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REVIEW SUMMARY
◥
OCEANS
Declining oxygen in the global ocean
and coastal waters
Denise Breitburg,*Lisa A. Levin, Andreas Oschlies, Marilaure Grégoire,
Francisco P. Chavez, Daniel J. Conley, Véronique Garçon, Denis Gilbert,
Dimitri Gutiérrez, Kirsten Isensee, Gil S. Jacinto, Karin E. Limburg, Ivonne Montes,
S. W. A. Naqvi, Grant C. Pitcher, Nancy N. Rabalais, Michael R. Roman,
Kenneth A. Rose, Brad A. Seibel, Maciej Telszewski, Moriaki Yasuhara, Jing Zhang
BACKGROUND: Oxygen concentrations in
both the open ocean and coastal waters have
been declining since at least the middle of the
20th century. This oxygen loss, or deoxygenation,
is one of the most important changes occurring
in an ocean increasingly modified by human
activities that have raised temperatures, CO
2
levels, and nutrient inputs and have altered the
abundances and distributions of marine species.
Oxygen is fundamental to biological and bio-
geochemical processes in the ocean. Its decline
can cause major changes in ocean productivity,
biodiversity, and biogeochemical cycles. Analy-
ses of direct measurements at sites around the
world indicate that oxygen-minimum zones in
the open ocean have expanded by several million
square kilometers and that hundreds of coast-
al sites now have oxygen concentrations low
enough to limit the distribution and abundance
of animal populations and alter the cycling of
important nutrients.
ADVANCES: In the open ocean, global warm-
ing, which is primarily caused by increased
greenhouse gas emissions, is considered the
primary cause of ongoing deoxygenation. Nu-
merical models project further oxygen declines
during the 21st century, even with ambitious
emission reductions. Rising global temperatures
decrease oxygen solubility in water, increase
the rate of oxygen consumption via respira-
tion, and are predicted to reduce the introduc-
tion of oxygen from the atmosphere and surface
waters into the ocean interior by increasing
stratification and weakening ocean overturn-
ing circulation.
In estuaries and other coastal systems strongly
influenced by their watershed, oxygen declines
have been caused by increased loadings of nu-
trients (nitrogen and phosphorus) and organic
matter, primarily from agriculture; sewage; and
the combustion of fossil fuels. In many regions,
further increases in nitrogen discharges to coast-
al waters are projected as human populations
and agricultural production rise. Climate change
exacerbates oxygen decline in coastal systems
through similar mechanisms as those in the
open ocean, as well as by increasing nutrient
delivery from watersheds that will experience
increased precipitation.
Expansionoflow-oxygenzonescanincrease
production of N
2
O, a potent greenhouse gas;
reduce eukaryote biodiversity; alter the structure
of food webs; and negatively affect food secu-
rity and livelihoods. Both acidification and in-
creasing temperature are mechanistically linked
with the process of deoxygenation and combine
with low-oxygen conditions to affect biogeo-
chemical, physiological, and
ecological processes. How-
ever, an important paradox
to consider in predicting
large-scale effects of future
deoxygenation is that high
levels of productivity in
nutrient-enriched coastal systems and upwell-
ing areas associated with oxygen-minimum zones
also support some of the world’smostprolific
fisheries.
OUTLOOK: Major advances have been made
toward understanding patterns, drivers, and
consequences of ocean deoxygenation, but there
is a need to improve predictions at large spatial
and temporal scales important to ecosystem ser-
vices provided by the ocean. Improved numerical
models of oceanographic processes that control
oxygen depletion and the large-scale influence
of altered biogeochemical cycles are needed
to better predict the magnitude and spatial pat-
terns of deoxygenation in the open ocean, as
well as feedbacks to climate. Developing and
verifying the next generation of these models
will require increased in situ observations and
improved mechanistic understanding on a va-
riety of scales. Models useful for managing
nutrient loads can simulate oxygen loss in coast-
al waters with some skill, but their ability to
project future oxygen loss is often hampered
by insufficient data and climate model projec-
tions on drivers at appropriate temporal and
spatial scales. Predicting deoxygenation-induced
changes in ecosystem services and human wel-
fare requires scaling effects that are measured
on individual organisms to populations, food
webs, and fisheries stocks; considering com-
bined effects of deoxygenation and other ocean
stressors; and placing an increased research
emphasis on developing nations. Reducing the
impacts of other stressors may provide some
protection to species negatively affected by low-
oxygen conditions. Ultimately, though, limiting
deoxygenation and its negative effects will ne-
cessitate a substantial global decrease in green-
house gas emissions, as well as reductions in
nutrient discharges to coastal waters.▪
RESEARCH
Breitburg et al., Science 359, 46 (2018) 5 January 2018 1of1
The list of author affiliations is available in the full article online.
*Corresponding author. Email: breitburgd@si.edu
Cite this article as D. Breitburg et al., Science 359, eaam7240
(2018). DOI: 10.1126/science.aam7240
Low and declining oxygen levels in the open ocean and coastal waters affect processes
ranging from biogeochemistry to food security. The global map indicates coastal sites
where anthropogenic nutrients have exacerbated or caused O
2
declines to <2 mg liter
−1
(<63 mmol liter
−1
) (red dots), as well as ocean oxygen-minimum zones at 300 m of depth
(blue shaded regions). [Map created from data provided by R. Diaz, updated by members of
the GO
2
NE network, and downloaded from the World Ocean Atlas 2009].
ON OUR WEBSITE
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at http://dx.doi.
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science.aam7240
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REVIEW
◥
OCEANS
Declining oxygen in the global ocean
and coastal waters
Denise Breitburg,
1
*Lisa A. Levin,
2
Andreas Oschlies,
3
Marilaure Grégoire,
4
Francisco P. Chavez,
5
Daniel J. Conley,
6
Véronique Garçon,
7
Denis Gilbert,
8
Dimitri Gutiérrez,
9,10
Kirsten Isensee,
11
Gil S. Jacinto,
12
Karin E. Limburg,
13
Ivonne Montes,
14
S. W. A. Naqvi,
15
†Grant C. Pitcher,
16,17
Nancy N. Rabalais,
18
Michael R. Roman,
19
Kenneth A. Rose,
19
Brad A. Seibel,
20
Maciej Telszewski,
21
Moriaki Yasuhara,
22
Jing Zhang
23
Oxygen is fundamental to life. Not only is it essential for the survival of individual animals,
but it regulates global cycles of major nutrients and carbon. The oxygen content of the
open ocean and coastal waters has been declining for at least the past half-century, largely
because of human activities that have increased global temperatures and nutrients
discharged to coastal waters. These changes have accelerated consumption of oxygen by
microbial respiration, reduced solubility of oxygen in water, and reduced the rate of oxygen
resupply from the atmosphere to the ocean interior, with a wide range of biological and
ecological consequences. Further research is needed to understand and predict long-term,
global- and regional-scale oxygen changes and their effects on marine and estuarine
fisheries and ecosystems.
