Extreme sea temperatures: Corals around the world are in danger of bleaching

One consequence of climate change is the loss of coral reefs. According to an expert, development could accelerate this year.

The global sea temperature has been extremely high for almost half a year. For many coral reefs – especially in the Atlantic Ocean – the consequences are becoming increasingly noticeable. Reefs in the Caribbean and the Gulf of Mexico are currently being hit hard, Christian Wild, head of the marine ecology working group at the University of Bremen, told the German news agency DPA. The water temperatures are currently extremely high there.

“Some corals there have been bleaching since June and some corals will die as the bleaching continues.” Bleaching is also expected for many other reefs in the Indian and Pacific Oceans in the coming weeks and months.

Hard corals are more like plants, but they are so-called cnidarians. Bleaching is the bleaching of the often colorful corals. They live with differently colored algae in a mutually beneficial community. However, at high temperatures, the corals repel the algae – and lose their color as a result. They stop growing and are less able to defend themselves against enemies and competitors. If the microalgae return within a certain amount of time as water temperatures cool again, the coral can recover – otherwise it will die.

“In recent decades, the frequency of coral bleaching has increased dramatically, giving corals less and less recovery time,” Wild explained. Conditions are particularly bad this year: according to the American platform ‘Climate Reanalyzer’, the surface of the oceans has been showing global record temperatures for the month in question since March. In April and for some time now, daily values ​​of 21.1 degrees were recorded – this has never been the case in the 40 years of registration. Regionally, water temperatures can be much higher.

An extreme with consequences: in the Gulf of Mexico and the Caribbean, coral bleaching started two months earlier than normal this year. “The bleaching is currently expanding and will certainly continue for several weeks to months until temperatures cool enough,” Wild emphasizes. It normally only takes a few days for corals to begin bleaching as water temperatures slowly rise above their species-specific comfort zone. “For most coral species, bleaching starts at about 32 degrees.”

In addition to the warming of the seawater, acidification and factors such as overfertilization and overfishing also play a role in coral mortality, says Wild. Certain measures are crucial to support the Cnidarians: “Stop or at least severely mitigate climate change and strengthen reefs by greatly reducing other stressors such as overfishing and overfertilization”. In addition, scientific approaches such as the targeted introduction of heat-tolerant hard corals and modern reforestation methods are helpful.

According to a study recently published in the journal Nature, global warming could also lead to a regional shortage of coral nutrients. “Warmer surface waters are less able to absorb nutrients from deeper water layers,” explains co-author Cecilia D’Angelo of the University of Southampton. “The reduced water productivity could lead to less nutrients for the symbionts and thus less food for the coral animals.” The analysis suggests that while coral animals can survive brief periods of starvation by feeding on their symbionts, at least some coral reefs are at risk of starvation due to continued nutrient depletion.

(yam/sda/dpa)