produced in the surface layer by phytoplankton and by the trophic network, 40°N which when degraded, consumes O2 during the process of remineralization. As a result, the rate of dissolved 0°

O2 can be used to reconstruct the his- tory of a water mass and as a tracer of its evolution. Remineralization 40°S mainly occurs between the surface and a depth of 1,000 m, caused by microbial communities that can travel 80°S attached to particles, thereby creating 100°W 0°W 100°E an O2 minimum in intermediate ocean waters (Fig. 1). 0 10 20 45 70 95 120 145 170 195

Fig. 2 – Distribution of areas of oxygen minimum concentration (O2) in In the most sluggish waters, the ocean (µmol/liter). Oxygen minimum zones (OMZs) in dark blue-gray which are relatively old since they shading. Coastal sites where hypoxic events have been reported are in have not been in contact with the orange. Data from the World Ocean Atlas 2013 and Diaz and Rosenberg, 2008. atmosphere for between10 and 100 © A. PAULMIER and S. ILLIG. n years, the O2 minimum may inten- sify (suboxia, anoxia). Often out- revolution, and especially since the marine aerosols, and regulation of lined by eddies, these waters form end of World War II, global warming albedo. OMZs, which have long the oxygen minimum zone (OMZ), has led to warmer surface temperatures been classified as dead zones, also reaching from about 10 to 1,000 m and,as a direct consequence, oceans have an impact on ecosystems and depth and extending up to 3,000 km tend to deoxygenate more. In coastal fisheries via the respiratory barrier, from the continental margins in the areas, where sediments play an nutrient losses (nitrate), acidifica- open ocean like in the eastern Pacific important role in remineralization, the tion, production of toxic gases, and Ocean, or in more closed configura- frequency at which hypoxic events are limitation of biodiversity. OMZs are tions (e.g. North Indian Ocean, Black recorded has increased exponentially natural laboratories to study how life Sea, deep trenches, estuaries, for ins- in response to effluent discharges. adapts to climate change, as they are tance, Fig. 2). However, the roles of both natural refuges for unsuspected marine life, and anthropic causes still need to be favoring the emergence of ecosystems better understood. that are among the most active and abundant of the ocean in the tran- Evolution and impacts To conclude, let us emphasize the sition zone between oxygenated and on climate and importance of the oxygen minimum deoxygenated environments. We must zones (OMZs), given their poten- therefore be vigilant and maintain a ecosystems tially major feedback to the Earth’s stable balance in oxygen dynamics biogeochemical cycles impacting the between the various compartments In response to climate variations, climate: source of greenhouse gases, of our planet and the ocean, in full periods of oxygenation in the ocean destruction of stratospheric ozone, interaction with the different forms naturally alternated with periods role in the formation of clouds via of living organisms. of deoxygenation: from geological time-scales (~million years) to hourly References fluctuations. These complex changes • G. COPIN-MONTEGUT – Chimie de l’eau de mer, Institut result from ventilation and ocean Océanographique, 1996. mixing processes, but also from the • K. ISENSEE, L. A. LEVIN, D. BREITBURG, M. GREGOIRE, V. GARCON production of planktonic particles and L. VALDES – L’Océan est à bout de souffle, In Ocean and Climate – occasionally fertilized by upwelling. Scientific sheets, 2016. However, since the industrial

Oxygen and the ocean | 65

Acknowledgements

We have received a great deal of support, without which we could never have produced this book. We would like to thank the chairman and members of the AllEnvi research alliance for their backing of the project, and the exe- cutive secretariat of AllEnvi for their assistance, the CNRS Ecology and Environment Institute for their precious support and the scientific communities at the various research performing organizations involved.

The publication’s coordinators also express their warmest thanks to the members of the editorial committee for all their hard work and responsiveness, and to all the contributors for their enthusiasm in sharing their knowledge, even within some very tight deadlines! The English version could not have been produced without the efficiency and meticulousness of the translators. The coordinators would also like to thank CNRS Éditions who have pro- vided their support throughout the publication process. Finally, this book could not have been published without the energy and professionalism of Elsa Godet, who threw herself into the challenge of producing the book in both languages.

Steering Committee

Philippe CURY, IRD.

Agathe EUZEN, CNRS.

Françoise GAILL, CNRS.

Denis LACROIX, Ifremer.

Stéphanie THIÉBAULT, CNRS, Vice-président of AllEnvi.

Multidisciplinary Editorial Committee

Sophie ARNAUD-HAOND. Ifremer. Françoise GOURMELON. CNRS.

Cédric BACHER. Ifremer. Catherine JEANDEL. CNRS.

Jacques BERTRAND. Ifremer. Denis LACROIX. Ifremer. Laurent BOPP. CNRS . Gérard LE BOUEDEC. University of Bretagne Sud. Catherine BOYEN. CNRS. Nadine LEBRIS. Pierre et Marie Curie University. Chantal CAHU. Ifremer. Pierre-Yves LE TRAON. Ifremer. Sylvie LAPÈGUE. Ifremer. Marina LÉVY. IRD. Philippe CURY. IRD. Catherine MÉVEL. CNRS. Pascale DELECLUSE. CNRS. Frédéric MÉNARD. IRD. Agathe EUZEN. CNRS. Éric RIETH. CNRS. Lise FECHNER. Ifremer.

Françoise GAILL. CNRS. Thierry PEREZ. CNRS.

François GALGANI. Ifremer. Sarah SAMADI. MNHN.

Jean-Pierre GATTUSO. CNRS. Patrick VINCENT. Ifremer. Editorial and graphic design: Elsa Godet (www.sciencegraphique.com)

Translation: Morris Traduction (www.andrewmorris.fr) and Daphne Goodfellow p. 166 Graham Maclachlan (Mactrad)