Corals sustain for some of the most diverse marine wildlife on the planet, and unfortunately rising sea temperatures, driven by climate change, are threatening coral reefs around the world through a phenomenon known as ‘coral bleaching’.
Marked by a whitening in colour, coral bleaching starves the coral reefs of their predominant source of food, algae. In some cases, corals can recuperate from bleaching, but a recent study has found an increased frequency of bleaching events is not giving them a chance to do so.
As temperatures rise, massive coral bleaching events have become much more frequent, with the period between events decreasing fivefold since the 1980s.
Small, fast-growing species of coral mature much quicker than bigger coral structures, which can take decades to recover from a bleaching event. And yet, even the fastest developing corals still need at least a decade to regrow after a bleaching event. However, the reefs aren't getting that amount of time.
In the past, reefs endured bleaching events every 25 to 30 years, but since 2010, the gap between events has contracted to six years, according to a study published in January 2018. The study included a recent large bleaching event that was the first to last greater than 12 months.
Corals have symbiotic algae that becomes toxic when temperatures become too warm. When this happens, the corals eject their algae. In the process, cells that were taking in nutrients from the algae are ripped out. Ejecting toxic algae that was a food source leaves the corals injured and starving. Without time to recuperate, bleached out corals leave behind a barren region that had been host to a wide variety of marine life.
In the past, bleaching events happened regionally during El Niño events. This phenomenon is a part of the natural El Niño-Southern Oscillation climate cycle. However, sea temperatures have been rising during La Niña, El Niño’s cooler equivalent. According to the new study, La Niña events today are actually warmer than El Niño events from three decades ago. This change means that corals are no longer secure during what used to be a regular recovery period.
Are Corals Adapting?
While the increasing frequency of mass bleaching events is grim news for coral reefs around the world, another recent study has indicated that corals may be adapting to rising average water temperatures.
Published in August 2018, the study duplicated trials on coral from the 1970s and discovered coral's ability to withstand warmer ocean waters may have heightened drastically.
When the original study was carried out, the researchers modelled rising temperatures and discovered that coral had a small temperature tolerance, able to endure a 1 to 2 degrees Celsius rise above the normal maximum. Although temperatures vary across the oceans, the majority of coral species would start to bleach with that small rise in temperature. Only up to 40 percent of the coral in the various trials were capable of tolerating the warmer waters.
In the new study, about five decades after the original, the scientists exposed coral to temperatures similar to those in the original study for one month. Then, the coral was given 28 days to recuperate. A surprising 60 to 90 percent of the coral made it through.
The coral was capable of improving its capacity to tolerate warmer waters either because the algae adapted or because natural selection has led to the most temperature-tolerant corals surviving within the last few half-century, the study scientists said.
Despite the promising outcome of the new study, the team behind it said this level of resilience isn't sufficient to ensure the survival of coral reefs.