Oxygen levels have been decreasing in the
open ocean and coastal waters since at least
the middle of the 20th century (1–3). This
ocean deoxygenation ranks among the most
important changes occurring in marine eco-
systems (1,4–6) (Figs. 1 and 2). The oxygen content
of the ocean constrains productivity, biodiversity,
and biogeochemical cycles. Major extinction events
in Earth’s history have been associated with warm
climates and oxygen-deficient oceans (7), and un-
der current trajectories, anthropogenic activities
coulddrive the ocean towardwide spread oxygen
deficiency within the next thousand years (8). In
this Review, we refer to “coastal waters”as systems
that are strongly influenced by their watershed,
and the “open ocean”as waters in which such in-
fluences are secondary.
The open ocean lost an estimated 2%, or 4.8 ±
2.1 petamoles (77 billion metric tons), of its oxy-
gen over the past 50 years (9). Open-ocean oxygen-
minimum zones (OMZs) have expanded by an
area about the size of the European Union (4.5 mil-
lion km
2
,basedonwaterwith<70mmol kg
−1
oxy-
gen at 200 m of depth) (10), and the volume of
water completely devoid of oxygen (anoxic) has
more than quadrupled over the same period (9).
Upwelling of oxygen-depleted water has intensi-
fied in severity and duration along some coasts,
with serious biological consequences (11).
Since 1950, more than 500 sites in coastal waters
have reported oxygen concentrations ≤2mgliter
−1
(=63 mmol liter
−1
or ≅61 µmol kg
-1
), a threshold
often used to delineate hypoxia (3,12) (Fig. 1A).
Fewer than 10% of these systems were known to
have hypoxia before 1950. Many more water bodies
may be affected, especially in developing nations
where available monitoring data can be sparse
and inadequately accessed even for waters receiv-
ing high levels of untreated human and agricul-
tural waste. Oxygen continues to decline in some
coastal systems despite substantial reductions in
nutrient loads, which have improved other water
quality metrics (such as levels of chlorophyll a)
that are sensitive to nutrient enrichment (13).
Oxygen is naturally low or absent where bio-
logical oxygen consumption through respiration
exceeds the rate of oxygen supplied by physical
transport, air-sea fluxes, and photosynthesis for
sufficient periods of time. A large variety of such
systems exist, including the OMZs of the open
ocean, the cores of some mode-water eddies, coastal
upwelling zones, deep basins of semi-enclosed seas,
deep fjords, and shallow productive waters with
restricted circulation (14,15). Whether natural or
anthropogenically driven, however, low oxygen
levels and anoxia leave a strong imprint on bio-
geochemical and ecological processes. Electron ac-
ceptors, such as Fe(III) and sulfate, that replace
oxygen as conditions become anoxic yield less energy
than aerobic respiration and constrain ecosystem
energetics (16). Biodiversity, eukaryotic biomass,
and energy-intensive ecological interactions such
as predation are reduced (17–19), and energy is
increasingly transferred to microbes (3,16). As
oxygen depletion becomes more severe, persistent,
and widespread, a greater fraction of the ocean is
losing its ability to support high-biomass, diverse
animal assemblages and provide important eco-
system services.
But the paradox is that these areas, sometimes
called dead zones, are far from dead. Instead they
contribute to some of the world’smostproduc-
tive fisheries harvested in the adjacent, oxygenated
waters (20–22) and host thriving microbial assem-
blages that utilize a diversity of biogeochemical
pathways (16).Eukaryoteorganismsthatuselow-
oxygen habitats have evolved physiological and
behavioral adaptations that enable them to extract,
transport, and store sufficient oxygen, maintain
aerobic metabolism, and reduce energy demand
(23–26). Fishes, for example, adjust ventilation
rate, cardiac activity, hemoglobin content, and O
2
binding and remodel gill morphology to increase
lamellar surface area (27). For some small taxa,
including nematodes and polychaetes, high surface
area–to–volume ratios enhance diffusion and con-
tribute to hypoxia tolerance (26). Metabolic depres-
sion (23,25,28) and high H
2
S tolerance (24)are
also key adaptations by organisms to hypoxic and
anoxic environments.
Causes of oxygen decline
Global warming as a cause of oxygen
loss in the open ocean
The discovery of widespread oxygen loss in the
open ocean during the past 50 years depended
on repeated hydrographic observations that re-
vealed oxygen declines at locations ranging from
the northeast Pacific (29) and northern Atlantic
(30) to tropical oceans (2). Greenhouse gas–driven
global warming is the likely ultimate cause of this
ongoing deoxygenation in many parts of the open
RESEARCH
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 1of11
1
Smithsonian Environmental Research Center, Edgewater, MD 21037, USA.
2
Center for Marine Biodiversity and Conservation and Integrative Oceanography Division, Scripps Institution of
Oceanography, University of California, San Diego, CA 92093, USA.
3
GEOMAR Helmholtz Centre for Ocean Research Kiel, 24105 Kiel, Germany.
4
Department of Astrophysics, Geophysics and
Oceanography, MAST-FOCUS Research Group, Université de Liège, 4000 Liège, Belgium.
5
Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA.
6
Department of Geology,
Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden.
7
CNRS/Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, 31401 Toulouse, CEDEX 9, France.
8
Maurice-Lamontagne
Institute, Fisheries and Oceans Canada, Mont-Joli, Québec G5H 3Z4, Canada.
9
Instituto del Mar del Perú (IMARPE), Esquina Gamarra y General Valle s/n, Callao, Peru.
10
Facultad de Ciencias y
Filosofıa, Programa de Maestrıa en Ciencias del Mar, Universidad Peruana Cayetano Heredia, Lima 31, Peru.
11
Intergovernmental Oceanographic Commission of UNESCO, 75732 Paris, CEDEX 7,
France.
12
The Marine Science Institute, University of the Philippines, Diliman, Quezon City, Philippines.
13
State University of New York College of Environmental Science and Forestry, Syracuse,
NY 13210, USA.
14
Instituto Geofísico del Perú, Lima, Perú.
15
Environment and Life Sciences Research Center, Kuwait Institute for Scientific Research, Salmiya, 22017 Kuwait.
16
Fisheries Research
and Development, Department of Agriculture, Forestry and Fisheries, Cape Town, South Africa.
17
Department of Biological Sciences, University of Cape Town, South Africa.
18
Department of
Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA.
19
University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, MD
21613, USA.
20
College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA.
21
International Ocean Carbon Coordination Project, Institute of Oceanology of Polish
Academy of Sciences, Ul. Powstancow Warszawy 55, 81-712 Sopot, Poland.
22
School of Biological Sciences and Swire Institute of Marine Science, University of Hong Kong, Hong Kong SAR,
China.
23
State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China.
*Corresponding author. Email: breitburgd@si.edu †Present address: Council of Scientific and Industrial Research, Rafi Marg, New Delhi, India.
on January 4, 2018 http://science.sciencemag.org/Downloaded from
ocean (31). For the upper ocean over the period
1958–2015, oxygen and heat content are highly cor-
related with sharp increases in both deoxygenation
and ocean heat content, beginning in the mid-
1980s (32).
Ocean warming reduces the solubility of oxy-
gen. Decreasing solubility is estimated to account
for ~15% of current total global oxygen loss and
>50% of the oxygen loss in the upper 1000 m of
the ocean (9,33). Warming also raises metabolic
rates, thus accelerating the rate of oxygen con-
sumption. Therefore, decomposition of sinking
particles occurs faster, and remineralization of
these particles is shifted toward shallower depths
(34), resulting in a spatial redistribution but not nec-
essarily a change in the magnitude of oxygen loss.
