Coral reefs are underwater ecosystems built by colonies of tiny animals called coral polyps. As biodiversity hotspots, their survival is delicately balanced with their environment. Water temperature is a foundational aspect of coral health, as the stability of the ocean’s temperature is directly linked to the vitality of these marine habitats.
Ideal Thermal Conditions for Coral Reefs
Most reef-building corals flourish in water temperatures between 23°C and 29°C (73°F and 84°F). The formation of large reefs is constrained to areas where the water temperature does not drop below 18°C (64°F) for long durations. This thermal range is dictated by the needs of microscopic algae called zooxanthellae.
These algae live symbiotically within the coral’s tissues, providing up to 90% of its nutritional needs through photosynthesis. The coral, in return, provides the algae with a protected environment. This relationship is efficient only within the specified temperature window, and temperature stability is often more impactful than the exact value within this range.
The Phenomenon of Coral Bleaching
When ocean temperatures rise for a prolonged period, the symbiotic relationship between corals and zooxanthellae breaks down. An increase of just 1°C above the usual summer maximum for a few weeks can trigger this stress response. Under thermal stress, the zooxanthellae produce reactive oxygen species that are toxic to the coral.
As a survival mechanism, the coral expels the toxic algae. Since zooxanthellae give coral its color, their expulsion leaves the coral’s tissues transparent, revealing the white calcium carbonate skeleton beneath. This process is known as coral bleaching. A bleached coral is not dead but is starving and more susceptible to disease.
If high water temperatures subside quickly, corals can recover by reacquiring zooxanthellae. However, if the thermal stress is prolonged or the temperature increase is too great, the coral will likely die. Mass bleaching events, where large areas of a reef bleach simultaneously, are becoming more common due to elevated ocean temperatures.
Effects of Cooler Waters on Coral Health
Temperatures that fall below the optimal range also pose a threat to coral health. When water becomes too cold, a coral’s metabolic activity slows, leading to reduced growth rates and impaired reproductive capacity.
Temperatures dipping below the 18°C (64°F) threshold inhibit the coral’s ability to secrete the calcium carbonate that forms its skeleton, which halts reef growth. Prolonged exposure to cold conditions can weaken corals, making them more vulnerable to disease.
Primary Drivers of Ocean Temperature Shifts
The primary cause of the sustained rise in global ocean temperatures is climate change, driven by the accumulation of greenhouse gases from human activities. The ocean has absorbed significant excess heat from global warming, leading to a steady increase in average sea surface temperatures. This trend makes marine ecosystems less tolerant to temperature fluctuations.
Marine heatwaves, which are prolonged periods of abnormally high ocean temperatures, are superimposed on this warming trend. These events have become more frequent and intense due to climate change. Natural climate patterns like the El Niño-Southern Oscillation (ENSO) also cause temporary regional warming. When these natural cycles occur on top of the warmer baseline, they can push temperatures beyond the threshold for coral survival and trigger mass bleaching.
Coral Adaptation and Varying Temperature Tolerances
Not all coral species respond to temperature changes in the same way, as some are naturally more robust. There is evidence of local adaptation, where coral populations in historically warmer waters, like the Red Sea, have evolved higher thermal tolerance than corals in cooler regions.
Deep-water corals are a different group, existing in colder, stable environments where temperatures can be as low as 4°C. These corals lack zooxanthellae and are adapted to different conditions, making them vulnerable to changes in deep-ocean temperatures.
Scientists are studying if shallow-water corals can adapt quickly enough to keep pace with ocean warming. While some adaptation is occurring, many researchers believe the pace of climate change is too rapid for natural evolution to prevent widespread coral loss. This has led to research into methods like selectively breeding more heat-tolerant corals for reef restoration.