Florida’s recent bleaching sees severe decline in elkhorn coral diversity
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Author: Mark ‘Crowley’ Russell
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During the summer of 2023, a marine heatwave surrounding Florida and the Gulf of Mexico caused widespread coral bleaching, part of the global mass bleaching event that occurred towards the tail end of 2023 and the first months of 2024.
NOAA scientists monitoring the mass bleaching event have reported that elkhorn coral has experienced a huge depletion in genetic biodiversity as a result of coral mortality caused by the bleaching.
The scientists say that at least 160 elkhorn coral genotypes – unique genetic makeups – were documented alive before the heatwave. By Autumn 2023, only 37 remained – a loss of 77 per cent of regional genotypic diversity. The remaining genotypes were found on just 16 reefs in the upper Florida Keys, Miami-Dade and Broward Counties.
Although the loss of genotypes in the wild is a blow to Florida’s reefs, there remains some hope that the populations may be restored as there are more than 180 elkhorn genotypes present in regional coral nurseries.
Elkhorn coral (Acropora palmata) is one of the most important reef-building corals in the Caribbean. Like other species of branching Acropora, it is relatively quick to grow but – as with similar fast-growing corals such as staghorn and table corals – they are far more susceptible to mortality as a result of bleaching.
Genetic diversity among coral is as important as it is among other forms of life. Genetically identical members of the same species are equally vulnerable to the same diseases and, in the case of coral, heat stress resulting in bleaching.
Coral bleaching occurs when elevated temperatures cause the coral’s symbiotic zooxanthellae – a form of algae known as dinoflagellates – to be ejected from its host. Zooxanthellae photosynthesise and provide the coral polyps with nutrients, and also colour – hence their loss reveals the white reef-building substrate secreted by the coral colonies through their naturally translucent polyps, giving them the appearance of being ‘bleached’.
Should temperatures return to normal, the coral will reacquire its zooxanthellae and survive. Prolonged periods of bleaching, however, can eventually cause the coral to die.
As a result of their individual genotypes, corals of the same species can be differently affected by the same heat stress. Where one colony might fail, another of a different genetic makeup may not even become bleached. Genetic diversity means that coral species have a better chance of survival over the longer term, with the more resilient able to survive and sustain the reef.
Elkhorn coral colonies are often formed when larger structures are broken. Providing the fragments remain in a sunlit position where the zooxanthellae can continue to photosynthesise and the coral polyps can reach into the water column to extract food supplies, the fragment will eventually grow into a new elkhorn colony.
This means, however, that a lot of elkhorn coral in a given area remains genetically identical, even through sexual reproduction with its close neighbours. Genetic diversity, and therefore sustainability, can only be increased by interbreeding with other genotypes, but as elkhorn colonies on the Florida reefs become further removed from each other, the ability to sexually reproduce with genetically distinct colonies through natural means becomes increasingly unlikely.
NOAA says its scientists and restoration managers are ‘working diligently to find ways to assist reproduction to ensure the population can sustain itself into the future.’
‘Producing new genotypes through assisted reproduction of unrelated members of the original, naturally occurring population could result in offspring that may be able to better survive future bleaching events,’ say the scientists.
‘This will require the use of wild and nursery genotypes. This can help coral restoration partners determine which genotypes are missing from nurseries and genebanks being used in coral restoration efforts.’
Scuba divers have an important part to play in the recovery of Florida Keys coral, and are being encouraged to report elkhorn colony locations to the NOAA elkhorn conservation team in case they are diving over a colony with a previously undocumented genotype.
‘We want people to know that there are many elkhorn coral outplants out there on the reefs as a result of restoration efforts,’ said Southeast Corals Recovery Coordinator, Jennifer Moore, ‘But we still welcome divers to report any sightings in case it’s something new. Every observation counts for this threatened species.’
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