Healthy coral reefs, such as those found here in the Indonesian seas, are biodiversity hotspots; however, they are increasingly exposed to stressors such as heat and cold events, which could be further exacerbated by climate change.
Photo Credit: © Takaaki K. Watanabe, Kiel University
Scientific Frontline: Extended "At a Glance" Summary: Extreme Cold-Induced Coral Bleaching
The Core Concept: Extreme cold water events in the ocean can trigger severe coral bleaching, rivaling the intensity and structural damage typically associated with marine heatwaves.
Key Distinction/Mechanism: While heat stress is often widespread and driven by phenomena like El Niño, cold stress is triggered by upwelling from a positive Indian Ocean Dipole. Although spatially limited, these cold events often achieve higher intensities and persist an average of 20 days longer than heatwaves, disrupting the coral-algae symbiosis when temperatures deviate by at least 1 degree Celsius.
Major Frameworks/Components:
- Symbiotic Disruption: The biological mechanism where corals expel photosynthetic, nutrient-providing single-celled algae in response to acute temperature deviations, leading to starvation.
- Positive Indian Ocean Dipole: A climatic framework responsible for driving cold deep water to the ocean surface, primarily affecting the coasts of Sumatra and Java.
- Compound Climate Events: The compounding stress of sequential climate anomalies, such as a strong El Niño followed by a negative Indian Ocean Dipole, which intensifies overall reef stress.
- Thermal Refuges: Oceanographic zones protected by complex currents (e.g., the Karimata and Makassar Straits) that buffer against temperature extremes and act as coral larvae reservoirs.
Branch of Science: Marine Biology, Climatology, Oceanography.
Future Application: The identification and targeted conservation of thermal refuges to supply coral larvae for the natural repopulation and restoration of severely damaged reef ecosystems across the archipelago.
Why It Matters: This research mandates a fundamental reassessment of coral stress factors, demonstrating that previously assumed climate refuges may still be critically vulnerable to overlooked cold-water bleaching, thereby complicating global reef conservation strategies.
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| Corals bleach and die when exposed to unusual stresses affecting ecosystems over extended periods of time. Researchers at Kiel University have now systematically investigated this phenomenon in the Indian Ocean. Photo Credit: © Saori Ito, Kiel University |
The Indonesian seas are a biodiversity hotspot, harboring the highest coral diversity in the tropics and home to an extraordinary variety of marine life. Yet these unique ecosystems have been under growing pressure for years, particularly from increasing ocean heat waves. However, heat stress is not the only stressor affecting these sensitive marine creatures. A new study led by researchers at Kiel University (CAU), the Kikai Institute for Coral Reef Sciences (Japan), and the Indonesian Research Institute (BRIN) shows that extreme cold events can also cause coral bleaching. These events are often more intense than heat waves and rival the negative records set by them. For the first time, researchers have systematically investigated the consequences of both extremes for Indonesia’s coral reefs over several decades. Their findings were published in the journal Geophysical Research Letters.
“Our findings show that we need to reassess stress factors affecting corals. Rising temperatures are not the only threat; unusually cold water can also cause massive damage to coral reefs,” explains lead author Dr. Takaaki K. Watanabe of the Institute of Geosciences at Kiel University.
Cold as a Previously Underestimated Stress Factor
In their new study, the international research team analyzed both heat and cold stress in Indonesian waters over the past 40 years. Heat stress events, triggered by El Niño, increased significantly during the study period and affected larger ocean areas. Cold stress, on the other hand, occurs primarily along the coasts of Sumatra and Java when a positive Indian Ocean Dipole brings cold deep water to the surface. Although these cold events are more spatially limited, they often reach higher intensities and last, on average, about 20 days longer than heat events.
Corals Are Sensitive to Temperature Fluctuations
Tropical coral reefs are the ecosystems most severely affected by rising temperatures. Corals live in symbiosis with single-celled algae, which provide them with nutrients through photosynthesis and give them their characteristic coloration. If the water temperature deviates by at least 1°C from the average summer temperature for several weeks, the corals expel these algae. The result is bleaching: without their algae, the corals starve and die.
However, unusually cold water also has consequences for the sensitive ecosystems in Indonesia’s seas. The cold stress levels identified in the study reached intensities comparable to the most extreme heat events documented to date—including the one off the coast of Florida in 2023, which led to the functional extinction of certain coral species there. “What is surprising is that these extreme cold events are nearly equivalent in intensity to extreme heat waves. They occur in regions that were previously considered refuges, as the bleaching caused by cold water has largely been overlooked until now,” says Professor Miriam Pfeiffer, head of the Paleontology and Historical Geology research group and spokesperson for the DFG Priority Program Tropical Climate Variability and Coral Reefs (SPP 2299).
Climate Change Amplifies Impacts on Coral Reefs
Tropical coral reefs suffer particularly in years when multiple climate phenomena occur in succession. Strong El Niño events, followed by a negative Indian Ocean Dipole, can significantly prolong and intensify periods of heat stress. Such combinations have already been linked to widespread coral bleaching, most recently in 2016—the worst heat stress event in the region since records began. Global climate change further exacerbates this dynamic. Previous studies have shown that the tipping point for coral reefs has already been exceeded at 1.5°C of global warming. “We don’t yet know how cold stress events will develop in the future. To assess whether they will decrease or intensify, we need even more reliable long-term data,” says Watanabe, who is also conducting research in the priority program.
Protected Regions for Coral Reef Conservation
In addition to the risks, the study shows that regions such as the Karimata and Makassar Straits are comparatively well protected from extreme temperature fluctuations due to their complex ocean currents. These straits in the Indonesian archipelago could play a key role in the recovery of damaged reefs as thermal refuges. They serve not only as refuges but also as sources of coral larvae, which could contribute to the repopulation of endangered reefs throughout the region via ocean currents.
“These refuges are crucial for the preservation of these valuable ecosystems. Their targeted protection could help ensure that more severely affected reefs are repopulated and recover in the long term,” says Watanabe.
Published in journal: Geophysical Research Letters
Title: Coral Reefs in the Indonesian Seas Threatened by Heat and Cold Stress
Authors: Takaaki K. Watanabe, Saori Ito, Ayu Utami Nurhidayati, Sri Yudawati Cahyarini, and Miriam Pfeiffer
Source/Credit: Kiel University
Reference Number: mb051226_01