Hope for the Great Barrier Reef – Knowledge

The Great Barrier Reef (GBR) off the northeast coast of Australia is the largest and probably the best-studied coral reef in the world. The findings are usually frustrating: year after year, scientists have to watch as the GBR, which actually consists of more than 2,900 individual reefs, continues to bleach and slowly die.

But now there is finally good news about the reef, which is considered one of the seven natural wonders of the world: Researchers from the University of Exeter and the University of Queensland have discovered that there are areas on the Great Barrier Reef in water depths of 30 to 50 meters , where the corals are probably protected even during marine heat waves. “These areas could be a refuge under certain conditions,” write the scientists in the current issue of the journal PNAS.

Corals are extremely sensitive to heat and are therefore, according to most experts, among the first victims of climate change. Global warming is also causing the water in the oceans to warm up and so-called marine heat waves are becoming more frequent. The corals react to this by shedding so-called zooxanthellae that live inside them. These small, colorful algae carry out photosynthesis and can thus supply the corals with important nutrients. At water temperatures above 30 degrees Celsius, the algae lose this ability, so that the corals reject their useless tenants and bleach them. Mass bleaching is currently occurring on the Great Barrier Reef, the fifth in the last eight years.

The corals can survive in this state for a few weeks. If the water cools down in time, the lodgers move in again, the corals become colorful again and survive. If the heat wave lasts too long, they die. According to the World Biodiversity Council (IPBES), 99 percent of all coral reefs could be gone by the end of the century – and with them many of the marine creatures that live in the reefs.

According to the current study, the heat does not penetrate into certain, deeper areas of the Great Barrier Reef. Most studies in which researchers try to predict the fate of corals as climate change progresses and temperatures rise only take into account the increase in temperature on the water surface, the authors write in PNAS. This does not take into account the “stratification” in the water column, i.e. the phenomenon that the upper layer of water is warmer than the lower layer.

Some coral species still don’t stand a chance

To compensate for this shortcoming, the researchers incorporated the effect of stratification into common climate models and calculated the effects on areas of the Great Barrier Reef that are at a depth of between 30 and 50 meters. It found that “stratification isolates many offshore regions of the Great Barrier Reef from surface heat waves.” In other words: even if the water on the surface warms up significantly, it remains pleasantly cool further down. The reason for this is that climate change is increasing the effect of stratification, the researchers write in PNAS. “Stable stratification prevents vertical mixing and can thus reduce heat exchange.”

So will coral reefs survive climate change? “Our study is both a ray of hope and a warning,” said Jennifer McWorther, who led the study, according to a University of Exeter press release. There is hope that some corals may be able to survive the current stage of climate change. However, this does not apply to all species: “Some shallow water species are not found in deeper areas,” says McWorther. These species are therefore unprotected from the warming of the upper water layer.

In addition, according to the researchers’ calculations, the protective effect of stratification has its limits: it breaks down when the temperature rises by more than three degrees compared to pre-industrial levels. By then, the lower parts of the Great Barrier Reef will also bleach and die.

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