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Coral reefs are incredibly diverse ocean environments, home to turtles, dolphins, and anemone as well as thousands of different species of fish. But they are under threat; the Great Barrier Reef, the largest of all coral reefs, experienced back-to-back bleaching events in 2016 and 2017, causing a third of the reef to die.
Coral reefs occupy less than a tenth of a percent of the seafloor but are home to a quarter of all known marine species. Corals build limestone homes in clear, warm, shallow waters of the tropics; each coral is made up of thousands of tiny living coral creatures called polyps. These polyps are fed by symbiotic photosynthesizing algae, which also gives the coral their bright and fascinating colors.
When healthy, corals are a biologically diverse and economically valuable ecosystem. They are a habitat and spawning/nursery ground for thousands of fish, they are a source of food, protect the coastline from storms and erosion and provide jobs and income for local economies.
When unhealthy, this diversity and economy are threatened. If coral reefs are not revived, coastal residents and properties are exposed to rising seas and stronger storms, and the number of fish severely diminishes.
Marine Heat Waves
Global warming and climate change are causing the ocean to warm; these marine heatwaves affect the behavior of the coral polyps. Scientists are unsure why, but elevated sea temperatures cause the coral polyps to expel their algae, leading to loss of color in what is known as a bleaching event, and ultimately death of the reef.
Australia’s independent climate communication organization has estimated that that large-scale bleaching event used to occur every 27 years along the Great Barrier Reef, but currently occur every six years. By 2030 this is estimated to be a biannual event and by the 2050s an annual occurrence.
Australia’s Great Barrier Reef is the world’s largest structure created by living organisms, an area of 2,900 individual reefs. In August 2017, scientists transplanted hundreds of nursery-grown coral fragments on to the reef in an attempt to replenish what had been lost. The idea is nothing new, it has been used on damaged reefs for decades, but this was the first time that human intervention was deemed necessary to ensure the survival of the reef.
Fragments of the surviving coral at Opal Reef were taken to a nearby sandy lagoon and encouraged then to grow on mesh platforms. Scientists chose 12 different species covering a range of different coral forms. After a few months of growth and stabilization, the fragments were transferred back to the reef. Scientists are hoping that propagation and outplanting of these stress-surviving corals will speed up the recovery process; however, how successful this has been won’t be known until another marine heatwave, which is likely to occur sooner rather than later.
Another technique employed is to seed the damaged area with millions of lab-raised coral larvae. Coral eggs and sperm had previously been collected and cultivated, before being returned to the wild. Scientists believe several have survived and continued to grow and regenerate the reef, although it will take about three years before this coral will be mature enough to reproduce.
But that’s not the only issue facing coral reefs; as carbon dioxide levels increase, water acidity is also increasing to a level not experienced by coral before. Scientists have been searching for hot spots of resilience, spaces where coral has already adapted to extreme heat and acidity such as volcanic vents and submarine springs where carbon dioxide naturally bubbles through the seafloor.
Scientists have discovered some shallow water corals with an exceptional tolerance for heat and have been able to identify some of the genes responsible. Some coral also has heat-tolerant algae, which could be introduced into other corals to increase their bleaching resistance. There is however a trade-off to be made – although more resistant to bleaching, coral growth is also slower so it will contribute less to the reef’s recovery and diversity.
Corals are also abundant in mangroves, where the water is hotter than in open reefs and more acidic due to the presence of trees. Scientists have been able to transplant coral from mangroves to reefs further offshore and vice versa. While most coral polyps are fed by their algae and supplement their food at night be extending from their skeleton, polyps in the mangroves are also extended during the day suggesting that the metabolic demands of living in such harsh environments may require extended feeding activity. As life on open reefs becomes harsher, and increased feeding activity may become necessary.
Coral has the ability to recover – unfortunately just not at the same rate as which it is being destroyed by human activities. Therefore, we must find ways to boost their abundance, whether that be cultivating coral in nurseries and transplanting it back on to damaged areas of the reef, or discovering ways in which to boost the resilience of corals. Otherwise, the planet will lose these hotspots of diversity which act as barriers against the elements and are home to thousands of marine species.
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