including biogeochemical cycling that range from mild to
severe disruption.
Dead zone is an area of hypoxia or anoxia that is related
to anthropogenic activity.
Introduction
Oxygen is necessary to sustain the life of fishes and virtually all higher invertebrates. When the supply of oxygen is
cut off from bottom waters, usually from temperature
and/or salinity stratification of the water column that
separates surface and bottom layers, or consumption of
oxygen through respiration exceeds resupply, oxygen concentrations can decline below levels that will sustain animal life. This condition of low oxygen is known as
hypoxia. Water devoid of oxygen is referred to as
anoxic. Ecologists have borrowed the term hypoxia from
the medical community, but the meaning and processes
for the environment are the same. The medical condition
is a deficiency in the amount of oxygen reaching tissues.
Similarly, a water body can be deprived of adequate
oxygen for proper ecosystem functioning.
Hypoxic areas are sometimes referred to as dead zones.
A term first applied to the northern Gulf of Mexico hypoxic area, which is related to excess agricultural and
municipal nutrients discharged from the Mississippi and
Atchafalaya Rivers (Rabalais et al., 2002, Rabalais et al.,
2010), and refers to the fact that fish and shrimp avoid
and migrate out of hypoxic areas. When fishermen trawl
in these zones, little to nothing is caught. The term dead
zone is best applied to coastal waters affected by anthropogenic activities where oxygen depletion occurs in otherwise normoxic (well oxygenated) waters and not to
naturally occurring oceanic oxygen minimum zones.
The migration of mobile fishes and invertebrates can
be obvious, and mortality of smaller sessile invertebrates
can be overlooked, but in the absence of larger fauna,
smaller species (e.g., foraminiferans and nematodes) and
microbes persist and thrive, such that hypoxic areas are
not truly dead or devoid of life. In addition, the fully oxygenated upper water column continues to support diverse
communities, including productive fisheries. Globally,
there are four basic types of low-oxygen marine
environments:
• Naturally occurring oceanic oxygen minimum zones
• Naturally occurring coastal upwellings
• Naturally occurring anoxic deep basins
• Anthropogenic-related coastal, estuarine, and brackish
water hypoxia
Dead zones are just one of many consequences of
anthropogenic activities (MA, 2005). Human population
is expanding exponentially, recently passing six billion,
and will likely exceed 8-10 billion by the year 2050. This
expansion has lead to an exponential modification of landscapes at the expense of ecosystem function and services
including pervasive effects from fuelling coastal primary
production with excess nutrients, the primary factor
leading to formation of hypoxia, to fishing down the food
web (Vitousek et al., 1997; Jackson et al., 2001; Foley
et al., 2005; Lotze et al., 2006). Long-term records of
nutrient discharges provide compelling evidence of
a rapid increase in the fertility of many temperate coastal
ecosystems starting about 50 years ago (Galloway et al.,
2004; Galloway et al., 2013). On a global basis, by
2050, coastal marine systems are expected to experience,
from today’s levels, a 2.4-fold increase in nitrogen and
2.7-fold increase in phosphorus loading from this population expansion (Tilman et al., 2001), with serious
consequences to ecosystem structure and function. The
question asked by Foley et al. (2005) is as follows: are
land-based activities degrading the global environment
in ways that undermine ecosystem services, which in turn
undermine human welfare? When it comes to dissolved
oxygen and the development of dead zones, the answer
is yes.
Dissolved oxygen concentrations
While many authors and water quality regulations focus on
concentrations of oxygen below 2–3 mg O 2 /L (see Figure 1
for conversions to other units) as a threshold value for
hypoxia in marine and brackish water environments, such
arbitrary limits may be unsuitable when examining potential impacts of hypoxia on any one given species (VaquerSonyer and Duarte, 2008). Hypoxia becomes detrimental
when behavioral and physiological responses result in
altered behavior or negative impacts, such as reduced
growth, loss of reproductive capacity, mortality, reduced
biodiversity, loss of secondary production, and stressed
fisheries. For example, Atlantic cod (Gadus morhua)
growth in St. Laurence is reduced below about 7 mg O 2 /
L or 70 % air saturation (Chabot and Dutil, 1999). Shrimp
and fish avoid dissolved oxygen below 2 mg O 2 /L
(approximately 30 % air saturation) in the northern Gulf
of Mexico hypoxic zone, while sharks and rays emigrate
from the area at oxygen concentrations below 3 mg O 2 /L
(Rabalais et al., 2001).
Because of the low solubility of oxygen in water, small
changes in the absolute amount of oxygen dissolved in
water lead to large differences in percent air saturation.
For freshwater at 20 ºC, 9.1 mg of oxygen (O 2 ) will
dissolve in a liter of water, so a 1 mg O 2 /l drop is about
an 11 % decline in saturation. In addition, oxygen solubility is strongly dependent on temperature and the amount
of salt dissolved in the water. Saturation declines about
1 mg O 2 /L from 20 to 26 ºC and about 2 mg O 2 /L from
freshwater to seawater at similar temperatures (Benson
and Krausse, 1984). Therefore, depending on temperature
and salinity, water contains 20–40 times less oxygen by
volume and diffuses about 10,000 times more slowly
through water than air (Graham, 1990). Thus, what appear
to be small changes in oxygen can have major consequences to animals living in an oxygen-limited milieu
(Rabalais and Gilbert, 2009). Physiologically, higher temperatures also increase metabolic requirements for oxygen
20
ANOXIA, HYPOXIA, AND DEAD ZONES
severe disruption.
