OXYGEN DEPLETION
Evgeniy Yakushev
Section of Oceanography and Remote Sensing,
Norwegian Institute for Water Research (NIVA), Oslo,
Norway
Synonyms
Deoxygenation
Definition
Oxygen depletion in the aquatic environment is a process
involving the decrease in the concentration of dissolved
oxygen caused by its consumption during the oxidation
of organic matter and/or reduced compounds of other
chemical elements. Oxygen depletion leads to changes in
redox conditions.
Description
There is a normal sequence or succession of processes during the transition from oxic to anoxic conditions. The bacterial decomposition of organic matter is an oxidative process.
If there is an excess of organic matter to be decomposed
once the dissolved oxygen has been used, bacterial activity
will substitute the dissolved oxygen with a series of other
electron acceptors (i.e., nitrate through denitrification) and
metals that usually end with the reduction of sulfate,
a major constituent in seawater. This last process produces
hydrogen sulfide, which is toxic to aerobic life forms. Conversely, the oxidation of reduced inorganic compounds
under anoxic conditions also fuels the microbial production
of organic matter via chemosynthesis (Nealson and Stahl,
1997; Sorokin, 2002; Canfield et al., 2009).
The occurrence of oxygen-depleted and anoxic water
depends on the combined influence of eutrophication
(organic matter and nutrient loads) and hydrodynamics
(intensity of mixing and water renewal) (Yakushev and
Newton, 2013). Oxygen depletion zones form when there
is an imbalance between the supply of organic matter and
the supply of dissolved oxygen for its decomposition. This
may occur when a hydrophysical structure, such as temperature stratification, restricts the aeration of the water
column. The existence of low oxygen structures can be
temporary or permanent, correspondingly creating zones
of temporary or permanent hypoxia and anoxia.
Oxygen depletion and anoxia in the water column are
increasingly common features observed in the world
ocean, inland seas, estuaries, and coastal marine waters.
Observations show a decline in the dissolved oxygen concentrations at the continental margins in many regions,
and these are related to both an increase in anthropogenic
nutrient loading and a decrease in vertical mixing (e.g.,
Richardson and Jorgensen, 1996; Diaz, 2001; Rabalais
et al., 2002; Diaz and Rosenberg, 2008; Savchuk, 2013).
Decreases in oxygen are also reported in the tropical
oceans (Emerson et al., 2004; Whitney et al., 2007;
Paulmier and Ruiz-Pino, 2009; Keeling et al., 2010;
Deutsch et al. 2011; Falkowski et al., 2011). Low oxygen
“dead zones” have spread exponentially since the 1960s
(Diaz and Rosenberg, 2008). The decrease in dissolved
oxygen throughout the open ocean basins may be a longterm, nonperiodic trend related to climate change, or the
result of natural cyclical processes, or a combination of
both (Falkowski et al., 2011). In the coastal regions, oxygen depletion events are connected with anthropogenic
forces such as eutrophication. However, climatic forces
may further stimulate the formation of oxygen-depleted
zones. Within the marine science community, there is
increasing interest in these events because of their global
occurrence.
The scale of processes that affect the formation of each
system’s hydrophysical structure varies from molecular diffusion to climatic variability. However, in lakes and fjords,
some processes, such as the transport of water with geostrophic currents or mesoscale eddies, are not important,
leading to less intense mixing than under marine conditions. Therefore, the chemical structure of redox interfaces
in fjords and lakes is characterized by sudden changes in
redox conditions and steep chemical gradients. In comparison to lakes and fjords, oxidation-reduction features at
marine redox interfaces are characterized by gradual gradients, with varying temporal changes as well. For example,
the boundary of the anoxic zone of the Cariaco Basin is
influenced by mesoscale eddies that periodically supply
dense water with high oxygen content to the anoxic zone
(Scranton et al., 2006). In the Baltic Sea, under certain winter weather conditions, there is an influx of oxygen-rich
saline Northern Sea waters to the deep anoxic layers
(Schneider et al., 2002; Feistel et al., 2008). The Black
Sea oxic/anoxic interface appears to be more stable,
because the Bosphorus plume waters influence only the
southwestern part of the Black Sea. The central and peripheral Black Sea is characterized by the stability of the chemical features (i.e., maximum positions, onset levels) in the
density field (Vinogradov and Nalbandov, 1990; Murray
et al., 1995). Such a chemotropicity implies that there are
no horizontal gradients of chemical variables along the
same density surface, supporting the use of a “onedimensional modelling” for describing the processes
responsible for the maintenance of the redox interface
chemical structure (Yakushev et al., 2005).
The redox interface is a layer where oxic and anoxic
conditions are adjacent. It is very complex because of
the many chemical reactions and biogeochemical mineralization processes that can be oxic, suboxic, and
anoxic. The depletion of oxygen concentration affects
the biogeochemical cycles of N, P, and S, as well as the
carbonate system and trace metal equilibria. Water layers
that are already depleted with respect to dissolved oxygen
may be close to the threshold between suboxic and anoxic
conditions and are the water bodies most vulnerable to the
effects of the globally observed deoxygenation trend
(Jost and Pollehne, 2013). It is necessary to better
OXYGEN DEPLETION
469
Evgeniy Yakushev
Section of Oceanography and Remote Sensing,
Norwegian Institute for Water Research (NIVA), Oslo,
Norway
Synonyms
Deoxygenation
Definition
Oxygen depletion in the aquatic environment is a process
involving the decrease in the concentration of dissolved
oxygen caused by its consumption during the oxidation
of organic matter and/or reduced compounds of other
chemical elements. Oxygen depletion leads to changes in
redox conditions.