Intensified stratification may account for the
remaining 85% of global ocean oxygen loss by re-
ducing ventilation—the transport of oxygen into
the ocean interior—andbyaffectingthesupplyof
nutrients controlling production of organic mat-
ter and its subsequent sinking out of the surface
ocean. Warming exerts a direct influence on ther-
mal stratification and indirectly enhances salinity-
driven stratification through its effects on ice melt
and precipitation. Increased stratification alters
the mainly wind-driven circulation in the upper
few hundred meters of the ocean and slows the
deep overturning circulation (9). Reduced venti-
lation, which may also be influenced by decadal
to multidecadal oscillationsinatmosphericforcing
patterns (35), has strong subsurface manifestations
at relatively shallow ocean depths (100 to 300 m)
in the low- to mid-latitude oceans and less pro-
nounced signatures down to a few thousand meters
at high latitudes. Oxygen declines closer to shore
have also been found in some systems, including
the California Current andlowerSaintLawrence
Estuary, where the relative strength of various
currents have changed and remineralization has
increased (36,37).
There is general agreement between numerical
models and observations about the total amount
of oxygen loss in the surface ocean (38). There is
also consensus that direct solubility effects do
not explain the majority of oceanic oxygen decline
(31). However, numerical models consistently sim-
ulate a decline in the total global ocean oxygen
inventory equal to only about half that of the most
recent observation-based estimate and also pre-
dict different spatial patterns of oxygen decline or,
in some cases, increase (9,31,39). These discrep-
ancies are most marked in the tropical thermo-
cline (40). This is problematic for predictions of
future deoxygenation, as these regions host large
open-ocean OMZs, where a further decline in oxy-
gen levels could have large impacts on ecosystems
and biogeochemistry (Fig. 2A). It is also unclear
how much ocean oxygen decline can be attributed
to alterations in ventilation versus respiration.
Mechanisms other than greenhouse gas–driven
global warming may be at play in the observed
ocean oxygen decline that are not well represented
in current ocean models. For example, internal os-
cillations in the climate system, such as the Pacific
Decadal Oscillation, affect ventilation processes
and, eventually, oxygen distributions (35).
Models predict that warming will strengthen
winds that favor upwelling and the resulting
transport of deeper waters onto upper slope and
shelf environments in some coastal areas (41,42),
especially at high latitudes within upwelling sys-
tems that form along the eastern boundary of
ocean basins (43). The predicted magnitude and
direction of change is not uniform, however, either
within individual large upwelling systems or
among different systems. Upwelling in the south-
ern Humboldt, southern Benguela, and northern
Canary Eastern Boundary upwelling systems is
predicted to increase in both duration and inten-
sity by the end of the 21st century (43). Where the
oxygen content of subsurface source waters de-
clines, upwelling introduces water to the shelf that
isbothlowerinoxygenandhigherinCO
2
.Along
the central Oregon coast of the United States in
2006, for example, anoxic waters upwelled to
depths of <50 m within 2 km of shore, persisted for
4months,andresultedinlarge-scalemortalityof
benthic macro-invertebrates (11). There are no prior
records of such severe oxygen depletion over the
continental shelf or within the OMZ in this area (11).
Nutrient enrichment of coastal waters
Sewage discharges have been known to deplete
oxygen concentrations in estuaries since at least
thelate1800s(44),andbythemid1900sthelink
to agricultural fertilizer runoff was discussed (45).
Nevertheless, the number and severity of hypoxic
sites has continued to increase (Fig. 2B). The hu-
man population has nearly tripled since 1950 (46).
Agricultural production has greatly increased to
feed this growing population and meet demands
for increased consumption of animal protein, re-
sulting in a 10-fold increase in global fertilizer use
over the same period (47). Nitrogen discharges
from rivers to coastal waters increased by 43% in
just 30 years from 1970 to 2000 (48), with more
than three times as much nitrogen derived from
agriculture as from sewage (49).
Eutrophication occurs when nutrients (primar-
ily N and P) and biomass from human waste and
agriculture, as well as N deposition from fossil
fuel combustion, stimulate the growth of algae
and increase algal biomass. The enhanced primary
and secondary production in surface waters in-
creases the delivery rate of degradable organic
matter to bottom waters where microbial decom-
position by aerobic respiration consumes oxygen.
Once oxygen levels are low, behavioral and bio-
geochemical feedbacks can hinder a return to
higher-oxygen conditions (50). For example, bur-
rowing invertebrates that introduce oxygen to
sediments die or fail to recruit, and sediment phos-
phorus is released, fueling additional biological
production in the water column and eventual in-
creased oxygen consumption.
Coastal systems vary substantially in their sus-
ceptibility to developing low oxygen concentrations.
Low rates of vertical exchange within the water
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 2of11
Fig. 1. Oxygen has declined in both the open ocean and coastal
waters during the past half-century. (A) Coastal waters where oxygen
concentrations ≤61 mmol kg
−1
(63 mmol liter
−1
or 2 mg liter
−1
) have been
reported (red) (8,12). [Map created from data in (8) and updated by
R. Diaz and authors] (B) Change in oxygen content of the global ocean
in mol O
2
m
−2
decade
−1
(9). Most of the coastal systems shown here
reported their first incidence of low oxygen levels after 1960. In some
cases, low oxygen may have occurred earlier but was not detected
or reported. In other systems (such as the Baltic Sea) that reported
low levels of oxygen before 1960, low-oxygen areas have become
more extensive and severe (59). Dashed-dotted, dashed, and solid
lines delineate boundaries with oxygen concentrations <80, 40, and
20 mmol kg
−1
, respectively, at any depth within the water column (9).
[Reproduced from (9)]
RESEARCH |REVIEW
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column reduce rates of oxygen resupply (51), and
long water-retention times favor the accumulation
of phytoplankton biomass (14) and its eventual
subsurface degradation. Chesapeake Bay develops
hypoxia and anoxia that persist for several months
during late spring through early autumn and cover
up to 30% of the system area. In contrast, the
nearby Delaware Bay, which has weaker strat-
ification and a shorter retention time, does not
develop hypoxia, in spite of similar nutrient loads
(52). Manila Bay is adjacent to a mega-
city and also receives similar loads
on an annual basis, but it becomes
hypoxic principally during the wet
southwest monsoon period, when
rainfall increases nutrient loads and
stratification (53).
Lowoxygenincoastalwatersand
semi-enclosed seas can persist for
minutes to thousands of years and
may extend over spatial scales rang-
ing from less than one to many thou-
sands of square kilometers. Both local
and remote drivers lead to tempo-
ral and spatial variations in hypoxia.
Local weather can influence oxy-
gen depletion in very shallow water
through wind mixing and the effect
of cloud cover on photosynthesis (54).
At larger spatial scales, variations in
wind direction and speed (55), pre-
cipitation and nutrient loads (56),
sea surface temperature (57), and
nutrient content of water masses
transported into bottom layers of
stratified coastal systems contribute
to interannual and longer-period var-
iations in hypoxic volume, duration,
and rate of deoxygenation (14).
