2023 from the corals’ perspective: heatwaves and backlashes

In 2023, there have been reports on mass bleaching in, for instance, Central America, North America and the Caribbean. Confirmed events included reefs in Panama, Colombia, El Salvador, Costa Rica, Mexico, and Florida, as well as the Bahamas and Cuba alongside with several other Caribbean countries1.

In Florida, the bleaching started already in mid-July, with temperatures that were not only extremely high (32°C), but also occurred unusually early in the year. Towards the end of July, a monitoring buoy in Manatee in the Florida Keys measured an ocean water temperature of more than 38°C! In the same time period, the reports on signs of coral bleaching from Central America, South America and the Caribbean also started to unfold. Overall, the global surface ocean temperatures reached record high values during this year’s northern hemisphere summer.

Reef-building corals live in symbiosis with microscopic algae that provide nutrition to the corals through their photosynthetic activity. These algae also provide the beautiful coloration of the corals.

Andrew Baker, a marine biology and ecology professor at the University of Miami, who was cited in the Guardian2 explained: “Unfortunately, that sort of special superpower that they have, which is this partnership with the algae, is also their Achilles heel, because the algae are very heat sensitive. And what happens when the corals get too hot is the algae, instead of producing food for the coral host, end up producing toxins and these toxins trigger a response out of the coral that causes the coral to try to get rid of the symbionts, the algae, as quickly as possible.” Hence, when exposed to prolonged heat, the corals loose both their coloration (they bleach) and the symbionts that provide them with nutrition.

Whereas corals take up only about 0.1 per cent of the ocean floor, they are the home of about a quarter of all marine species1. Many of the reef-building corals discussed here optimally prefer temperatures between 22 and 29°C2. Prolonged time-periods with heatwaves such as the ones described above are naturally very bad news.

Globally – and in consequence – there is a large number of alarming records on declining coral reefs. For instance, compared to the late 1970s, the Florida Keys’ healthy coral cover has declined with 90 per cent2. Another example comes from Dr Lorenzo Álvarez-Filip, a coral scientist at the National Autonomous University of Mexico, who is monitoring bleaching of corals at Puerto Morelos near Cancún. In 2023, he noted that corals started to bleach already in early June, which reportedly least three months earlier than had been observed previously. Cited in an article in the Guardian1, he said: “We have reports of bleaching from here to Belize – that’s [more than] 400km”.

Rising water temperatures are not the only reason to the dramatic decline of coral reefs, since ocean acidification, pollution, overfishing, storms and disease act in parallel. However, rising temperatures are impossible to fend off.

Alongside with firstly the high temperatures as such, and secondly their earlier occurrence, there is also a great risk stemming from the increased frequency of heatwaves. It is not impossible for coral reefs to recover from bleaching events, but they remain vulnerable and less functional for years. Frequent heatwaves reduce the recovery time, or as Dr Sean Connolly, of the Smithsonian Tropical Research Institute, explained to the Guardian: “We are seeing a world where these kinds of temperature extremes are happening so frequently that it’s going to become increasingly hard for reefs to recover in the time periods in between” 1.

The challenges in terms of reef-recovery were recently evidenced following an annual survey of corals in the Great Barrier Reef, which has suffered from four mass bleaching events in the past seven years. A period of recovery, with three years of relatively healthy conditions, was found to have been paused due to bleaching, attacks by starfish, and disease. Dr Mike Emslie from the Australian Institute of Marine Science commented on this backlash to the Guardian and concluded that “… even less-severe bleaching events are enough to cause a pause in coral cover” 3, hence highlighting the problems associated with reduced resilience of coral reefs following previous bleaching events, and with new events unfolding before the reefs have regained strength.

With heatwaves becoming both more common and setting off earlier, attempts to rescue corals increasingly include the collection of key genotypes of corals from their natural nurseries and relocating them into land-based holding systems in order to preserve the species and their genotypes, as well as to allow for restoration efforts. For instance, several such projects are ongoing in Florida. Phanor Montoya-Maya from the Coral Restoration Foundation (CRF) described to the Guardian that such efforts “…are potentially the last lifeline” and continued that “Given the severity of this event, our immediate focus is on the rescue mission at hand. We are concentrating on preserving and protecting as much of the genetic diversity of our reefs as possible” 2.

Montoya-Maya’s statements were echoed by CRF’s chief executive Scott Winters: “There is still time to intervene, but our window for action is rapidly closing. To save coral reefs, we must mitigate the effects of climate change, preserve the corals and coral reefs that we have, and engage in restoration to rebuild remnant populations and their genetic diversity so that coral reefs can evolve to meet changing climate conditions”.

Marko Reinikainen

This article is a compilation based on interpretations of the sources below. Possible misinterpretations are the sole responsibility of the author of this article.

1 https://www.theguardian.com/environment/2023/aug/11/coral-bleaching-cent...

2 https://www.theguardian.com/environment/2023/aug/04/florida-coral-bleach...

3 https://www.theguardian.com/environment/2023/aug/09/recovery-of-great-ba...