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Increasing hypoxia on global coral reefs under ocean warming

Nature Climate Change volume 13, pages 403–409 (2023)Cite this article

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Abstract

Ocean deoxygenation is predicted to threaten marine ecosystems globally. However, current and future oxygen concentrations and the occurrence of hypoxic events on coral reefs remain underexplored. Here, using autonomous sensor data to explore oxygen variability and hypoxia exposure at 32 representative reef sites, we reveal that hypoxia is already pervasive on many reefs. Eighty-four percent of reefs experienced weak to moderate (≤153 µmol O2 kg−1 to ≤92 µmol O2 kg−1) hypoxia and 13% experienced severe (≤61 µmol O2 kg−1) hypoxia. Under different climate change scenarios based on four Shared Socioeconomic Pathways (SSPs), we show that projected ocean warming and deoxygenation will increase the duration, intensity and severity of hypoxia, with more than 94% and 31% of reefs experiencing weak to moderate and severe hypoxia, respectively, by 2100 under SSP5-8.5. This projected oxygen loss could have negative consequences for coral reef taxa due to the key role of oxygen in organism functioning and fitness.

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Fig. 1: Oxygen sensor deployment sites and locations, hourly oxygen climatologies and oxygen distributions on global coral reefs.
Fig. 2: Shifts in DO concentration, hypoxic event duration and occurrence of hypoxia exposure under warming on global coral reef sites.
Fig. 3: Timing of present-day hypoxic observations across global coral reef sites.

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Data availability

All data included in this study (for all figures and statistics56) are freely available on Dryad (https://doi.org/10.5061/dryad.41ns1rnj7). Data may be used if cited appropriately.

Code availability

All code files written and used for analyses in this study57 are freely available on GitHub (https://github.com/apezner/GlobalReefOxygen). Code may be used if cited appropriately.

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Acknowledgments

We thank K. Inoha, R.-W. Syu and all field station administrators and field assistants who were essential in collecting these datasets. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government. Funding was provided by the National Science Foundation: OCE-1255042 (A.J.A.), OCE-1829778 (A.J.A.), OCE-1538495 (D.I.K. and M.T.), OCE-1459255 (M.D.D) and OPP-1951294 (M.D.D); Belmont Forum/NSF ICER 2029205 (A.J.A.); UCSD Marine Sciences grant no. A105437 (A.J.A.); the National Science Foundation Graduate Research Fellowship DGE-2038238 (A.K.P.); a Philanthropic Educational Organization International Scholar Award (A.K.P.); the NOAA Coral Reef Conservation Program and NOAA Ocean Acidification Program, through the NOAA National Coral Reef Monitoring Program (H.C.B.); the US Geological Survey Coastal and Marine Hazards and Resources Program-funded data collection at Crocker Reef, Florida, USA (K.K.Y.46); internal funding from the Okinawa Institute of Science and Technology (S.M.); and the Ministry of Science and Technology of Taiwan grant no. 107-2611-M-019-001-MY3 (W.-C.C.). Funding for the long-term monitoring programme on Palmyra Atoll was provided to Smith Lab (J.E.S.) from the Bohn Family Foundation and the Bill and Kathy Scripps Family Foundation.

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Authors and Affiliations

  1. Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA

    Ariel K. Pezner, Travis A. Courtney, Samantha M. Clements, Samuel A. H. Kekuewa, Todd R. Martz, Heather N. Page, Max S. Rintoul, Jennifer E. Smith, Martin Tresguerres & Andreas J. Andersson

