Dead Zones Mark Latest Threat to Coral Reefs

Andrew Altieri surveys a reef littered with dead corals where over 90 percent died due to hypoxia. (Cindy Gonzalez/STRI)

(CN) – Dead zones that snuff out marine life may affect dozens of coral reefs around the world, and threaten hundreds more.

A new study released Monday by Smithsonian Institution scientists is the first to link these low-oxygen areas with coral reef die-offs, which researchers have previously attributed to ocean warming and acidification stemming from global climate change.

“Ocean warming and acidification are recognized global threats to reefs and require large-scale solutions, whereas the newly recognized threats to coral reefs caused by dead zones are more localized,” said lead author Andrew Altieri of the Smithsonian Tropical Research Institute in Panama.

“Fortunately dead zones can be reduced by controlling sewage and agricultural runoff into the ocean.”

After seeing coral reefs in Almirante Bay in Panama’s Bocas del Toro province showed severe signs of stress, Altieri and his team went to work in September 2010, measuring several aspects of water quality. This led the researchers to notice extremely low oxygen levels in deeper water and high oxygen levels in shallow waters, where the corals were still healthy.

In addition to varying oxygen levels at different water depths, the team found thick mats of bacterial slime, and sea urchins and sponges lay scattered on the ocean floor. The combination of these factors led the team to determine a dead zone was the culprit, rather than a warming event.

The researchers believe dead zones may be common in the tropics but have gone unreported because scientists never looked.

“The number of dead zones currently on our map of the world is 10 times higher in temperate areas than it is in the tropics, but many marine biologists work out of universities in Europe and North America and are more likely to find dead zones close to home,” Altieri said.

A living, vibrant coral reef.

The team identified 20 instances when dead zones were connected to the mass mortality of coral reefs worldwide.

“Hypoxia (low oxygen) isn’t even mentioned in several of the most important academic reviews of threats to coral reefs and is rarely discussed at scientific meetings,” Altieri said. “Even worse, many coral-reef monitoring efforts do not include measurement of oxygen levels, making it nearly impossible to identify low oxygen as the cause of mass coral mortality after the fact.”

The team pointed to the case of a 2016 coral reef die-off at the Flower Garden Reefs in the Gulf of Mexico. The reason for the die-off remains unclear, but some of the photographs look quite similar to what was observed in Panama.

Study co-author Nancy Knowlton of the Smithsonian’s National Museum of Natural History highlighted the potential extent of the threat of dead zones.

“Based on our analyses, we think dead zones may be underreported by an order of magnitude,” she said.

“For every one dead zone in the tropics, there are probably 10 – nine of which have yet to be identified.”

 

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