Climate change in
coastal waters
Warming is predicted to exacerbate
oxygen depletion in many nutrient-
enriched coastal systems through
mechanisms similar to those of the
open ocean: increased intensity and
duration of stratification, decreased
oxygen solubility, and accelerated res-
piration (4,58,59). The current rate
of oxygen decline in coastal areas ex-
ceeds that of the open ocean (60),
however, likely reflecting the com-
bined effects of increased warming
of shallow water and higher con-
centrations of nutrients. Higher air
temperatures can result in earlier
onset and longer durations of hypoxia
in eutrophic systems through effects
on the seasonal timing of stratifica-
tion and the rate of oxygen decline
(58). An ensemble modeling study of
the Baltic Sea projects declining oxy-
gen under all but the most aggres-
sive nutrient-reduction plans, owing
to increased precipitation and con-
sequent nutrient loads, decreased
flux of oxygen from the atmosphere, and increased
internal nutrient cycling. Even aggressive nutrient
reduction is projected to yield far less benefit under
climatechangethanundercurrentconditions(61).
Because of regional variations in the effects of
global warming on precipitation and winds, the
rate and direction of change in oxygen content is
expected to vary among individual coastal water
bodies (4,58). Where precipitation increases, both
stratification and nutrient discharges are expected
to increase, with the reverse occurring in regions
where precipitation decreases. Changes in seasonal
patterns of precipitation and rates of evaporation
can also be important. Coastal wetlands that re-
move nutrients before they reach open water are
predicted to be lost as sea levels rise, decreasing
capacity to remove excess nitrogen, but the rate
of wetland inundation and the ability of wetlands
to migrate landward will vary.
Effects of ocean deoxygenation
Oxygen influences biological and bio-
geochemical processes at their most
fundamental level (Fig. 3). As re-
search is conducted in more habitats
and using new tools and approaches,
the range of effects of deoxygenation
that have been identified, and the
understanding of the mechanisms
behind those effects, has increased
substantially. Although 2 mg liter
−1
(61 mmol kg
−1
) is a useful threshold
for defining hypoxia when the goal is
to quantify the number of systems or
the spatial extent of oxygen-depleted
waters, a more appropriate approach
when considering biological and eco-
logical effects is to simply define hy-
poxia as oxygen levels sufficiently low
to affect key or sensitive processes.
Organisms have widely varying oxy-
gentolerances,eveninshallowcoastal
systems (19). In addition, because tem-
perature affects not only oxygen sup-
ply (through its effect on solubility
and diffusion) but also the respira-
tory demand by organisms, oxygen
limitation for organisms is better
expressed as a critical oxygen partial
pressure below which specific or-
ganisms exhibit reduced metabolic
functions than in terms of oxygen
concentration (62,63).
Biological responses
Ocean deoxygenation influences life
processes from genes to emergent
properties of ecosystems (Fig. 4). All
obligate aerobic organisms have limits
to the severity or duration of oxygen
depletion for which they can com-
pensate. Low oxygen levels can re-
duce survival and growth and alter
behavior of individual organisms
(3,4,26,64). Reproduction can be
impaired by reduced energy alloca-
tion to gamete production, as well
as interference with gametogenesis,
neuroendocrine function, and hor-
mone production, and can ultimate-
ly affect populations and fisheries
(65–67). Exposure to hypoxia can
trigger epigenetic changes expressed
in future generations, even if these
generations are not exposed to hy-
poxia (68). Brief, repeated exposure
to low oxygen can alter immune
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 3of11
Fig. 2. Dissolved oxygen concentrations in the open ocean and the
Baltic Sea. (A) Oxygen levels at a depth of 300 m in the open ocean.
Major eastern boundary and Arabian Sea upwelling zones, where oxygen
concentrations are lowest, are shown in magenta, but low oxygen levels
can be detected in areas other than these major OMZs. At this depth, large
areas of global ocean water have O
2
concentrations <100 mmol liter
−1
(outlined and indicated in red). ETNP, eastern tropical North Pacific; ETSP,
eastern tropical South Pacific; ETSA, eastern tropical South Atlantic;
AS, Arabian Sea. [Max Planck Institute for Marine Microbiology, based on
data from the World Ocean Atlas 2009] (B) Oxygen levels at the bottom
of the Baltic Sea during 2012 (59). In recent years, low-oxygen areas
have expanded to 60,000 km
2
as a result of limited exchange, high
anthropogenic nutrient loads, and warming waters (59)(red,O
2
concentration
≤63 mmol liter
−1
[2 mg liter
−1
]; black, anoxia). [Reproduced from (59)]
RESEARCH |REVIEW
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responses, increase disease, and reduce growth
(69,70).
In both oceanic and coastal systems, vertical
and horizontal distributions of organisms follow
oxygen gradients and discontinuities, and migra-
tory behavior is constrained in response to both
oxygen availability and the ways that oxygen alters
the distributions of predators and prey (64,71). Be-
cause oxygen tolerances and behavioral responses
to low oxygen levels vary among species, taxa,
trophic groups, and with mobility (19), encounter
rates, feeding opportunities, and the structure of
marine food webs change. Movement to avoid low
oxygen can result in lost feeding opportunities on
low-oxygen–tolerant prey and can increase energy
expended in swimming (19,70). Hypoxia effects
on vision, a function that is highly oxygen intensive,
may contribute to these constraints, in part through
changing light requirements (72).
The presence and expansion of low–water col-
umn oxygen reduces diel migration depths, com-
pressing vertical habitat and shoaling distributions
of fishery species and their prey (73–75). For pelagic
species, habitat compression can increase vulner-
ability to predation as animals are restricted to
shallower, better-lit waters and can increase vul-
nerability to fishing by predictably aggregating
individuals at shallower or lateral edges of low-
oxygen zones (71,76–78). For demersal species,
hypoxia-induced habitat compression can lead to
crowding and increased competition for prey (73),
potentially resulting in decreased body condition
of important fishery species such as Baltic cod (79).
In contrast, migration into and out of hypoxic
waters can allow some animals to utilize oxygen-
depleted habitats for predator avoidance or to
feed on hypoxia-tolerant prey, and then to return
to more highly oxygenated depths or locations
(23,80). Habitat compression may also enhance
trophic efficiency in upwelling regions, contrib-
uting to their extraordinary fish productivity
(20,21). Some hypoxia-tolerant fish and inverte-
brate species expand their ranges as OMZs expand
(28,81), and their predators and competitors are
excluded.
Multiple stressors
Deoxygenation is mechanistically linked to other
ocean stressors, including warming (82)andacidi-
fication (83), and thus it is often their combined
effects that shape marine ecosystems (84,85).
Because hypoxia limits energy acquisition, it is es-
pecially likely to exacerbate effects of co-occurring
stressors that increase energy demands (65). The
thermal tolerance of ectotherms is limited by their
capacity to meet the oxygen demands of aerobic
metabolism (62). Increased temperature elevates
oxygen demand while simultaneously reducing
oxygen supply, thus expanding the area of the
oceans and coastal waters where oxygen is insuf-
ficient. Through this mechanism, ocean warming
is predicted to result in shifts in the distribution
of fishes and invertebrates poleward by tens to
hundreds of kilometers per decade, shifts into
deeper waters, and local extinctions (63,86). Mod-
els project that warming combined with even
modest O
2
declines (<10 mmol kg
−1
) can cause
declines in important fishery species that are
sensitive to low oxygen levels (87). Physiological
oxygen limitation in warming waters is also pre-
dicted to reduce maximum sizes of many fish
species, including some that support important
fisheries (88).