  2. Department of Marine Sciences, University of Puerto Rico at Mayagüez, Mayagüez, Puerto Rico

    Travis A. Courtney

  3. NOAA Pacific Islands Fisheries Science Center, Honolulu, HI, USA

    Hannah C. Barkley

  4. Institute of Marine Environment and Ecology, National Taiwan Ocean University, Keelung, Taiwan

    Wen-Chen Chou & Hui-Chuan Chu

  5. Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, Taiwan

    Wen-Chen Chou

  6. Institute for Coastal Plain Science, Georgia Southern University, Statesboro, GA, USA

    Tyler Cyronak

  7. Department of Chemistry and Biochemistry, University of Montana, Missoula, MT, USA

    Michael D. DeGrandpre

  8. Smithsonian Tropical Research Institute, Ancón, Republic of Panama

    David I. Kline

  9. Department of Oceanography, National Sun Yat-sen University, Kaohsiung, Taiwan

    Yi-Bei Liang, Keryea Soong & Yi Wei

  10. Marine Biophysics Unit, Okinawa Institute of Science and Technology, Okinawa, Japan

    Satoshi Mitarai

  11. Sea Education Association, Woods Hole, MA, USA

    Heather N. Page

  12. Monterey Bay Aquarium Research Institute, Moss Landing, CA, USA

    Yuichiro Takeshita

  13. Institute of Oceanography, National Taiwan University, Taipei, Taiwan

    Yi Wei

  14. United States Geological Survey, St. Petersburg Coastal and Marine Science Center, St Petersburg, FL, USA

    Kimberly K. Yates

Authors
  1. Ariel K. Pezner
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  2. Travis A. Courtney
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  9. Samuel A. H. Kekuewa
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  22. Andreas J. Andersson
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Contributions

A.K.P., T.A.C. and A.J.A. conceptualized the paper and methodology, with contributions from M.S.R. to methodology. A.K.P. performed the formal analysis and visualization, under supervision of T.A.C. and A.J.A. A.K.P. and A.J.A. wrote the original draft of the paper. All authors (A.K.P., T.A.C., H.C.B., W.-C.C., H.-C.C., S.M.C., T.C., M.D.G, S.A.H.K., D.I.K., Y.-B.L., T.R.M., S.M., H.N.P., M.S.R., J.E.S., K.S., Y.T., M.T., Y.W., K.K.Y. and A.J.A.) contributed to investigation as well as review and editing of the paper.

Corresponding author

Correspondence to Ariel K. Pezner.

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Extended data

Extended Data Fig. 1 Dissolved oxygen and temperature time series for global coral reef sites.

Dissolved oxygen (µmol O2 kg−1; black, left y-axis) and temperature (°C; blue, right y-axis) as a function of time in order of sites with increasing deployment length, ranging from 3 to 309 days (Supplementary Table 1). For each location, different instrument deployment sites are represented by numbers (for example, Dongsha 1 and Dongsha 2), or a combination of letters and numbers where letters represent either different depths at the same site (for example, Bocas 1a and 1b) or different deployments at the same site over time (for example, Crocker 1a, 1b, and 1c) (see Supplementary Table 1 for specific site information).

Extended Data Fig. 2 Nonlinear regressions between dissolved oxygen metrics, depth, and flow speed categorized by reef type.

(a) Dissolved oxygen (DO) variability (mean daily range in DO; µmol O2 kg−1; ± 1 standard deviation (s.d.)) as a function of mean depth (m; ± 1 s.d.) and (b) mean flow speed (m s−1; ± 1 s.d.) at global coral reef sites categorized by reef type (colors; Supplementary Table 1). (c) Mean daily minimum DO (µmol O2 kg−1; ± 1 s.d.) as a function of mean depth (m; ± 1 s.d.) and (d) mean flow speed (m s−1; ± 1 s.d.). For DO metrics, error bars represent ± 1 s.d. (n varies by site, see Supplementary Table 2). Measurements of flow speed and depth were only made at a subset of locations (n varied by site, see Supplementary Table 3). Error bars also represent ± 1 s.d. for the sites where these data were recorded. For sites where no current meter was deployed, recorded deployment depth was used instead of a calculated mean depth (no error bars plotted). Regression lines were plotted using power model regression statistics reported in Supplementary Table 4. No regression line is plotted in B due to poor fit.

Extended Data Fig. 3 Sea surface temperature predictions for global coral reef locations.

Mean monthly Coupled Model Intercomparison Project 6 (CMIP6) ensemble member Community Earth System Model 2 Whole Atmosphere Community Climate Model (CESM2-WACCM)63,64 sea surface temperature (SST) projections at global coral reef sites for the Shared Socioeconomic Pathways (SSPs) SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios (blue to red) between 2015 and 2100.

