The study focuses on a coral reef in the Andaman Sea in the northeastern Indian Ocean that regularly experiences cooling from subsurface ocean waters driven by internal waves and changes in thermocline depth – the boundary separating warm surface water from cooler deeper water. These oceanographic processes can temporarily relieve heat stress on corals, potentially reducing the impacts of marine heatwaves. However, the research shows that this natural protection is far from constant. Instead, it varies with large-scale climate oscillations that shape conditions across the tropical Indo-Pacific.
Coral skeletons reveal the ocean’s hidden history
To understand how this natural cooling has changed over time, the researchers combined long-term subsurface temperature measurements with geochemical analyses of coral skeletons. Corals act as natural environmental archives, recording changes in seawater conditions as they grow.
By analysing strontium-to-calcium (Sr/Ca) ratios preserved within the skeleton, the team reconstructed past subsurface temperatures. They complemented this with measurements of carbon isotopes, which reveal how corals adjusted their metabolism during periods of heat stress. Together, these records allowed the scientists to reconstruct both the history of cooling events and the biological responses of the corals themselves.
Climate patterns determine the strength of natural refuges
One of the study’s key findings is that the cooling experienced at this reef refuge is strongly controlled by two major modes of tropical climate variability: the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). These climate phenomena influence regional wind patterns and the depth of the thermocline, ultimately determining how effectively cool subsurface water can reach shallow coral reefs.
The researchers found that the most pronounced cooling occurred during the exceptional 1997/1998 El Niño, which coincided with a strong positive Indian Ocean Dipole. During this period, the thermocline became unusually shallow, allowing internal waves to transport greater amounts of cool water onto the reef. Although this event triggered one of the most severe global coral bleaching episodes on record, the enhanced cooling substantially reduced thermal stress at the study site.
Corals change their strategy during heat stress
The chemical signatures preserved within the coral skeletons also reveal how corals cope with heat stress conditions. Under normal circumstances, reef-building corals rely primarily on microscopic algae living within their tissues for energy. During bleaching events, however, this partnership can break down.
The carbon isotope analyses suggest that, during many bleaching episodes, the corals compensated by increasing heterotrophy – that is, by capturing more food particles from the surrounding seawater. Interestingly, this shift was much less evident during the 1998 event, suggesting that the unusually strong natural cooling helped corals maintain their normal nutritional strategy despite widespread regional heat stress.
Why this matters for coral conservation
The findings highlight that coral refuges should not be viewed as permanently protected locations. Instead, their ability to buffer heat stress depends on the state of the climate system. As large-scale ocean-atmosphere circulation changes from year to year and decade to decade, so too does the effectiveness of these naturally cooled environments.
Understanding this dynamic relationship will help scientists better predict where corals are most likely to survive future marine heatwaves. It also provides valuable guidance for conservation planning, allowing managers to identify reef systems that are most likely to function as climate refuges under future warming.
A dynamic view of reef resilience
Rather than offering permanent shelter from climate change, naturally cooled reefs provide protection that waxes and wanes with tropical climate variability. By combining coral geochemistry with decades of oceanographic observations, this study demonstrates how processes operating across entire ocean basins can determine the fate of individual coral reefs.
As marine heatwaves become increasingly frequent, understanding when and where these natural refuges are most effective will be essential for protecting coral reef ecosystems in a warming world.
Research collaboration and support
This study was carried out within the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Priority Programme “Tropical Climate Variability & Coral Reefs” (SPP 2299). Led by MARUM researchers Hana Camelia and Thomas Felis, the study was conducted in close collaboration with researchers from Thailand, bringing together expertise in coral geochemistry, coral reef ecology, oceanography, and climate science to better understand how coral reefs respond to a changing climate.
Further Information:
DFG Priority Programme “Tropical Climate Variability & Coral Reefs” (SPP 2299): https://www.spp2299.tropicalclimatecorals.de/
MARUM Coral Paleoclimatology: https://www.marum.de/en/Coral-Paleoclimatology.html
Contact:
Dr. Hana Camelia
Coral Paleoclimatology
MARUM – Center for Marine Environmental Sciences, University of Bremen
Phone: +49 421 218-65758
Email: hcamelia@marum.de
Dr. Thomas Felis
Coral Paleoclimatology
MARUM – Center for Marine Environmental Sciences, University of Bremen
Phone: +49 421 218-65751
Email: tfelis@marum.de
Participating Organizations:
MARUM – Center for Marine Environmental Sciences, University of Bremen
The University of Western Australia
Chulalongkorn University, Thailand
Phuket Marine Biological Center, Thailand
GEOMAR Helmholtz Centre for Ocean Research Kiel
MARUM produces fundamental scientific knowledge about the role of the ocean and the ocean floor in the total Earth system. The dynamics of the ocean and the ocean floor significantly impact the entire Earth system through the interaction of geological, physical, biological and chemical processes. These influence both the climate and the global carbon cycle, and create unique biological systems. MARUM is committed to fundamental and unbiased research in the interests of society and the marine environment, and in accordance with the Sustainable Development Goals of the United Nations. It publishes its quality-assured scientific data and makes it publicly available. MARUM informs the public about new discoveries in the marine environment and provides practical knowledge through its dialogue with society. MARUM cooperates with commercial and industrial partners in accordance with its goal of protecting the marine environment.
Scientific Reports
Tropical climate modes control strength and distribution of thermal stress mitigation in a coral reef refugia.