HYPOXIA
N. N. Rabalais, Louisiana Universities Marine
Consortium, Chauvin, LA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction: Definitions
Hypoxic (low-oxygen) and anoxic (no-oxygen)
waters have existed throughout geologic time. Presently, hypoxia occurs in many of the ocean’s deeper
environs, open-ocean oxygen-minimum zones
(OMZs), enclosed seas and basins, below western
boundary current upwelling zones, and in fjord.
Hypoxia also occurs in shallow coastal seas and estuaries, where their occurrence and severity appear
to be increasing, most likely accelerated by human
activities (Figure 1). A familiar term used in the
popular press and literature, ‘dead zone’, used for
coastal and estuarine hypoxia, refers to the fish and
shellfish killed by the suffocating conditions or the
failure to catch these animals in bottom waters when
the oxygen concentration in the water covering the
seabed is below a critical level.
Based on laboratory or field observations or both,
the level of oxygen stress and related responses of invertebrate and fish faunas vary. The units are often
determined by oxygen conditions that are physiologically stressful, but these levels also differ depending
on the organisms considered, and the pressure, temperature, and salinity of the ambient waters. The numerical definition of hypoxia varies as do the units
used, but hypoxia has mostly been defined as dissolved
oxygen levels lower than a range of 3–2 ml l
À 1
, with
the consensus being in favor of 1.4 ml l
À 1 ( ¼ 2 mg l
À 1
or ppm). This value is approximately equivalent to
30% oxygen saturation at 25 1C and 35 salinity (psu).
Below this concentration, bottom-dragging trawl nets
fail to capture fish, shrimp, and swimming crabs.
Other fishes, such as rays and sharks, are affected by
oxygen levels below 3 mg l
À 1
, which prompts a behavioral response to evacuate the area, up into the
water column and shoreward. Water-quality standards
in the coastal waters of Long Island Sound, New York,
and Connecticut, USA, consider that dissolved oxygen
conditions below 5 mg l
À 1 result in behavioral effects
in marine organisms and fail to support living resources at sustainable levels.
Oxygen depletion
Annual
Episodic
Periodic
Persistent
Unknown
Figure 1 Distribution of coastal ocean hypoxic areas; excludes naturally occurring hypoxia, such as upwelling zones and OMZs.
Reproduced from Dı ´az RJ, Nestlerode J, and Dı ´az ML (2004) A global perspective on the effects of eutrophication and hypoxia on
aquatic biota. In: Rupp GL and White MD (eds.) Proceedings of the 7th International Symposium on Fish Physiology, Toxicology and
Water Quality, Tallinn, Estonia, May 12-15, 2003. EPA 600/R-04/049, pp. 1–33. Athens, GA: Ecosystems Research Division, US
Environmental Protection Agency, with permission from Robert J. Dı ´az.
306
N. N. Rabalais, Louisiana Universities Marine
Consortium, Chauvin, LA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction: Definitions
Hypoxic (low-oxygen) and anoxic (no-oxygen)
waters have existed throughout geologic time. Presently, hypoxia occurs in many of the ocean’s deeper
environs, open-ocean oxygen-minimum zones
(OMZs), enclosed seas and basins, below western
boundary current upwelling zones, and in fjord.
Hypoxia also occurs in shallow coastal seas and estuaries, where their occurrence and severity appear
to be increasing, most likely accelerated by human
activities (Figure 1). A familiar term used in the
popular press and literature, ‘dead zone’, used for
coastal and estuarine hypoxia, refers to the fish and
shellfish killed by the suffocating conditions or the
failure to catch these animals in bottom waters when
the oxygen concentration in the water covering the
seabed is below a critical level.
Based on laboratory or field observations or both,
the level of oxygen stress and related responses of invertebrate and fish faunas vary. The units are often
determined by oxygen conditions that are physiologically stressful, but these levels also differ depending
on the organisms considered, and the pressure, temperature, and salinity of the ambient waters. The numerical definition of hypoxia varies as do the units
used, but hypoxia has mostly been defined as dissolved
oxygen levels lower than a range of 3–2 ml l
À 1
, with
the consensus being in favor of 1.4 ml l
À 1 ( ¼ 2 mg l
À 1
or ppm). This value is approximately equivalent to
30% oxygen saturation at 25 1C and 35 salinity (psu).
Below this concentration, bottom-dragging trawl nets
fail to capture fish, shrimp, and swimming crabs.
Other fishes, such as rays and sharks, are affected by
oxygen levels below 3 mg l
À 1
, which prompts a behavioral response to evacuate the area, up into the
water column and shoreward. Water-quality standards
in the coastal waters of Long Island Sound, New York,
and Connecticut, USA, consider that dissolved oxygen
conditions below 5 mg l
À 1 result in behavioral effects
in marine organisms and fail to support living resources at sustainable levels.
Oxygen depletion
Annual
Episodic
Periodic
Persistent
Unknown
Figure 1 Distribution of coastal ocean hypoxic areas; excludes naturally occurring hypoxia, such as upwelling zones and OMZs.
Reproduced from Dı ´az RJ, Nestlerode J, and Dı ´az ML (2004) A global perspective on the effects of eutrophication and hypoxia on
aquatic biota. In: Rupp GL and White MD (eds.) Proceedings of the 7th International Symposium on Fish Physiology, Toxicology and
Water Quality, Tallinn, Estonia, May 12-15, 2003. EPA 600/R-04/049, pp. 1–33. Athens, GA: Ecosystems Research Division, US
Environmental Protection Agency, with permission from Robert J. Dı ´az.
306