Description
There is a normal sequence or succession of processes during the transition from oxic to anoxic conditions. The bacterial decomposition of organic matter is an oxidative process.
If there is an excess of organic matter to be decomposed
once the dissolved oxygen has been used, bacterial activity
will substitute the dissolved oxygen with a series of other
electron acceptors (i.e., nitrate through denitrification) and
metals that usually end with the reduction of sulfate,
a major constituent in seawater. This last process produces
hydrogen sulfide, which is toxic to aerobic life forms. Conversely, the oxidation of reduced inorganic compounds
under anoxic conditions also fuels the microbial production
of organic matter via chemosynthesis (Nealson and Stahl,
1997; Sorokin, 2002; Canfield et al., 2009).
The occurrence of oxygen-depleted and anoxic water
depends on the combined influence of eutrophication
(organic matter and nutrient loads) and hydrodynamics
(intensity of mixing and water renewal) (Yakushev and
Newton, 2013). Oxygen depletion zones form when there
is an imbalance between the supply of organic matter and
the supply of dissolved oxygen for its decomposition. This
may occur when a hydrophysical structure, such as temperature stratification, restricts the aeration of the water
column. The existence of low oxygen structures can be
temporary or permanent, correspondingly creating zones
of temporary or permanent hypoxia and anoxia.
Oxygen depletion and anoxia in the water column are
increasingly common features observed in the world
ocean, inland seas, estuaries, and coastal marine waters.
Observations show a decline in the dissolved oxygen concentrations at the continental margins in many regions,
and these are related to both an increase in anthropogenic
nutrient loading and a decrease in vertical mixing (e.g.,
Richardson and Jorgensen, 1996; Diaz, 2001; Rabalais
et al., 2002; Diaz and Rosenberg, 2008; Savchuk, 2013).
Decreases in oxygen are also reported in the tropical
oceans (Emerson et al., 2004; Whitney et al., 2007;
Paulmier and Ruiz-Pino, 2009; Keeling et al., 2010;
Deutsch et al. 2011; Falkowski et al., 2011). Low oxygen
“dead zones” have spread exponentially since the 1960s
(Diaz and Rosenberg, 2008). The decrease in dissolved
oxygen throughout the open ocean basins may be a longterm, nonperiodic trend related to climate change, or the
result of natural cyclical processes, or a combination of
both (Falkowski et al., 2011). In the coastal regions, oxygen depletion events are connected with anthropogenic
forces such as eutrophication. However, climatic forces
may further stimulate the formation of oxygen-depleted
zones. Within the marine science community, there is
increasing interest in these events because of their global
occurrence.
The scale of processes that affect the formation of each
system’s hydrophysical structure varies from molecular diffusion to climatic variability. However, in lakes and fjords,
some processes, such as the transport of water with geostrophic currents or mesoscale eddies, are not important,
leading to less intense mixing than under marine conditions. Therefore, the chemical structure of redox interfaces
in fjords and lakes is characterized by sudden changes in
redox conditions and steep chemical gradients. In comparison to lakes and fjords, oxidation-reduction features at
marine redox interfaces are characterized by gradual gradients, with varying temporal changes as well. For example,
the boundary of the anoxic zone of the Cariaco Basin is
influenced by mesoscale eddies that periodically supply
dense water with high oxygen content to the anoxic zone
(Scranton et al., 2006). In the Baltic Sea, under certain winter weather conditions, there is an influx of oxygen-rich
saline Northern Sea waters to the deep anoxic layers
(Schneider et al., 2002; Feistel et al., 2008). The Black
Sea oxic/anoxic interface appears to be more stable,
because the Bosphorus plume waters influence only the
southwestern part of the Black Sea. The central and peripheral Black Sea is characterized by the stability of the chemical features (i.e., maximum positions, onset levels) in the
density field (Vinogradov and Nalbandov, 1990; Murray
et al., 1995). Such a chemotropicity implies that there are
no horizontal gradients of chemical variables along the
same density surface, supporting the use of a “onedimensional modelling” for describing the processes
responsible for the maintenance of the redox interface
chemical structure (Yakushev et al., 2005).
The redox interface is a layer where oxic and anoxic
conditions are adjacent. It is very complex because of
the many chemical reactions and biogeochemical mineralization processes that can be oxic, suboxic, and
anoxic. The depletion of oxygen concentration affects
the biogeochemical cycles of N, P, and S, as well as the
carbonate system and trace metal equilibria. Water layers
that are already depleted with respect to dissolved oxygen
may be close to the threshold between suboxic and anoxic
conditions and are the water bodies most vulnerable to the
effects of the globally observed deoxygenation trend
(Jost and Pollehne, 2013). It is necessary to better
OXYGEN DEPLETION
469