Elsevier

Marine Chemistry

Volume 177, Part 3, 20 December 2015, Pages 472-475
Marine Chemistry

Photo-oxidation: Major sink of oxygen in the ocean surface layer

https://doi.org/10.1016/j.marchem.2015.06.003Get rights and content

Highlights

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    Evidence is presented that the oxygen demand associated with photochemical processes in the surface layer of oceans and seas worldwide is far higher than is generally accepted.

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    Oxygen loss due to photochmical processes is of the same order of magnitude as the amount of oxygen released by photosynthesis of the world’s marine phytoplankton.

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    Stratification is the conditio sine qua non for photo-oxidation to occur.

Abstract

Evidence is presented that the oxygen demand associated with photochemical processes in the surface layer of oceans and seas worldwide is of the same order of magnitude as the amount of oxygen released by photosynthesis of the world's marine phytoplankton. Both estimates are of necessity quite rough and therefore the agreement between oxygen loss and production, earlier found only locally in the Atlantic Ocean and the North Sea, came as a surprise. The heavy photochemical oxygen demand of the world's oceans must be explored further in oxygen budget studies by biogeochemical modelling in which special attention is paid to regional and temporal differences in the vertical stratification regime at the surface. Indeed, stratification is the conditio sine qua non for photochemical processes that are driven by ultraviolet radiation. Oxygen cannot be expected to be released from the sea to the atmosphere in the permanently stratified open ocean when year-round the oxygen loss associated with photochemistry is so high. In the upper layer of sea and ocean regions of the temperate zone vertical stratification is only pronounced during spring, summer and early autumn, so photochemical oxygen demand there is restricted to those months. We argue that the constancy of the oxygen concentration observed at the surface of the oceans in the tropics and subtropics (80% of Earth's marine regions) is the consequence of the balance between three processes that influence oxygen dissolved in seawater: loss due to photochemical processes and to the activity of microheterotrophs on the one hand, photosynthetic oxygen production by phytoplankton on the other.

Section snippets

Introduction: aquatic-marine versus terrestrial primary productivity

One fundamental aspect of aquatic primary production is hardly ever mentioned in recent publications in the field of marine chemistry: the oxygen that is produced during photosynthesis of microalgae (to be more precise: by cyanobacteria) was 3 billion years ago the conditio sine qua non for the development of life on Earth thanks to its release to the atmosphere. Nowadays, oxygen supersaturation at the surface of coastal seas during microalgal blooms (up to 140%, e.g., Gieskes and Kraay, 1977) no

Oxygen cycling in the ocean: sources and sinks

In recent reviews of the world's aquatic oxygen dynamics and its biogeochemical consequences (Blough, 2001, Testa and Kemp, 2011, Haeder et al., 2014) the molecular diffusion at the water–air interface is correctly described as the mechanism responsible for the exchange of oxygen to the atmosphere. In these reviews the concentration of oxygen in the water is basically considered to be determined at the level of photosynthesis on the one hand, and the counteracting process of respiration by

Photochemistry and oxygen loss in the ocean's surface layer: role of stratification

Photo-oxidative reactions in aquatic environments are widely known to be driven mainly by the effect of ultraviolet radiation that can penetrate to much greater depths than was thought: even UV-B can reach to depths of over 25 m in clear ocean water (Smith and Baker, 1979). Interactions of this radiation with the photo-reactive substances just mentioned should lead to a considerable photochemical oxygen demand (Amon and Benner, 1996, Andrews et al., 2000), a process that was documented

Quantifying photochemical oxygen demand in the ocean's surface layer, globe-wide

Thus, stratification is the normal situation at the surface of most of the world's tropical and subtropical oceans and seas throughout the year, only disturbed during short periods of strong winds. In the upper layer of sea and ocean regions of the temperate zone vertical stratification is normally pronounced only during spring, summer and early autumn. Since the tropical and subtropical oceans comprise 80% of Earth's marine regions we can extrapolate the photochemically-induced loss of oxygen

Discussion and conclusion

We conclude that the oceans can, as is often put forward in the popular press, not be a source of atmospheric oxygen because losses are so close to photosynthetic oxygen release in the upper mixed layer. In this respect oceans resemble tropical rain forests, where the high primary production and the subsequent oxygen release is counteracted by the equally high heterotrophic activity of all organisms living in the soil and the vegetation: that tropical rain forests are “the lungs of planet

Acknowledgments

Thanks are due to H.W.J. de Baar for stimulating discussions, inspiration and friendship throughout our careers of over 30 years in oceanography. G.W. Kraay was for the same length of time the most valuable of all colleagues in the laboratory and during numerous cruises.

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