Increased respiration that causes deoxygenation
also amplifies the problem of ocean acidification
because the by-product of aerobic respiration is
CO
2
. Temporal and spatial variations in oxygen
in subpycnocline and shallow eutrophic waters
are accompanied by correlated fluctuations in
CO
2
. In highly productive estuarine, coastal, and
upwelling regions, oxygen concentrations and pH
can exhibit extreme fluctuations episodically and
on diel, tidal, lunar, and seasonal cycles (83,89).
Elevated CO
2
can sometimes decrease the oxygen
affinity of respiratory proteins (90), reduce toler-
ance to low oxygen by increasing the metabolic
cost of maintaining acid-base balance (91), and
reduce responses to low oxygen that would other-
wise increase survival (92). Neither the occurrence
nor the magnitude of cases in which acidification
exacerbates the effects of low oxygen are currently
predictable (83).
Other covarying factors, such as nutrients and
fisheries dynamics, can mask or compensate for
effects of deoxygenation, complicating manage-
ment decisions. Fisheries management is designed
to adjust effort and catch as population abundance
changes (93). Thus, direct and indirect effects of
deoxygenation on a harvested population may
not be easily traceable in monitoring or catch data
because management actions adjust for the loss
in abundance. In addition, high nutrient loads can
stimulate production in a habitat that remains
well oxygenated, at least partially offsetting lost
production within a hypoxic habitat (52). Total
landings of finfish, cephalopods, and large mobile
decapods are positively correlated with nitrogen
loads (22), in spite of hypoxia in bottom waters
(52). The conflation of habitat loss and nutrient
enrichment is prominent in upwelling zones, as
well as eutrophic coastal waters. Increased upwell-
ing of nutrient-rich, oxygen-depleted waters from
the 1820s to the 20th century has increased pri-
mary and fish productivity off the coast of Peru,
for example (94). However, there are limits to
the extent of hypoxia that can form before total
system-wide fishery landings decline. In addition,
individual species dependent on a degraded habitat
may decline, whereas other species able to use
more highly oxygenated habitats within the same
system thrive (52).
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 4of11
Fig. 3. Life and death at low oxygen leve ls. (A) Animals using low-oxygen habitats exhibit
a range of physiological, morphological, and behavioral adaptations. For example, teribellid
worms (Neoamphitrite sp., Annelida) with large branchaea and high hemoglobin levels can
survive in the extremely low oxygen levels found at 400 m depth in the Costa Rica Canyon.
(B) Fish kills in aquaculture pens in Bolinao, Philippines, had major economic and health
consequences for the local population. (C) The ctenophore Mnemiopsis leidyi is more
tolerant of low oxygen than trophically equivalent fishes in its nat ive habitat in the Chesapeake
Bay and can use hypoxic areas from which fish are excluded. (D) A low-oxygen event
caused extensive mortality of corals and associated organisms in Bocas del Toro, Panama.
These events may be a more important source of mortality in coral reefs than previously
assumed.
PHOTOS: (CLOCK WISE FROM TOP LEFT) GREG ROUS E/SCRIP PS INSTITU TION OF OCEA NOGRAPH Y; PHILIPPI NE DAILY INQUI RER/OPINI ON/MA. CER ES P. DOYO; PETRA URBANEK/ WIKIME DIA COMMON S/HTTPS: //CREATIVECOMM ONS.ORG/LI CENSES/ BY-SA/4.0/; ARACDIO CASTI LLO/SMITHS ONIAN
INSTITUTIO N
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Biogeochemistry
Oxygen availability affects remineralization pro-
cesses and associated sources and sinks of im-
portant nutrient elements, such as nitrogen,
phosphorus, and iron. Even when occurring in
relatively small, low-oxygen regions, the effects of
oxygen-dependent nutrient-cycling processes are
communicated to the wider ocean by circulation.
Hence, local changes within OMZs can influence
nutrient budgets, biological productivity, and
carbon fixation on regional to global scales, and
changes in oxygen-depleted bottom waters of coast-
al systems can affect entire water bodies.
In addition to nitrogen, phosphorus, and iron,
which are discussed in more detail below, a wide
range of other elements are affected by oxygen
conditions. Hydrogen sulfide, which is toxic to
most aerobic organisms, is produced in anoxic
sediments and can be released to the overlying
water column, especially during upwelling events
(16). Methane, a potent greenhouse gas, is also
produced in anoxic sediments, but methanotro-
phic activity limits its release to the atmosphere
(95). Hypoxia increases conversion of As(V) to the
more toxic As(III) (96). Cadmium, copper, and zinc
form sulfide precipitates in the presence of anoxic
or extremely oxygen-deficient waters and sulfides
(97). This process may affect the global distribu-
tion of trace metals, some of which serve as micro-
nutrients for plankton growth, but the importance
of such controls is yet to be fully evaluated.
Where oxygen levels are extremely low or ab-
sent, anaerobic remineralization of organic matter
by denitrification and anaerobic ammonium oxi-
dation (anammox) leads to a net loss of bioavailable
nitrogen through the formation of dinitrogen gas
(N
2
). Recent investigations have reported func-
tionally anoxic conditions within open-ocean OMZs
(98) and have shown that traces of oxygen at nano-
molar levels can inhibit anaerobic processes, such
as denitrification (99). Total loss of bioavailable
nitrogen from the open ocean is currently esti-
mated to be 65 to 80 Tg year
−1
from the water
column and 130 to 270 Tg year
−1
from sediments
(100). Analysis and modeling of global benthic
data also indicate that denitrification in sediments
underlying high-nutrient and low-oxygen areas
(such as OMZs) removes around three times as
much nitrogen per unit of carbon deposited as
sediments underlying highly oxygenated water
and accounts for ~10% (i.e., 15 Tg year
−1
)ofglobal
benthic denitrification (101). Similarly enhanced
benthic denitrification has been observed at very
low bottom-water oxygen concentrations in eutro-
phic coastal systems (102,103)andintheoxycline
of the water column, comparable to OMZs (104).
Certainly, there is genetic potential for water col-
umn denitrification to occur once anoxic conditions
are reached.
A by-product of both nitrification and de-
nitrification is nitrous oxide, N
2
O, a potent green-
house gas (105). The amount of N
2
Oproducedis
strongly dependent on prevailing oxygen condi-
tions. Production of N
2
Oisenhancedattheoxic-
suboxic boundaries of low-oxygen waters, but N
2
O
is further reduced to N
2
in anoxic conditions (95),
so small differences in oxygen concentration de-
termine whether there is net production or con-
sumption of this gas. Low-oxygen zones (including
shelf and coastal areas) contributealargefraction
of the total oceanic N
2
O emission to the atmo-
sphere, and expansion of these systems may sub-
stantially enhance oceanic N
2
O emissions (95).
Record air-sea N
2
O fluxes have recently been ob-
served above the OMZ in the eastern tropical South
Pacific (106).
