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1 Living with the oceans. – A report on the state of the world's oceans

Oxygen

Oxygen in the ocean

> Scientists have been routinely measuring oxygen concentrations in the ocean for more than a hundred years. With growing concerns about climate change, however, this parameter has suddenly become a hot topic. Dissolved oxygen in the ocean provides a sensitive early warning system for the trends that climate change is causing. A massive deployment of oxygen ­sensors is projected for the coming years, which will represent a renaissance of this parameter.

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Oxygen – product and elixir of life

Carbon dioxide, which occurs in relatively small amounts in the atmosphere, is both a crucial substance for plants, and a climate-threatening gas. Oxygen, on the other hand, is not only a major component of the atmosphere, it is also the most abundant chemical element on Earth. The emergence of oxygen in the atmosphere is the result of a biological success model, photosynthesis, which helps plants and bacteria to convert inorganic materials such as carbon dioxide and water to biomass. Oxygen was, and continues to be generated by this process. The biomass produced is, for its part, the nutritional foundation for consumers, either bacteria, animals or humans. These consumers cannot draw their required energy from sunlight as the plants do, rather they have to obtain it by burning biomass, a process that consumes oxygen. Atmospheric oxygen on our planet is thus a product, as well as the elixir of life.

2.13 > Marine animals react in different ways to oxygen ­deficiency. Many species of snails, for instance, can tolerate lower O2 levels than fish or crabs. The diagram shows the concentration at which half of the animals die under experimental conditions. The average value is shown as a red line for each animal group. The bars show the full spectrum: some crustaceans can tolerate much lower O2 concentrations than others. /dt>

2.13 > Marine animals react in different ways to oxygen ­deficiency. Many species of snails, for instance, can tolerate lower O₂ levels than fish or crabs. The diagram shows the concentration at which half of the animals die under experimental conditions. The average value is shown as a red line for each animal group. The bars show the full spectrum: some crustaceans can tolerate much lower O₂ concentrations than others. © maribus (after Vaquer-Sunyer und Duarte, 2008)

Oxygen budget for the world ocean

Just like on the land, there are also photosynthetically active plants and bacteria in the ocean, the primary producers. Annually, they generate about the same amount of oxygen and fix as much carbon as all the land plants together. This is quite amazing. After all, the total living biomass in the ocean is only about one two-hundredth of that in the land plants. This means that primary pro­ducers in the ocean are around two hundred times more productive than land plants with respect to their mass. This reflects the high productivity of single-celled algae, which contain very little inactive biomass such as, for example, the heartwood in tree trunks.

2.14 > Oxygen from the atmosphere enters the near-surface waters of the ocean. This upper layer is well mixed, and is thus in chemical equilibrium with the atmosphere and rich in O2. It ends abruptly at the pyncnocline, which acts like a barrier. The oxygenrich water in the surface zone does not mix readily with deeper water layers. Oxygen essentially only enters the deeper ocean by the motion of water currents, especially with the formation of deep and intermediate waters in the polarregions. In the inner ocean, marine organisms consume oxygen. This creates a very sensitive equilibrium.
2.14 > Oxygen from the atmosphere enters the near-surface waters of the ocean. This upper layer is well mixed, and is thus in chemical equilibrium with the atmosphere and rich in O₂. It ends abruptly at the pyncnocline, which acts like a barrier. The oxygen-rich water in the surface zone does not mix readily with deeper water layers. Oxygen essentially only enters the deeper ocean by the motion of water currents, especially with the formation of deep and intermediate waters in the polar ­regions. In the inner ocean, marine organisms consume oxygen. This creates a very sensitive equilibrium. © maribus
Photosynthetic production of oxygen is limited, however, to the uppermost, sunlit layer of the ocean. This only extends to a depth of around 100 metres and, because of the stable density layering of the ocean, it is largely separated from the enormous underlying volume of the deeper ocean. Moreover, most of the oxygen generated by the primary producers escapes into the atmosphere within a short time, and thus does not contribute to oxygen enrichment in the deep water column. This is because the near-surface water, which extends down to around 100 metres, is typically saturated with oxygen by the supply from the atmosphere, and thus cannot store additional oxygen from biological production. In the inner ocean, on the other hand, there is no source of oxygen. Oxygen enters the ocean in the surface water through contact with the atmosphere. From there the oxygen is then brought to greater depths through the sinking and circulation of water masses. These, in turn, are dynamic processes that are strongly affected by climatic conditions. (Chapter 1).

Three factors ultimately determine how high the concentration of dissolved oxygen is at any given point within the ocean:

  1. The initial oxygen concentration that this water possessed at its last contact with the atmosphere.
  2. The amount of time that has passed since the last contact with the atmosphere. This can, in fact, be decades or centuries.
  3. Biological oxygen consumption that results during this time due to the respiration of all the consumers. These range from miniscule bacteria to the zooplankton, and up to the higher organisms such as fish.

The present-day distribution of oxygen in the internal deep ocean is thus determined by a complicated and not fully understood interplay of water circulation and bio­logical productivity, which leads to oxygen consumption in the ocean’s interior. Extensive measurements have shown that the highest oxygen concentrations are found at high latitudes, where the ocean is cold, especially well-mixed and ventilated. The mid-latitudes, by contrast, especially on the western coasts of the continents, are characterized by marked oxygen-deficient zones. The oxygen supply here is very weak due to the sluggish water circulation, and this is further compounded by ­elevated oxygen consumption due to high biological productivity. This leads to a situation where the oxygen is almost completely depleted in the depth range between 100 and 1000 metres. This situation is also observed in the northern Indian Ocean in the area of the Arabian Sea and the Bay of Bengal.
Different groups of marine organisms react to the ­oxygen deficiency in completely different ways, because of the wide range of tolerance levels of different marine animals to oxygen-poor conditions. For instance, crustaceans and fish generally require higher oxygen concentrations than mussels or snails. The largest oceanic oxygen minimum zones, however, because of their extremely low concentrations, should be viewed primarily as natural dead zones for the higher organisms, and by no means as caused by humans. >

2.15 > Marine regions with oxygen deficiencies are completely natural. These zones are mainly located in the mid-latitudes on the west sides of the continents. There is very little mixing here of the warm surface waters with the cold deep waters, so not much oxygen penetrates to greater depths. In addition, high bioproductivity and the resulting large amounts of sinking biomass here lead to strong oxygen consumption at depth, ­especially between 100 and 1000 metres.
2.15 > Marine regions with oxygen deficiencies are completely natural. These zones are mainly located in the mid-latitudes on the west sides of the continents. There is very little mixing here of the warm surface waters with the cold deep waters, so not much oxygen penetrates to greater depths. In addition, high bioproductivity and the resulting large amounts of sinking biomass here lead to strong oxygen consumption at depth, ­especially between 100 and 1000 metres.  © maribus (nach Keeling et al., 2010)

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