12.9: Dissolved Oxygen and Carbon Dioxide

Oxygen is released to solution during photosynthesis and consumed during respiration. The distribution of dissolved oxygen is controlled by these processes and by exchanges between atmosphere and ocean.

Oxygen is exchanged continuously between the atmosphere and ocean. Hence, dissolved oxygen concentrations in the upper few meters of ocean water almost always equal the saturation solubility for the water temperature (Fig. 5-7). However, oxygen is produced by photosynthesis much faster than it is consumed by respiration in this layer, particularly when primary productivity is intense. As a result, oxygen concentration increases and the water becomes supersaturated. The depth and concentration of the oxygen maximum in the photic zone depend on the depth of maximum primary productivity and the depth and intensity of wind mixing that brings water to the surface, where excess oxygen can be released to the atmosphere. By many estimates, marine photosynthesizers contribute more oxygen to our atmosphere than land plants.

As they release oxygen to ocean waters and atmosphere, phytoplankton also remove dissolved carbon dioxide. Carbon dioxide is highly soluble in seawater as it reacts to form carbonate and bicarbonate ions (Chap. 5). The carbon dioxide concentration in seawater is so high that its removal by photosynthesis does not produce a marked minimum concentration of carbon dioxide at the oxygen maximum. Nevertheless, much of the carbon dioxide that primary producers remove from ocean waters is carried down into the deep ocean, where it is removed from contact with the atmosphere for hundreds of years.

It has been estimated that almost half of all carbon dioxide released to the atmosphere by human activity since the Industrial Revolution has entered the oceans. Substantial research is now focused on identifying the fate of this carbon dioxide. Predictions of future climate change depend on the rate at which carbon dioxide is transported into the deep ocean. Global climate models are limited by a lack of understanding of this rate and of various other critical ocean processes. For example, we do not know whether and to what degree increased carbon dioxide concentrations might increase ocean primary productivity, or how much of the additional organic matter that might be produced by such an increase would be transported below the thermocline. Proposals have been made to lessen or avoid global climate changes due to the increasing carbon dioxide concentrations in the atmosphere by injecting some of the carbon dioxide into the deep. The effectiveness of placing more carbon dioxide in the deep oceans would depend on the mode and location of introduction and would need to ensure that the carbon dioxide was stored in sediments or rocks as, otherwise ocean circulation would simply return the carbon dioxide to the atmosphere. The likely effectiveness and possible adverse side effects of implementing such carbon dioxide storage in the deep oceans cannot be properly assessed, given the present state of knowledge of the biogeochemical cycles of the oceans. 

Organic matter transported below the thermocline as detritus or through animal migration is decomposed, releasing nutrients and carbon dioxide. At the same time, the processes that decompose this organic matter consume dissolved oxygen. The continuous decomposition of organic matter progressively depletes dissolved oxygen beneath the thermocline, where it reaches a minimum. Below the minimum, oxygen increases slightly at greater depths due to the influx of cold, oxygenated deep water masses originating from polar regions. The oxygen concentration minimum below the bottom of the thermocline coincides with the nutrient maximum (Fig. 12-8).

At present, oxygen is fully depleted only in limited areas of the oceans, mainly where the residence time of subthermocline water is extremely long and/or where mixed-layer primary productivity is exceptionally high. Such conditions are present, for example, in the Baltic Sea, where water residence time is long and primary productivity high because of large anthropogenic nutrient inputs. Oxygen depletion also occurs in many fjords, where water residence time is very long. Once dissolved oxygen has been completely depleted, the water is anoxic. Most marine species cannot survive in anoxic water. However, certain bacteria and archaea in anoxic water can obtain energy by reducing molecules that contain oxygen. First, bacteria that reduce nitrate (NO^3–) to ammonium (NH⁴⁺) thrive, and then, when all nitrogen compounds are reduced, they are replaced by other species that reduce sulfate (SO₄^2–) to sulfide (S^2–). Sulfide is highly toxic and can be released from anoxic bottom waters into the overlying photic zone if vertical mixing is temporarily enhanced. In some cases, water containing hydrogen sulfide reaches the surface, and its foul smell is released to the atmosphere. Anoxic conditions are becoming more common and widespread in some coastal-ocean regions and estuaries because of inputs of nutrients from sewage and agricultural chemicals. In addition, there is a large area of the North Pacific Ocean where there is a strong oxygen minimum zone below the pycnocline. Similar strong oxygen minimum zones have also been observed in the Atlantic Ocean off the coast of Southern Africa (Angola and Namibia) and in the Indian Ocean off both coasts of India (Arabian Sea and Bay of Bengal).

There is evidence that the North Pacific oxygen minimum zone is expanding and deepening. Oxygen depleted water from this water mass now seasonally moves onto the Oregon continental shelf in most years to form a dead zone (Chap. 13). Expansion of the oxygen minimum zone in the North Pacific could be related to a slowdown of the ocean conveyor belt circulation (Chap. 8) since this would increase the age of this water mass and, therefore, the length of time during which oxygen was consumed by respiration and decomposition. Alternatively, the expansion could be related to an increase in overall ocean primary productivity that would increase the rate of transport of oxidizable organic matter below the thermocline. Both a slowdown of the ocean conveyor belt circulation and an increase in primary production are effects of anthropogenic carbon dioxide and nutrient releases that are predicted by some of the mathematical climate models. 

It should also be noted that one proposed approach to mitigation of climate change is that we should fertilize the oceans (primarily with iron in areas where it is the limiting nutrient) so that some of our released carbon dioxide would be transported into the deep oceans as organic detritus. If this were to be done successfully one of the undesirable side effects would be deoxygenation. The oxygen minimum zone would almost certainly deepen and expand further, perhaps eventually causing mass extinctions of ocean species as has occurred in the past when oxygen has become depleted in the deep oceans. In any event, ocean fertilization is a temporary solution at best. The deep oceans would only be a temporary storage location for our excess carbon dioxide since the excess carbon dioxide would be released back to the atmosphere within the next few generations as the deep water was mixed back to the surface.

Anoxic conditions have been more widespread in the oceans in the past. During anoxia, detritus that falls into the anoxic layer is no longer subject to decomposition by aerobic respiration, because there is no oxygen. Also, animals and other species that require dissolved oxygen can no longer live in the deep oceans. In such circumstances, organic matter can accumulate in large quantities in sediments. Oil and gas deposits are probably the result of diagenetic changes to such sediments over the millennia.