Although our understanding of the relationships
among oxygen, remineralization of bioavailable
N, and production of N
2
O has greatly increased,
the consequences of a shift in these relationships
in a warming world with increased O
2
-depleted
waters are less well understood. Continued de-
oxygenation of OMZ waters is expected to increase
the volume of water where denitrification and
anammox occur and may lead to increased marine
nitrogen loss (99). This could alter the ocean’sni-
trogen inventory and, eventually, biological pro-
duction on millennial time scales if nitrogen losses
are not compensated for by increases in nitrogen
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 5of11
Eukaryotic biomass and diversity not
limited by oxygen unless increasing
temperature increases oxygen demand
above oxygen supply
Fishing boats target nsh and
invertebrates found at high densities at
the edge of low-oxygen zones where they
escape physiologically stressful
conditions and take advantage of prey
that use this edge as a refuge habitat
Upwelling of low-O2, high-CO2 waters
can kill and displace sh and benthic
invertebrates, but high nutrients in
upwelled waters fuel high productivity
Organisms inhabiting low-oxygen
habitats have evolved physiological
and behavioral adaptations, but when
tolerances are exceeded, survival,
growth, and reproduction decline
Global warming is expected to continue
to worsen deoxygenation in the open
ocean, and both increasing nutrient
loads and warming could worsen future
deoxygenation in coastal waters
Absence of eukaryotes dependent on
aerobic respiration; increased
dentrication, production of N2O, and
release of Fe and P from sediments
Well-
oxygenated
water
Hypoxia
Anoxia
Fig. 4. Oxygen exerts a strong control over biological and biogeochemical processes
in the open ocean and coastal waters. Whether oxygen patterns change over space, as with
increasing depth, or over time, as the effects of nutrients and warming become more
pronounced, animal diversity, biomass, and productivity decline with decreasing levels of oxygen.
At the edge of low-oxygen zones, where nutrients are high and predators and their prey are
concentrated into an oxygenated habitat, productivity can be very high, but even brief exposures
to low oxygen levels can have strong negative effects. (Top ) Well-oxygenated coral reef with
abundant fish and invertebrate assemblages. (Middle) Low-oxygen event in Mobile Bay, United
States, in which crabs and fish crowd into extreme shallows where oxygen levels are highest.
(Bottom) Anoxic mud devoid of macrofauna.
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RESEARCH |REVIEW
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fixation (107). However, the feedbacks that link
nitrogen loss and nitrogen fixationremain enig-
matic (101). The direction and magnitude of
change in the N
2
O budget and air-sea N
2
Oflux
are also unclear because increased stratification
could reduce the amount of N
2
O that reaches
the surface ocean and escapes to the atmo-
sphere (108).
The supply of phosphorus and iron released
from the sediments is generally enhanced under
anoxic conditions (109,110). These nutrients have
the potential to further stimulate biological pro-
duction if they reach well-lit surface waters, such
as above the OMZs associated with coastal up-
welling regions and the surface layer of coastal
waters. Elevated dissolved inorganic phosphorus
and chlorophyll are found in surface waters when
anoxia occurs in fjords and estuaries (111), and,
in some systems, deep waters supply as much
phosphorus to productive surface layers as do
watershed discharges (112). Increased productivity
will tend to increase oxygen consumption, may
increase the sediment area in contact with low-
oxygen waters, and may eventually lead to further
release of phosphorus and iron from the sediment.
There is evidence for this positive feedback in en-
closed seas such as the Baltic Sea, where enhanced
nitrogen fixation in response to deoxygenation
has led to the recent proliferation of undesirable
cyanobacterial blooms that can be toxic and have
adverse effects on ecosystems and society (102).
Enhanced phosphate and iron levels may gen-
erally favor nitrogen fixation by diazotrophs, es-
pecially in the presence of nitrogen loss when
ordinary plankton are driven toward nitrogen
limitation.
Predicting oxygen decline
Sound management of marine ecosystems is based
on reliable predictions under a range of future
scenarios and an understanding of associated
uncertainties. Numerical models that can project
effects of climate change and eutrophication on
oxygen availability in the open ocean and in coastal
systems can offer these predictions. Current state-
of-the-art global models generally agree that the
total amount of oxygen loss will be a few percent
by the end of the century (31), a decline that
could have substantial biogeochemical and eco-
logical effects. However, there is little agreement
among models about the spatial distribution of
future low-oxygen zones having <100 mmol O
2
kg
−1
(113) or the spatial patterns of O
2
changes that
have occurred over the past several decades (40).
This uncertainty currently limits our ability to
reliably predict the regional impact of climate
warming on open-ocean OMZs and, hence, on
oxygen-sensitive biogeochemical processes, in-
cluding the nitrogen budget. More realistic and
detailed inclusion of mechanisms other than
CO
2
-driven global warming—such as atmospheric
nutrient deposition and decadal- to multidecadal-
scale climate variability (especially fluctuations
in wind patterns)—may improve agreement among
models and, therefore, their ability to predict the
spatial distribution of past and future low-oxygen
areas.
Predicting oxygen levels in individual coastal
water bodies requires modeling the variability in
these systems, which is tightly governed by inter-
actions with the land, atmosphere, sediment, and
offshore waters at small space and time scales.
This can be achieved by current estuary-specific
and regional three-dimensional coupled hydro-
dynamic–water quality models (67); these and
other state-of-the-art modeling approaches de-
serve broader implementation. However, model
performance can be hampered by the use of forcing
data, such as river discharges and atmospheric
conditions, that lack sufficiently resolved spatial
and temporal detail. Projections of future de-
oxygenation also require reliable information on
changes in key parameters and interactions under
a range of climate change and nutrient manage-
ment scenarios and benefit from the use of ap-
proaches that explicitly model connections along
the river–estuary–adjacent ocean or sea continuum.
Projections of local changes in timing and magni-
tude of precipitation and warming are especially
important. Future characteristics of human pop-
ulations, such as rates of population growth, the
effectofclimatechangeonthegeographyofpop-
ulation centers, and the effects of education and
income on demands for improved sanitation and
animal protein are also needed because of their
influence on nutrient discharges at both local
and global scales.
Improving predictions critical for manage-
ment in both the open ocean and coastal systems
will require increased observations from field mea-
surements and experiments to constrain and refine
models. Ideally, such data should include repre-
sentations of future environmental conditions.
An improved mechanistic understanding of feed-
backs that limit or exacerbate oxygen depletion
and alter oxygen-sensitive biogeochemical cycles
is especially important. In the open ocean, infor-
mation is needed on transport mechanisms—such
as small-scale mixing processes (114), stirring, and
transport by mesoscale structures (115)—that influ-
ence oxygen distributions.
Advanced observation networks can provide
data to underpin the development of an improved
mechanistic understanding and the refinement
of current models. Drifters and autonomous plat-
forms ranging from Argo floats to tethered arrays
provide real-time data and have the potential to
increase knowledge of oxygen dynamics at the
smallspatialandtemporalscalesthatareultima-
tely needed for both regional and global models.
