The most commonly used definition for oceanic
waters is dissolved oxygen content less than 1 ml l
À 1
(or 0.7 mg l
À 1 ). Disoxyic or disaerobic refers to
oxygen levels between 0.1 and 1.0 ml l
À 1
. OMZs are
usually defined as waters less than 0.5 ml l
À 1 dissolved oxygen.
Causes
Hypoxia occurs where the consumption of oxygen
through respiratory or chemical processes exceeds
the rate of supply from oxygen production via
photosynthesis, diffusion through the water column,
advection, or mixing. The biological and physical
water-column characteristics that support the development and maintenance of hypoxia include (1) the
production, flux, and accumulation of organic-rich
matter from the upper water column; and (2) watercolumn stability resulting from stratification or long
residence time. Dead and senescent algae, zooplankton fecal pellets, and marine aggregates contribute significant amounts of organic detritus to the
lower water column and seabed. Aerobic bacteria
consume oxygen during the decay of the carbon and
deplete the oxygen, particularly when stratification
prevents diffusion of oxygen. Stratification is the
division of the water column into layers with different densities caused by differences in temperature or
salinity or both. Hypoxia will persist as long as
oxygen consumption rates exceed those of supply.
Oxygen depletion occurs more frequently in estuaries
or coastal areas with longer water residence times,
with higher nutrient loads and with stratified water
columns.
Hypoxia is a natural feature of many oceanic
waters, such as OMZs and enclosed seas, or forms in
coastal waters as a result of the decomposition of
high carbon loading stimulated by upwelled nutrientrich waters. Hypoxia in many coastal and estuarine
waters, however, is but one of the symptoms of eutrophication, an increase in the rate of production
and accumulation of carbon in aquatic systems. Eutrophication very often results from an increase in
nutrient loading, particularly by forms of nitrogen
and phosphorus. Nutrient over-enrichment from
anthropogenic sources is one of the major stressors
impacting estuarine and coastal ecosystems, and
there is increasing concern in many areas around the
world that an oversupply of nutrients is having pervasive ecological effects on shallow coastal waters.
These effects include reduced light penetration, increased abundance of nuisance macroalgae, loss of
aquatic habitat such as seagrass or macroalgal beds,
noxious and toxic algal blooms, hypoxia and anoxia,
shifts in trophic interactions and food webs, and
impacts on living resources.
Hypoxic Systems
Oxygen-minimum Zones
Persistent hypoxia is evident in mid-water OMZs,
which are widespread in the world oceans where
the oxygen concentrations are less than 0.5 ml l
À 1 (or
about 7.5% oxygen saturation, o22 mM). They occur
at different depths from the continental shelf to upper
bathyal zones (down to 1300 m). Many of the OMZs
form as a result of high primary production associated
with coastal upwelled nutrient-rich waters. Their
formation also requires stagnant circulation, long
residence times, and the presence of oxygen-depleted
source waters. The extensive OMZ development in
the eastern Pacific Ocean is attributed to the fact that
intermediate depth waters of the region are older and
have overall oxygen concentrations lower than other
water masses. The largest OMZs are at bathyal
depths in the eastern Pacific Ocean, the Arabian Sea,
the Bay of Bengal, and off southwest Africa. The
upper boundary of an OMZ may come to within 10
or 50 m of the sea surface off Central America, Peru,
and Chile. The OMZ is more than 1000-m thick off
Mexico and in the Arabian Sea, but off Chile, the
OMZ is o400-m thick. Along continental margins,
minimum oxygen concentrations occur typically between 200 and 700 m. The area of the ocean floor
where oceanic waters permanently less than
0.5 ml l
À 1 impinge on continental margins covers
10
6 km
2 of shelf and bathyal seafloor, with over half
occurring in the northern Indian Ocean. These permanently hypoxic waters account for 2.3% of the
ocean’s continental margin. These hypoxic areas are
not related to eutrophication, but longer-term shifts in
meteorological conditions and ocean currents may
increase their prevalence in the future with global
climate change. Shifts in ocean currents have been
implicated in the increased frequency of continental
shelf hypoxia along the northwestern US Pacific coast
of Oregon.
Deep Basins, Enclosed Seas, and Fjord
Many of the existing permanent or periodic anoxic
ocean environments occur in enclosed or semienclosed waters where a mass of deep water is bathymetrically isolated from main shelf or oceanic water
masses by surrounding landmasses or one or more
shallow sills. In conjunction with a pycnocline, the
bottom water volume is restricted from exchange
with deep open water. Examples of hypoxic and
HYPOXIA 307
waters is dissolved oxygen content less than 1 ml l
À 1
(or 0.7 mg l
À 1 ). Disoxyic or disaerobic refers to
oxygen levels between 0.1 and 1.0 ml l
À 1
. OMZs are
usually defined as waters less than 0.5 ml l
À 1 dissolved oxygen.