Dead zone is an area of hypoxia or anoxia that is related
to anthropogenic activity.
Introduction
Oxygen is necessary to sustain the life of fishes and virtually all higher invertebrates. When the supply of oxygen is
cut off from bottom waters, usually from temperature
and/or salinity stratification of the water column that
separates surface and bottom layers, or consumption of
oxygen through respiration exceeds resupply, oxygen concentrations can decline below levels that will sustain animal life. This condition of low oxygen is known as
hypoxia. Water devoid of oxygen is referred to as
anoxic. Ecologists have borrowed the term hypoxia from
the medical community, but the meaning and processes
for the environment are the same. The medical condition
is a deficiency in the amount of oxygen reaching tissues.
Similarly, a water body can be deprived of adequate
oxygen for proper ecosystem functioning.
Hypoxic areas are sometimes referred to as dead zones.
A term first applied to the northern Gulf of Mexico hypoxic area, which is related to excess agricultural and
municipal nutrients discharged from the Mississippi and
Atchafalaya Rivers (Rabalais et al., 2002, Rabalais et al.,
2010), and refers to the fact that fish and shrimp avoid
and migrate out of hypoxic areas. When fishermen trawl
in these zones, little to nothing is caught. The term dead
zone is best applied to coastal waters affected by anthropogenic activities where oxygen depletion occurs in otherwise normoxic (well oxygenated) waters and not to
naturally occurring oceanic oxygen minimum zones.
The migration of mobile fishes and invertebrates can
be obvious, and mortality of smaller sessile invertebrates
can be overlooked, but in the absence of larger fauna,
smaller species (e.g., foraminiferans and nematodes) and
microbes persist and thrive, such that hypoxic areas are
not truly dead or devoid of life. In addition, the fully oxygenated upper water column continues to support diverse
communities, including productive fisheries. Globally,
there are four basic types of low-oxygen marine
environments:
• Naturally occurring oceanic oxygen minimum zones
• Naturally occurring coastal upwellings
• Naturally occurring anoxic deep basins
• Anthropogenic-related coastal, estuarine, and brackish
water hypoxia
Dead zones are just one of many consequences of
anthropogenic activities (MA, 2005). Human population
is expanding exponentially, recently passing six billion,
and will likely exceed 8-10 billion by the year 2050. This
expansion has lead to an exponential modification of landscapes at the expense of ecosystem function and services
including pervasive effects from fuelling coastal primary
production with excess nutrients, the primary factor
leading to formation of hypoxia, to fishing down the food
web (Vitousek et al., 1997; Jackson et al., 2001; Foley
et al., 2005; Lotze et al., 2006). Long-term records of
nutrient discharges provide compelling evidence of
a rapid increase in the fertility of many temperate coastal
ecosystems starting about 50 years ago (Galloway et al.,
2004; Galloway et al., 2013). On a global basis, by
2050, coastal marine systems are expected to experience,
from today’s levels, a 2.4-fold increase in nitrogen and
2.7-fold increase in phosphorus loading from this population expansion (Tilman et al., 2001), with serious
consequences to ecosystem structure and function. The
question asked by Foley et al. (2005) is as follows: are
land-based activities degrading the global environment
in ways that undermine ecosystem services, which in turn
undermine human welfare? When it comes to dissolved
oxygen and the development of dead zones, the answer
is yes.
Dissolved oxygen concentrations
While many authors and water quality regulations focus on
concentrations of oxygen below 2–3 mg O 2 /L (see Figure 1
for conversions to other units) as a threshold value for
hypoxia in marine and brackish water environments, such
arbitrary limits may be unsuitable when examining potential impacts of hypoxia on any one given species (VaquerSonyer and Duarte, 2008). Hypoxia becomes detrimental
when behavioral and physiological responses result in
altered behavior or negative impacts, such as reduced
growth, loss of reproductive capacity, mortality, reduced
biodiversity, loss of secondary production, and stressed
fisheries. For example, Atlantic cod (Gadus morhua)
growth in St. Laurence is reduced below about 7 mg O 2 /
L or 70 % air saturation (Chabot and Dutil, 1999). Shrimp
and fish avoid dissolved oxygen below 2 mg O 2 /L
(approximately 30 % air saturation) in the northern Gulf
of Mexico hypoxic zone, while sharks and rays emigrate
from the area at oxygen concentrations below 3 mg O 2 /L
(Rabalais et al., 2001).
Because of the low solubility of oxygen in water, small
changes in the absolute amount of oxygen dissolved in
water lead to large differences in percent air saturation.
For freshwater at 20 ºC, 9.1 mg of oxygen (O 2 ) will
dissolve in a liter of water, so a 1 mg O 2 /l drop is about
an 11 % decline in saturation. In addition, oxygen solubility is strongly dependent on temperature and the amount
of salt dissolved in the water. Saturation declines about
1 mg O 2 /L from 20 to 26 ºC and about 2 mg O 2 /L from
freshwater to seawater at similar temperatures (Benson
and Krausse, 1984). Therefore, depending on temperature
and salinity, water contains 20–40 times less oxygen by
volume and diffuses about 10,000 times more slowly
through water than air (Graham, 1990). Thus, what appear
to be small changes in oxygen can have major consequences to animals living in an oxygen-limited milieu
(Rabalais and Gilbert, 2009). Physiologically, higher temperatures also increase metabolic requirements for oxygen
20
ANOXIA, HYPOXIA, AND DEAD ZONES