High-resolution measurements have revealed the
small-scale patchiness of oxygen-sensitive process-
es in space and time (99,106) and have provided
new insight into the biogeochemistry of OMZs
(98). Optical oxygen sensors mounted on Argo
floats or gliders can now use atmospheric oxygen
to perform ongoing, in situ calibrations through-
out the float (116) or glider lifetime. The accuracy
of autonomous measurements of in situ oxygen
concentrations ≤1mmol kg
−1
has been improved
by the development of STOX (switchable trace
amount oxygen) sensors (117), and novel trace-
oxygen optical sensors can now provide precise
oxygen quantification in OMZs and detect oxygen
concentrations as low as ~5 nmol kg
−1
(118). The
new platforms and sensors facilitate the implemen-
tation of regional and global oxygen observatories
targeted toward the much-improved monitoring
and, eventually, modeling and management of de-
oxygenation. For coastal waters, it is also important
to develop sensors that are affordable for use in
low-income developing countries (LIDCs) and
that can be used to generate reliable data from
citizen science.
Predicting effects at large scales of
space, time, and ecological organization
Improved management and conservation of open-
ocean and coastal systems requires predictions of
the effects of deoxygenation at spatial, temporal,
and ecological scales most relevant to the eco-
system services provided by these waters. Although
research has clearly shown that low-oxygen zones
reduce habitat for species dependent on aerobic
respiration and that exposure to suboptimal oxy-
gen levels leads to a host of negative effects on
individuals, identifying effects of expanding de-
oxygenation at the scale of populations or fish-
eries stocks has been difficult, particularly for
mobile species (52,119). A similar problem applies
to scaling up oxygen-sensitive biogeochemical pro-
cesses to predict feedbacks on global ocean nutrient
inventories and Earth’sclimate.
Scaling to predict effects on food webs and
fisheriesisconfoundedbycompensatorymech-
anisms; examples include increased production
of planktonic prey under high nutrient loads and
increased encounter rates between predators and
their prey when they are squeezed into smaller
oxygenated habitat space (52,119,120). In addi-
tion, populations maintained below their habitat-
dependent carrying capacity by fisheries or other
factorsmaynotbeasstronglyaffectedbytheloss
of habitat as species nearer their carrying capacity.
In these cases, habitats suitable for feeding and
other life functions may remain sufficient, even
when their size is reduced by low oxygen.
The most promising approaches to scaling em-
ploy a suite of methods ranging from detailed
mechanistic studies to large-scale field efforts, as
well as new and increasingly sophisticated analy-
ses and modeling tools that address spatial pro-
cesses (120), temporal fluctuations (121,122), and
the role of co-occurring stressors. Consideration
of the effects of early hypoxia exposure on later
life stages after organisms migrate to more highly
oxygenated habitats can indicate the large spatial
scales over which even spatially limited hypoxia
can have impacts (123). Paleoecological approaches
are critical for gaining a long-term perspective
beyond the time scale of biological and oceano-
graphic observation (94,124). Even sophisticated
approaches will not always provide support for
large-scale negative effects of deoxygenation, but
eliminating deoxygenation as a major cause of
population declines is also important to effective
management.
Increased research is most needed in locations
where deoxygenation is likely to affect local eco-
nomies and food security. Place-based, artisanal
fisheries with little capacity to relocate as local
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 6of11
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habitat degrades are more likely to suffer from
deoxygenation than industrialized fisheries with
highly mobile fishing fleets. Aquaculture, in par-
ticular, can be a critical intersection between de-
oxygenation and societal effects because aquaculture
itself can cause deoxygenation (125), and animals
restrained in nets and cages are unable to escape
harmful oxygen conditions. But critically, much
of the world’s marine aquaculture is done in
LIDCs. Fish kills in aquaculture pens (125) can
compromise livelihoods and can directly harm
human health when low incomes and food in-
security lead to consumption of fish killed by
low-oxygen conditions (126). Coral reefs contrib-
ute to food security and local economies through
their value to tourism and storm protection, as well
as food production. Recent research indicates that
low oxygen may be an increasingly important factor
in the mortality of corals and associated fauna in
some regions and that low-oxygen problems on coral
reefs are likely underreported (127).
Reducing deoxygenation and its
negative effects
Local, national, and global efforts are required
to limit further oxygen declines, restore oxygen
to previously well-oxygenated environments, and
enhance the resilience of ecosystems affected by
deoxygenation. At their most basic level, the actions
needed to address deoxygenation—reducing nutri-
ent loads to coastal waters and reducing green-
house gas emissions globally—have substantial
benefits to society above and beyond improving
oxygen conditions. Improved sanitation can ben-
efit human health directly while also reducing
coastal nutrient loads. Eliminating excess and
inefficiently applied fertilizer can reduce costs to
farmers (128) and emissions of N
2
O(129) and
may decrease nitrogen loads to waterways. Eli-
minating emissions from combustion of fossil
fuels can reduce greenhouse gas production and
may result in decreased atmospheric deposition
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 7of11
Deoxygenation management and policy strategies
Ecosystem-based mitigation to
restore and protect the environment
Implement and maintain monitoring and analysis programs
Adaptation to restore and protect
marine organisms and sheries
Reduce anthropogenic nutrients
reaching coastal waters to reduce
eutrophication-driven deoxygenation
Reduce greenhouse gas
emissions to reduce deoxygen-
ation due to climate change
Monitoring, data analysis, and
dissemination of results are
critical to detect problems and
determine the eectiveness of
management and restoration
eorts.
Create marine protected areas and no-catch zones in well-oxygenated
areas that can serve as refugia; protect populations when oxygen is low
Consider eects of low
oxygen on production,
non-shing mortality, and
shing mortality in setting
catch limits
Reduce shing pressure on hypoxia-intolerant species. Utilize shing
gear that minimizes additional stress on oxygen-impacted sh stocks
and ecosystems.
Develop aquaculture practices and limits to protect oxygen
content of waters
Fig. 5. Strateg ies for deoxygenati on management and
policy-making. (Left) Multiple management actions can help to mitigate
deoxygenation. Key among these are reductions in (i) anthropogenic
nutrient inputs from land, which will reduce algal blooms and
subsequent oxygen drawdown; (ii) greenhouse gas emissions, which
will slow warming; and (iii) waste production from aquaculture, which
will contribute to oxygen consumption. (Right) Adaptive measures
can reduce stress and may increase resilience of marine ecosystems
that face deoxygenation. Examples include creating protected
areas that can serve as refugia in hypoxic areas or during hypoxic
events; incorporating oxygen effects on population distribution
and dynamics into catch limits and closures, as has been done for
rockfish; and adopting gear regulations that reduce stress on vulnerable
fisheries or ecosystems. (Bottom) Both types of actions benefit
from enhanced oxygen and biological monitoring, including access
to real-time data that can elicit quick management responses, as
well as more synthetic analyses that might reveal spatial and
temporal trends.
PHOTO: (TOP LE FT) DAVID DIXON/WI KIMEDIA COMMON S/HTTPS://CREATIVECOMMONS.OR G/LICENS ES/BY-SA/2.0/; (B OTTOM RIGHT) SMITHSONI AN ENVIRONMENTAL RESEA RCH CENTER; (B OTTOM LEFT) NOAA
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of nitrogen that stimulates primary production
in coastal waters (130). Reducing or eliminating
greenhouse gas emissions can, more generally,
lower the threats from global warming and ocean
acidification and, simultaneously, reduce ocean
deoxygenation. Improved management of fish-
eries and marine habitats that are sensitive to
the development and effects of low oxygen helps
to protect economies, livelihoods, and food secu-
rity (Fig. 5).
