Review

Declining oxygen in the global ocean and coastal waters

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Science  05 Jan 2018:
Vol. 359, Issue 6371, eaam7240
DOI: 10.1126/science.aam7240

Figures

  • 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 O2 declines to <2 mg liter−1 (<63 μmol 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 GO2NE network, and downloaded from the World Ocean Atlas 2009].

  • 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 μmol kg−1 (63 μmol 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 O2 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 μmol kg−1­, respectively, at any depth within the water column (9). [Reproduced from (9)]

  • 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 O2 concentrations <100 μmol 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 km2 as a result of limited exchange, high anthropogenic nutrient loads, and warming waters (59) (red, O2 concentration ≤63 μmol liter−1 [2 mg liter−1]; black, anoxia). [Reproduced from (59)]

  • Fig. 3 Life and death at low oxygen levels.

    (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 native 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: (CLOCKWISE FROM TOP LEFT) GREG ROUSE/SCRIPPS INSTITUTION OF OCEANOGRAPHY; PHILIPPINE DAILY INQUIRER/OPINION/MA. CERES P. DOYO; PETRA URBANEK/WIKIMEDIA COMMONS/HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY-SA/4.0/; ARACDIO CASTILLO/SMITHSONIAN INSTITUTION
  • 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.

    PHOTOS: (TOP) UXBONA/WIKIMEDIA COMMONS/HTTP://CREATIVECOMMONS.ORG/LICENSES/BY/3.0; (BOTTOM) B. FERTIG/COURTESY OF THE INTEGRATION AND APPLICATION NETWORK, UNIVERSITY OF MARYLAND CENTER FOR ENVIRONMENTAL SCIENCE
  • Fig. 5 Strategies for deoxygenation 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 LEFT) DAVID DIXON/WIKIMEDIA COMMONS/HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY-SA/2.0/; (BOTTOM RIGHT) SMITHSONIAN ENVIRONMENTAL RESEARCH CENTER; (BOTTOM LEFT) NOAA
  • 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.

    IMAGES: (TOP ROW, LEFT) © CSIRO AUSTRALIA; (TOP ROW, MIDDLE) OCEAN OBSERVATORIES INITIATIVE/NSF; (SECOND ROW, MIDDLE) NOAA; (THIRD ROW, LEFT) AUGUST LINNMAN/WIKIMEDIA COMMONS/HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY-SA/2.0/; (BOTTOM ROW) TUUKKA TROBERG/HELCOM

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