Causes
Hypoxia occurs where the consumption of oxygen
through respiratory or chemical processes exceeds
the rate of supply from oxygen production via
photosynthesis, diffusion through the water column,
advection, or mixing. The biological and physical
water-column characteristics that support the development and maintenance of hypoxia include (1) the
production, flux, and accumulation of organic-rich
matter from the upper water column; and (2) watercolumn stability resulting from stratification or long
residence time. Dead and senescent algae, zooplankton fecal pellets, and marine aggregates contribute significant amounts of organic detritus to the
lower water column and seabed. Aerobic bacteria
consume oxygen during the decay of the carbon and
deplete the oxygen, particularly when stratification
prevents diffusion of oxygen. Stratification is the
division of the water column into layers with different densities caused by differences in temperature or
salinity or both. Hypoxia will persist as long as
oxygen consumption rates exceed those of supply.
Oxygen depletion occurs more frequently in estuaries
or coastal areas with longer water residence times,
with higher nutrient loads and with stratified water
columns.
Hypoxia is a natural feature of many oceanic
waters, such as OMZs and enclosed seas, or forms in
coastal waters as a result of the decomposition of
high carbon loading stimulated by upwelled nutrientrich waters. Hypoxia in many coastal and estuarine
waters, however, is but one of the symptoms of eutrophication, an increase in the rate of production
and accumulation of carbon in aquatic systems. Eutrophication very often results from an increase in
nutrient loading, particularly by forms of nitrogen
and phosphorus. Nutrient over-enrichment from
anthropogenic sources is one of the major stressors
impacting estuarine and coastal ecosystems, and
there is increasing concern in many areas around the
world that an oversupply of nutrients is having pervasive ecological effects on shallow coastal waters.
These effects include reduced light penetration, increased abundance of nuisance macroalgae, loss of
aquatic habitat such as seagrass or macroalgal beds,
noxious and toxic algal blooms, hypoxia and anoxia,
shifts in trophic interactions and food webs, and
impacts on living resources.
Hypoxic Systems
Oxygen-minimum Zones
Persistent hypoxia is evident in mid-water OMZs,
which are widespread in the world oceans where
the oxygen concentrations are less than 0.5 ml l
À 1 (or
about 7.5% oxygen saturation, o22 mM). They occur
at different depths from the continental shelf to upper
bathyal zones (down to 1300 m). Many of the OMZs
form as a result of high primary production associated
with coastal upwelled nutrient-rich waters. Their
formation also requires stagnant circulation, long
residence times, and the presence of oxygen-depleted
source waters. The extensive OMZ development in
the eastern Pacific Ocean is attributed to the fact that
intermediate depth waters of the region are older and
have overall oxygen concentrations lower than other
water masses. The largest OMZs are at bathyal
depths in the eastern Pacific Ocean, the Arabian Sea,
the Bay of Bengal, and off southwest Africa. The
upper boundary of an OMZ may come to within 10
or 50 m of the sea surface off Central America, Peru,
and Chile. The OMZ is more than 1000-m thick off
Mexico and in the Arabian Sea, but off Chile, the
OMZ is o400-m thick. Along continental margins,
minimum oxygen concentrations occur typically between 200 and 700 m. The area of the ocean floor
where oceanic waters permanently less than
0.5 ml l
À 1 impinge on continental margins covers
10
6 km
2 of shelf and bathyal seafloor, with over half
occurring in the northern Indian Ocean. These permanently hypoxic waters account for 2.3% of the
ocean’s continental margin. These hypoxic areas are
not related to eutrophication, but longer-term shifts in
meteorological conditions and ocean currents may
increase their prevalence in the future with global
climate change. Shifts in ocean currents have been
implicated in the increased frequency of continental
shelf hypoxia along the northwestern US Pacific coast
of Oregon.
Deep Basins, Enclosed Seas, and Fjord
Many of the existing permanent or periodic anoxic
ocean environments occur in enclosed or semienclosed waters where a mass of deep water is bathymetrically isolated from main shelf or oceanic water
masses by surrounding landmasses or one or more
shallow sills. In conjunction with a pycnocline, the
bottom water volume is restricted from exchange
with deep open water. Examples of hypoxic and
HYPOXIA 307