Failure to reduce nutrient loads, at all or suf-
ficiently, is the primary reason that oxygen levels
have not improved in most coastal systems. But
some of the reasons for slow progress are inher-
ent in the problem itself. High sedimentary oxygen
demand can continue for decades as accumulated
organic matter degrades (57), phosphorus may con-
tinue to be released from sediments once oxygen
thresholds have been crossed (102), and nitrogen
leached from soils and dissolved in groundwater
continues to enter waterways for decades (131).
Increasing temperatures can require greater re-
ductions in nutrients to meet the same oxygen goals
(57,61). Because of changing conditions and the
nonlinearity of ecological processes, ecosystems
may not return to their predisturbed state even if
conditions that caused the initial deoxygenation
are eased (132).
To maintain the current conditions, per capita
reductions in nutrient discharges and greenhouse
gas emissions will need to increase as the global
population continues to grow. Nevertheless, con-
siderable improvements have been observed in
some coastal systems through implementation of
a wide range of strategies to reduce the input of
nutrients and biomass (133). Some of the most
notable improvements have occurred in systems
such as the Thames and Delaware River estuaries,
where steps to keep raw sewage out of the rivers
and, eventually, to treat wastewater substantially
decreased biological oxygen demand (133). In the
Maryland portion of the Chesapeake Bay, where
both point- and nonpoint-source nutrient reduc-
tion strategies have been implemented, oxygen
concentrations <0.1 mg liter
−1
(<3 mmol kg
−1
)have
rarely been measured since 2014—amarkedcon-
trast to the first 30 years of frequent monitoring
(1984–2013) (134). In one Chesapeake tributary,
the Potomac River, nitrogen reductions due to
better air quality have played the major role in
water quality improvements (135). Additionally,
better understanding of deoxygenation may
enable a range of adaptive, protective actions
for fisheries and the habitats that sustain them
(Fig. 5).
An integrated framework that combines mod-
eling, observations, and experiments in a multiple-
stressor environment and involves the full range
of stakeholders (e.g., scientists, local governments,
intergovernmental bodies, industrial sectors, and
the public) will facilitate the development and im-
plementation of the most ecologically and econo-
mically effective plans to reverse deoxygenation
(Fig. 6). Networks of research scientists, such as
the Intergovernmental Oceanographic Commis-
sion (IOC)–UNESCO Global Ocean Oxygen Network
(www.unesco.org/new/en/natural-sciences/ioc-
oceans/sections-and-programmes/ocean-sciences/
global-ocean-oxygen-network/), as well as groups
with more limited geographic and disciplinary
scope, can help to keep the process updated and
to build capacity in parts of the world where
improved technology and training are needed.
The key to effective management is raised aware-
ness of the phenomenon of deoxygenation, as well
as its causes, consequences, and remediation
measures.
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 8of11
Monitoring: sophisticated automated oceanographic sensor arrays and citizen scientists document
current and changing oxygen conditions.
Inclusion of data in regional and global databases allows local measurements to contribute to analyses of
local and large-scale patterns and trends.
Numerical models use monitoring and experiment results to predict future conditions and loss of
ecosystem services under a range of possible scenarios.
Research can inform
regulations for restoring
oxygen and reducing its
decline, as well as aid
sheries managment to
minimize eects on
economies and food
security.
Societal goals based on protection of ecosystem services
and historical conditions
D = [exp(–k1t) – exp(–k2t)]
k1L0
k2–k1
+ D0exp(–k2t)
el
s use mon
it
or
i
ng an
d
exper
i
men
t
resu
l
i
ces under a ran
ge
of
pos
sible scenarios
Fig. 6. Monitoring in coastal waters and the open ocean enables documentation of
deoxygenation and, in some cases, improved oxygen conditions. In shallow water, handheld,
continuous, and shipboard sensors are used worldwide. In the open ocean and nearshore waters,
global arrays of sensors (such as the Argo floats), shipboard measurements, and deep platforms and
profilers provide data to validate global models. Archiving data in well-documented databases
accessible by all stakeholders facilitates scientific and management advances and public
engagement. Experiments and field studies at scales ranging from genes to ecosystems provide
information to predict the effects of low oxygen levels on ecological processes and services and are
also used to develop fisheries and ecosystem models. Model projections and analyses of
deoxygenation and its effects inform management and policy at both local and multinational scales
and provide the basis for strategies to combat deoxygenation.
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ACKNOW LEDGM ENTS
We thank IOC-UNESCO for financial support and for initiating
and supporting the Global Ocean Oxygen Network. We also
thank R. Diaz for help with updating the list of coastal sites that
have reported hypoxia (Fig. 1A); B. Michael and M. Trice of
the Maryland Department of Natural Resources for help w ith the
Maryland water quality database; and our many current and
past collaborators on deoxygenation research in coastal
systems, OMZs, the Black Sea, and elsewhere. Funding was
provided by National Oceanic and Atmospheric Administration
(NOAA)–Center for Sponsored Coastal Ocean Research
grant NA10NOS4780138 and Maryland Sea Grant SA75281450-P
(to D.B.), NSF-EAR grant 1324095 (to L.A.L.), the Deutsche
Forschungsgemeinschaft via grant SFB754 (to A.O.), and the
Fonds National de la Recherche Scientifique and the BENTHOX
program grant T.1009.15 (to M.G.). This study was partly
supported by the BONUS COCOA project (grant 2112932-1),
funded jointly by the European Union and the Swedish
Research Council for Environment, Agricultural Sciences
and Spatial Planning.
10.1126/science.aam7240
Breitburg et al., Science 359, eaam7240 (2018) 5 January 2018 11 of 11
RESEARCH |REVIEW
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Declining oxygen in the global ocean and coastal waters
Yasuhara and Jing Zhang
Grant C. Pitcher, Nancy N. Rabalais, Michael R. Roman, Kenneth A. Rose, Brad A. Seibel, Maciej Telszewski, Moriaki
Garçon, Denis Gilbert, Dimitri Gutiérrez, Kirsten Isensee, Gil S. Jacinto, Karin E. Limburg, Ivonne Montes, S. W. A. Naqvi,
Denise Breitburg, Lisa A. Levin, Andreas Oschlies, Marilaure Grégoire, Francisco P. Chavez, Daniel J. Conley, Véronique
DOI: 10.1126/science.aam7240
(6371), eaam7240.359Science
, this issue p. eaam7240Science
which ultimately will cause societal and economic harm.
oxygen minimum zones. In the longer term, these conditions are unsustainable and may result in ecosystem collapses,
result in improvements in local fisheries, such as in cases where stocks are squeezed between the surface and elevated
sediments and fundamental shifts in the availability of key nutrients. In the short term, some compensatory effects may
are changing ocean biogeochemistry and increasing oxygen consumption. This results in destabilization of−−activities each resulting from human−−the open ocean and coastal waters. Rising nutrient loads coupled with climate change
review the evidence for the downward trajectory of oxygen levels in increasing areas of et al.deoxygenation. Breitburg
As plastic waste pollutes the oceans and fish stocks decline, unseen below the surface another problem grows:
Beneath the waves, oxygen disappears
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