As land-dwelling animals, we breathe plentiful oxygen from the atmosphere, except perhaps if we are climbing a high mountain summit where oxygen is more scarce. But ocean-dwelling animals are often not as fortunate as we are when it comes to oxygen abundance.
To begin with, oxygen as a gas is NOT easily dissolved in water. Though a liter of seawater weighs nearly 1000 times more than a liter of air, that very same liter of water contains about 40 times less oxygen than one liter of air (0.008 g versus 0.3g).
But things can get much worse than this, because below the surface layer, oxygen can litterally go down to zero. At 300 m depth, vast expanses of the ocean contain a very small percentage of the amount of oxygen these same waters contained when they were last found at the sea surface, in direct contact with the atmosphere. This percentage gets very close to zero in the northern Indian Ocean and the eastern equatorial Pacific, and actually reaches zero in the Black Sea.
You may ask yourself why oxygen can get so scarce in the ocean interior. The ocean’s surface mixed layer is well oxygenated because of photosynthesis and gas exchange with the atmosphere. But below the sunlit surface layer, there is no photosynthesis and no direct gas exchange with the atmosphere. Oxygen must then be carried away by ocean currents from the regions of the ocean where subtropical mode waters, intermediate waters, or deep waters are formed, towards the ocean interior. The main regions of the surface ocean that export recently ventilated and oxygenated waters to the ocean interior are shown below.
Waters of a given density (isopycnal) within the permanent thermocline of the subtropical gyres are ventilated at the latitude where that isopycnal intersects the base of the surface mixed layer during the winter. At the end of each winter, waters near the base of the mixed layer subduct into the ocean interior, and then begin to slowly diffuse equatorward. This ventilation process is particularly intense in the high latitude range of the subtropical gyres, where subtropical mode waters (STMW) and subantarctic mode waters (SAMW) are formed This process ventilates subtropical gyres to depths of about 500 to 900m.
In the
Southern Ocean, a surface convergence zone between about 50°S and 60°S causes
local sinking of surface waters and the formation of Antarctic Intermediate Water (AAIW) which
ventilates water depths between about 700 and 1200m. The Southern Ocean is
also home to the formation of the densest water mass of the world ocean, Antarctic Bottom Water (AABW), formed in
areas of intense sea ice freezing and brine rejection around the continent of
Antarctica. AABW sinks all the way to the bottom of the three major
ocean basins and propagates northward.
In the northern hemisphere, ventilation processes vary quite a lot between the three major ocean basins. The formation of North Pacific Intermediate Water (NPIW) in the mixed water region of the Oyashio and Kuroshio ventilates the upper 300 to 700 meters of the North Pacific subpolar gyre. In the North Atlantic, to the north of the Denmark-Faroe-Shetland ridge, deep convection in the Greenland-Iceland-Norwegian Sea (GINSEA) is the source of Denmark Strait Overflow Water (DSOW) that spills over the Denmark Strait sill (625 m deep) and then descends to the bottom of the North Atlantic. Some of this dense water also flows northward and propagates towards the Arctic Ocean. The Labrador Sea, and to a lesser extent the Irminger Sea, is another important site of deep water formation in the North Atlantic, with convection events reaching up to 2000 m depth. This leads to the formation of North Atlantic Deep Water (NADW) which then propagates southward. The ultimate fate of oxygen-rich NADW is to either 1) upwell into a surface divergence zone of the Antarctic Circumpolar Current; 2) ventilate the Indian Ocean; 3) ventilate the Pacific Ocean.
While ventilation provides new oxygen to the ocean interior, oxygen consumption processes (animal respiration, bacterial degradation of organic matter) remove oxygen from the water column. The rate of oxygen consumption is greatest near the ocean surface, and decreases exponentially with depth. Observed oxygen concentrations in the ocean reflect a fine balance between the ventilation supply mechanisms illustrated here and the oxygen consumption mechanisms. Some ocean regions have very high oxygen consumption rates, such as the Eastern Boundary Upwelling Systems off Peru-Chile or Namibia, and this is where we tend to find the most intense and thick oxygen minimum zones (OMZs).
During the very long journey from its formation region in the northern North Atlantic, NADW remains isolated from the atmosphere and its oxygen content becomes progressively lower as we get further away from its source region. In the Indian and Pacific Oceans, a mixture of NADW and predominantly AABW propagates northwards. No deep water formation takes place in the Pacific and Indian Ocean, so that they are less well ventilated than the North Atlantic and have lower oxygen content, as can be seen in the South to North sections of the three major ocean basins shown below.
Suggested further reading:
Bopp, L., Resplandy, L., Orr, J.C., Doney, S.C., Dunne, J.P., Gehlen, M., Halloran, P., Heinze, C., Ilyina, T., Seferian, R. and Tjiputra, J., Vichi, M., 2013. Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models, Biogeosciences, 10, 6225-6245.
Breitburg, D., Levin, L. A., Oschlies, A., Grégoire, M., Chavez, F. P., Conley, D. J., Garçon, V., Gilbert, D., Gutiérrez, D., Isensee, K., Jacinto, G. S., Limburg, K. E., Montes, I., Naqvi, S. W. A., Pitcher, G. C., Rabalais, N. N., Roman, M. R., Rose, K. A., Seibel, B. A., Telszewski, M., Yasuhara, M. & Zhang, J. 2018. Declining oxygen in the global ocean and coastal waters. Science, 359, 11p.
Gilbert, D., 2017. Oceans lose oxygen. Nature, 542: 303-304.
Gilbert, D., N.N. Rabalais, R.J. Díaz, and J. Zhang, 2010. Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean. Biogeosciences, 7: 2283-2296.
Schmidtko, S., Stramma, L. Visbeck, M. 2017. Decline in global oceanic oxygen content during the past five decades. Nature, 542, 335.