Heat accumulation on coral reefs mitigated by internal waves

Abstract

Coral reefs are among the most species-rich, productive and economically valuable ecosystems on Earth but increasingly frequent pantropical coral bleaching events are threatening their persistence on a global scale. The 2015–2016 El Niño led to the hottest sea surface temperatures on record and widespread bleaching of shallow-water corals. However, the causes of spatial variation in bleaching are poorly understood, and near-surface estimates of heat stress, such as those inferred from satellites, cannot be generalized across the broad depth ranges occupied by corals. Here, using in situ temperatures recorded across reefs from the near surface to 30–50 m depths in the western, central and eastern Pacific, we show that during the peak of the 2015–2016 anomaly, temperature fluctuations associated with internal waves reduced cumulative heat exposure by up to 88%. The durations of severe thermal anomalies above 8 °C-days, at which point widespread coral bleaching and mortality are likely, were also decreased by >36% at some sites and were prevented entirely at others. The impact of internal waves across depths on coral reefs has the potential to create and support thermal refuges in which heat stress and coral bleaching risk may be modulated, but future effects depend on the response of internal wave climates to continued warming and strengthening ocean stratification.

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Fig. 1: In situ summertime water temperature variation across depths over three coral reefs in the Pacific Ocean.
Fig. 2: In situ water temperature variation relative to SST for observed and NIW time series.
Fig. 3: Internal waves decreased the duration and severity of thermal anomalies across depths and sites during the 2015–2016 El Niño.
Fig. 4: Heat accumulation across depths during the 2015–2016 El Niño with and without internal waves.

Data availability

Satellite SST observations can be accessed at https://coralreefwatch.noaa.gov/satellite/index.php. In situ data can be accessed at http://mcr.lternet.edu/data for the Moorea LTER site and at https://doi.pangaea.de/10.1594/PANGAEA.906191 for Iriomote and the Gulf of Chiriquí.

Code availability

The Matlab code used to produce the NIW data from observed temperature time series in this paper is available from the corresponding author on request.

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Acknowledgements

Logistical support in Iriomote was provided by T. Naruse, Dive Lateeq and Sera MarineTaxi, Shirahama, with funding provided by the Japan Society for the Promotion of Science (JSPS; grants nos. 15F15904 and 15K12183), the Japan Science and Technology Agency (CREST grant no. JPMJCR13A4), the Sumitomo Foundation (Environmental Research Grant) and the Nissei Foundation (Environmental Research Grant–Young Researcher). In Moorea, K. Sydel and C. Gotshalk assisted with access and processing of data from the Moorea Coral Reef (MCR) LTER Site, which is funded by the US National Science Foundation (NSF) under grant no. OCE 16-37396 (and earlier awards) as well as a gift from the Gordon and Betty Moore Foundation. Research in Moorea was completed under permits issued by the French Polynesian Government (Délégation à la Recherche) and the Haut-commissariat de la République en Polynésie Francaise (DTRT) (Protocole d’Accueil 2005–2018). This work represents a contribution of the MCR LTER Site. In Panama, data were obtained under NSF grant no. OCE-1535203, with permits from MiAmbiente. Participation by J.J.L. was also supported by the Center for International Collaboration at the Atmosphere Ocean Research Institute, The University of Tokyo. L.T.T. and R.B.A. were supported by NSF grant no. OCE-1535007. A.S.J.W. was partially supported by a Pathway-to-Position Fellowship from JSPS and L.T.T. was supported by the Coastal/Marine Hazards and Resources Program of the US Geological Survey. This is contribution no. 221 from the Institute for Global Ecology at the Florida Institute of Technology. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.

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The study was conceived and carried out by A.S.J.W. and J.J.L., who wrote the initial draft of the paper. All authors contributed to writing and editing subsequent drafts.

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Correspondence to Alex S. J. Wyatt.

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Wyatt, A.S.J., Leichter, J.J., Toth, L.T. et al. Heat accumulation on coral reefs mitigated by internal waves. Nat. Geosci. 13, 28–34 (2020). https://doi.org/10.1038/s41561-019-0486-4

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