Apparent Oxygen Utilisation
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In
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does in ...
or
marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...
systems apparent oxygen utilization (AOU) is the difference between oxygen gas
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...
(i.e. the concentration at saturation) and the measured
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
concentration in water with the same physical and chemical properties. AOU= _2 \ solubility _2 \ observed


General influences

Differences in O2 solubility and measured concentration (AOU) typically occur when biological activity,
ocean circulation An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
, or ocean mixing act to change the ambient concentration of oxygen. For example,
primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
liberates oxygen and increases its concentration, while
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellula ...
consumes it and decreases its concentration. Consequently, the AOU of a water sample represents the sum of the biological activity that the sample has experienced since it was last in equilibrium with the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...
. In shallow water systems (e.g.
lakes A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
), the full
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
is generally in close contact with the atmosphere, and oxygen concentrations are typically close to saturation, and AOU values are near zero. In deep water systems (e.g.
oceans The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
), water can be out of contact with the atmosphere for extremely long periods of time (years, decades, centuries) and large positive AOU values are typical. On occasion, where near-surface primary production has raised oxygen concentrations above saturation, negative AOU values are possible (i.e. oxygen has not been utilized to below saturation concentrations).


O2 trends and AOU in the ocean

O2 concentrations in the ocean have decreased since the 1980s. Part of this decrease is due to increased
ocean heat content In oceanography and climatology, ocean heat content (OHC) is a term for the energy absorbed by the ocean, where it is stored for indefinite time periods as internal energy or enthalpy. The rise in OHC accounts for over 90% of Earth’s excess th ...
(OHC) from global warming decreasing O2 solubility. As solubility in surface oceans decreases, O2 out gasses to the atmosphere. Increased AOU is likely also contributing to declining ocean O2 concentrations. Changes in AOU in the ocean could be caused by multiple forcings, such as changes in subduction rates, changes in water mass boundaries, initial O2 from water mass formation, biochemical consumption of O2, or changes in eddy mixing. Based on observations, global AOU increase seems to be linked to increasing OHC.


Spatial trends in AOU and O2

Changes in AOU are not consistent across spatial domains. Notably, the Northern Hemisphere AOU spikes in the mid-1990s, while Southern Hemisphere AOU significantly decreases in the mid-2000s. This could be due to less dense data in the Southern Hemisphere. Since the 80s, regions of major O2 decrease (AOU increase) include the subpolar North Pacific, equatorial Atlantic, and eastern equatorial Pacific. Regions of O2 increase (AOU decrease) include the western subtropical North Pacific and eastern subpolar North Atlantic.


See also

*
Biological pump The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...
*
Eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
* Lake metabolism *
Stream metabolism Stream metabolism, often referred to as aquatic ecosystem metabolism in both freshwater (lakes, rivers, wetlands, streams, reservoirs) and marine ecosystems, includes gross primary productivity (GPP) and ecosystem respiration (ER) and can be express ...
* Hypoxia *
Remineralisation In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link ...
*
World Ocean Atlas The World Ocean Atlas (WOA) is a data product of the Ocean Climate Laboratory of the National Oceanographic Data Center ( U.S.). The WOA consists of a climatology of fields of ''in situ'' ocean properties for the World Ocean. It was first produ ...
*
Deep ocean water Deep ocean water (DOW) is the name for cold, salty water found deep below the surface of Earth's oceans. Ocean water differs in temperature and salinity. Warm surface water is generally saltier than the cooler deep or polar waters; in polar region ...
*
Thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...
*
Phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. P ...


References

Biological oceanography Geochemistry Limnology Oceanography Oxygen {{biochem-stub