Extended Data Fig. 4 Conceptual diagram of calculation approach used to estimate changes in coral reef dissolved oxygen under warming (Equations 6–13).

(a) Present-day dissolved oxygen (DO; µmol O2 kg−1; solid grey line), mean DO value across time series (purple dashed line; assumed to be close to equilibrium), approximation of drawdown of DO by respiration at night (DOoffset; yellow arrow) expressed as the difference between mean DO and mean daily minimum (DOmin; orange), and the projected increase in respiration under 3 °C warming using a Q10 relationship (∆DOQ10; pink arrow). (b) Present-day DO (solid grey line), present-day DO solubility (dashed dark blue line; DOsol present), DO solubility under 3 °C warming (dashed light blue line), and the calculated decrease in solubility under 3 °C warming (∆DOsol; green arrow). (c) Present-day DO (solid grey line) and new calculated DO under 3 °C warming (black solid line) due to increased respiration and decreased solubility (pink and green arrows, respectively).

Extended Data Fig. 5 Box model simulations and validation of calculation approach to estimate dissolved oxygen changes as a result of warming.

(a) Modeled variations in seawater dissolved oxygen (DO; µmol O2 kg−1) in a hypothetical coral reef system over 7 days under three temperature scenarios (25 °C, 28 °C, and 31 °C; purple, green, blue, respectively) and two residence times (1 hour and 5 hours; solid and dotted lines, respectively). The model is described in detail in the Supplementary Information Extended Methods. (b) Comparison between the box model-calculated changes in DO due to warming and the calculation approach employed for the global coral reef dataset (Extended Data Fig. 4; Equations 6–13) represented as the deviation of model DO estimates from calculation DO estimates (µmol O2 kg−1). Comparisons were made for two warming scenarios relative to the base scenario of 25 °C (+3 °C and +6 °C; blue and green, respectively) and two residence times (1 hour and 5 hours; solid and dotted, respectively) over 7 days.

Extended Data Fig. 6 Total number of hypoxic observations and events for global coral reef sites under different warming scenarios and hypoxia thresholds.

(a) Total number of observations and (b) total number of hypoxic events below different hypoxia thresholds: 153 µmol O2 kg−1, 122 µmol O2 kg−1, 92 µmol O2 kg−1, and 61 µmol O2 kg−1 (weak, mild, moderate, and severe; light blue to dark blue) for different warming scenarios including 4 Shared Socioeconomic Pathways (SSPs) and a heatwave event across global coral reef sites.

Extended Data Fig. 7 Changes in the duration, intensity, and severity of hypoxic events under warming for global coral reef sites.

Distributions of the (a) duration (hours), (b) intensity (µmol O2 kg−1), and (c) severity (µmol O2 kg−1 hr) of hypoxic events below different oxygen thresholds (≤153 µmol O2 kg−1, ≤122 µmol O2 kg−1, ≤92 µmol O2 kg−1, or ≤61 µmol O2 kg−1) under present-day conditions and 5 different warming projections (including 4 Shared Socioeconomic Pathways (SSPs); blue to red) across global coral reef sites.

Extended Data Fig. 8 Changes in the cumulative duration, intensity, and severity of hypoxic events under warming for global coral reef sites.

(a) Cumulative duration (hours), (b) cumulative intensity (µmol O2 kg−1), and (c) cumulative severity (µmol O2 kg−1 hr) of hypoxic events below different oxygen thresholds (≤153 µmol O2 kg−1, ≤122 µmol O2 kg−1, ≤92 µmol O2 kg−1, or ≤61 µmol O2 kg−1; shades of blue) for different warming scenarios including 4 Shared Socioeconomic Pathways (SSPs) and a heatwave event across global coral reef sites.

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Pezner, A.K., Courtney, T.A., Barkley, H.C. et al. Increasing hypoxia on global coral reefs under ocean warming. Nat. Clim. Chang. 13, 403–409 (2023). https://doi.org/10.1038/s41558-023-01619-2

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