Coral reefs are hotspots of marine biodiversity, but global warming is causing more and more of them to die off. Previously, it was thought that only tropical shallow-water reefs would bleach due to rising temperatures, but apparently even deep-water corals are not safe from climate change, as researchers have now discovered. In the Indian Ocean they discovered bleached coral reefs even at a depth of 90 meters. This means that significantly more reefs could be threatened by climate change than previously assumed.
Although coral reefs make up less than one percent of the ocean’s surface, they are home to 30 percent of all known species of marine fish. But these biodiversity hotspots are at risk because they have very little tolerance for rising temperatures. If the sea water around them warms up too much, the single-celled, symbiotic algae that live inside the corals suddenly produce toxins instead of nutrients. The coral rejects it, bleaches and dies because it cannot survive without the algae.
Coral bleaching in the depths
Until now, it was assumed that this fate could only affect tropical shallow-water corals that live at a depth of two to 30 meters. Corals in medium water depths of up to 150 meters, on the other hand, were considered to be significantly more resistant to climate change because the water at these depths is significantly cooler. In addition, these corals live at the level of the so-called thermocline, in which the water temperatures relatively abruptly become several degrees cooler. This means they are used to changing water temperatures – or so it was thought.
But researchers led by Clara Diaz from the University of Plymouth have now discovered bleached reefs at a depth of 90 meters for the first time – deeper than ever before. The affected mesophotic coral reefs are located in the Chagos Archipelago in the central Indian Ocean, where the team had already detected them in 2019 using a special underwater robot. Curiously, the higher reefs in the same area showed no signs of coral bleaching at that time. At a depth of 60 to 90 meters, however, between 75 and almost 100 percent of the corals had died, as the team determined. These corals were similarly affected by coral bleaching as some areas of the Great Barrier Reef and other shallow-water coral reefs.
Weather phenomenon shifted water zones
In order to find out why in the Chagos Archipelago all previously assumed laws appear to be upside down, Diaz and her colleagues evaluated numerous other data that they had collected on their research trip. This included satellite information as well as information on sea conditions and water temperature. The result: Apparently the temperature of the deep water layer at the level of the mesophotic coral reefs rose from 22 to 29 degrees within a very short time in 2019 – a huge jump even for temperature-tolerant deep-water corals. During the same period, the temperature of the sea surface near which the shallow-water corals grew had hardly changed, as Diaz and her colleagues report.
How is that possible? The team believes that the thermocline shifted more strongly and persistently than normal this year. As a result, the mesophotic corals were now well above this boundary layer and were therefore exclusively surrounded by warm water. They faded. According to the researchers’ records, the trigger for this thermocline jump was the so-called Indian Ocean Dipole (IOD), a weather phenomenon similar to the Pacific El Niño. The IOD strengthened easterly winds across the eastern and central Indian Ocean, Diaz and her colleagues report. Together with persistent westerly flow, they shifted the thermocline significantly downward in just a few months.
But apparently these changes didn’t last. When the researchers returned for further expeditions in the following years, the deep-water reefs had recovered again. Apparently the water temperature had returned to normal in the meantime and the symbiotic algae had returned to the corals just in time. Despite this happy ending, there is still no reason to give the all-clear, as Diaz explains: “In the future, bleaching in the depths of the ocean here and elsewhere will probably occur more regularly.” Because climate change is increasing weather phenomena such as IOD and El Niño and thus favoring thermocline formation. Shifts. Against this background, significantly more coral reefs are in danger in the long term than previously assumed.
Source: University of Plymouth; Specialist article: Nature Communications, doi: 10.1038/s41467-023-42279-2