Although coral reefs can recover from bleaching, those subjected to heat stress have a lower reproductive output, researchers have found. This means reefs may take longer to bounce back than previously assumed and are more vulnerable to future stressors.
Bleaching occurs when corals exposed to above-average ocean temperatures expel the symbiotic algae that live inside their tissues. These organisms give them their vibrant colours as well as providing most of their food via photosynthesis. Bleached coral remains alive and the symbionts may return if temperatures return to normal, but if it remains hot for prolonged periods, the coral dies from starvation.
The world is currently experiencing its fourth global coral bleaching event on record, the US National Oceanic Atmospheric Administration announced on 15 April. The Great Barrier Reef in Australia has been particularly affected due to record-level heat stress, with aerial surveys revealing mile-long stretches of stark white coral skeletons. Researchers have now explored how the colonies that survive bleaching will be fundamentally altered.
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In the aftermath of bleaching on the Great Barrier Reef in 2020, Nico Briggs at the Australian Institute of Marine Science and his colleagues followed the reef鈥檚 recovery and investigated the impact of heat stress on surviving colonies of Acropora millepora coral.
At the height of the event in April that year, 76 per cent of the corals they studied had a severe or catastrophic response, but nearly all had superficially recovered 鈥� regaining their algae and thus pigmentation 鈥� by October.
However, six months after the heatwave, the colonies experienced a 21 per cent decrease in egg production, a measure of their reproductive output, compared with previous years.
Earlier research has found that after a mass mortality event, a few fast-growing branching species tend to dominate the reef, while so-called boulder corals 鈥� which take decades to centuries to grow 鈥� are lost.
A decline in species diversity and lower reproductive rates are expected to make reefs less resilient and more susceptible to other stressors, such as disease and predation by crown-of-thorns starfish.
鈥淭ypical assessments of coral health after a bleaching event consist of surveys to determine how much coral lived or died, but these surveys miss longer-term impacts to health, such as impacts to reproduction and growth,鈥� says Briggs.
Coral Reefs
