Coral reefs are stressed out, and for good reason. Threats to corals are wide and varying. From increasing sea surface temperatures and sea level rise to ocean acidification, the coral struggle is real. And the biggest threat to the reefs right now are shifts in biophysical conditions, which often result in the corals being bleached.
Facing these stressors, there are two possible states for Anthropocene coral reefs, which exist today. Either they will fall apart—bleached, wracked by storms, drowned by sediment, acidified and cooked with steadily increasing ocean temperatures—or, they will reinvent themselves.
Photo: Chris Roelfsema
If they reinvent themselves, the individual coral animals, octopi, fish and starfish alike will fly apart, redistributing themselves across the globe as biophysical conditions shift under their feet. Coral animals that have symbioses with temperature-sensitive zooxanthellae algae will retreat from warmer waters. But rather than leaving behind a wasteland of rubble, they will be replaced by different coral species, or coral animals that share mutualisms with different, more temperature tolerant zooxanthellae algae.
In other words: some species will win and some will lose. There will still be coral reefs, they will just be radically different. Ecologists call these “novel” ecosystems.
But is it possible for coral to achieve such a “novel” feat? While most coral researchers agree that there are occasional winners among coral, fish and other species that make up the extraordinarily diverse and economically vital coral reef communities, the fact is: most species lose, and reef communities become more simple and less resilient as a result.
Photo: Chris Roelfsema
To speculate what novel coral reefs might look like, it is useful to consider ancient reefs that existed under dramatically different global climates. While the fossil record obscures our understanding of many of the ecological dynamics, it is clear that coral communities have existed under radically different environmental conditions in the past, even those with far higher average sea surface temperatures and atmospheric carbon dioxide levels brought on by today’s climate change.
In the middle Eocene epoch—around 40 million years ago—Earth lacked polar ice. Globally, sea level was more than 100 meters higher than it is today, and CO2 levels were likely in excess of 1,000 ppm. Yet, in the Caribbean, coral reef communities were equally diverse to those that thrive in the Caribbean today. Considering genera of reef-building coral species, these Eocene reefs were 31 percent similar to modern reefs. Some of them even included Acropora—a genus that is today under an extreme threat of bleaching, with 10 species globally listed as threatened.
As time passed, coral communities shifted in response to changes in climate and ocean circulation. Earth entered a prolonged period of cooling, and sea level dropped as polar ice re-appeared. Coral reefs incurred successive peaks of community assembly, driven by new genera and species arriving in the Caribbean. But there were also periods of marked extinction, most prominently during the last several million years, as the Earth has entered a period of successive glacial advances and retreats, which have caused sea levels to oscillate around ~100,000 year intervals. Biogeography played an outsize role: the relatively recent closure of the Panamanian Isthmus (about 3.5 million years ago), reduced dispersal from the Pacific Ocean and may have had a negative effect on reef biodiversity throughout the Caribbean.
There are also modern reef communities that appear to be tolerant of higher temperatures. The Red Sea and Arabian Gulf have recently emerged as having strains of zooxanthellae algae that are more tolerant of higher temperatures, giving their symbiotic coral partners greater resistance to changing climate. And there was a 2015 report describing a new species of zooxanthellae, Symbiodinium thermophilum in the Arabian Gulf, where corals survive with annual maximum temperatures as high as 34 to 36 degrees celsius.
Photo: Chris Roelfsema
If the evolutionary history and current geographic distributions of coral reefs suggest that certain reefs can, in principle, exist in a high carbon dioxide world, the prospects for modern reefs to re-organize to withstand such conditions is not encouraging.
If the majority of the world’s coral reefs enter a phase of chronic bleaching—as is now expected based on empirical trends—they will enter a corresponding period of community disassembly. While novel reef systems may emerge under high temperatures and carbon dioxide concentrations, such systems may require decades, centuries, or even longer to proliferate.
In 2018, researchers examined changes in coral communities following successive bleaching events on the Great Barrier Reef in 2016. They found that after bleaching reached severe levels (defined as more than 6 degrees celsius), coral communities fractured, with widespread coral mortality and dramatic reductions in coral cover. Faster-growing species of coral were preferentially extirpated from the Great Barrier Reef, leaving behind depauperate assemblages dominated largely by slower-growing species.
Fish communities around bleached reefs similarly undergo massive shifts in composition. As reefs bleached, their fish communities became more similar, with fish that were generalized consumers of algae (which often increases after bleaching) becoming more abundant, according to another 2018 study. And in 2011, researchers exploited unique, naturally occurring carbon dioxide vents in the Mediterranean to examine the effects of elevated CO2(and associated increases in ocean acidity) on coral reef communities. Wth reefs that had been bleached, researchers found reduced coral diversity, and shifts in coral communities toward slower growing species. Coral cover was a bright spot: it remained consistent, although at far lower structural and functional complexity.
All that said, there’s a light shining on the seemingly ominous waters of coral’s future.
The Allen Coral Atlas—in partnership with the satellite imagery and data company Planet—has established the first high-resolution map of coral reefs around the globe.
With access to the Allen Coral Atlas online, reef scientists will have a better understanding of the extent and condition of the world’s coral reefs, allowing them to improve analyses of how reefs are responding to climate change. Over time, we may gain a better understanding of what novel coral reefs will look like in the future, or if they will exist at all.
Moving forward, the Atlas partners are developing analytics to reveal changes that occur on reefs experiencing biophysical shifts. In the future, the Allen Coral Atlas will attempt to detect changes in coral reefs—most importantly bleaching of reefs.