While already threatened by ocean warming or acidification, coral reefs around the world are facing another adversity in the form of dead zones.

Hypoxia, or low oxygen levels, can create large dead zones that snuff out marine life and threaten dozens to hundreds of coral reefs worldwide, according to researchers from the Smithsonian Tropical Research Institute or STRI.

What Are Dead Zones?

Dead zones take place at the bottom of a body of water when there isn’t sufficient oxygen to sustain marine life. They form naturally but their number and severity have significantly increased in the recent decades, said lead study author Andrew Altieri.

They are worsened by eutrophication, or a density of nutrients, as well as sewage inputs in coastal waters. This leads phytoplankton blooms and plankton to die, decompose, and draw off oxygen in the process.

"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," the STRI scientist said in a statement.

Fortunately, while warming acidification demands large-scale solutions, dead zones are usually a localized issue and can be reduced through controlling sewage and agricultural runoff into the ocean, Altieri added.

The study focused on the huge, semi-enclosed Almirante Bay, located around 174 square miles in the Bocas del Toro province in Panama. When they saw a massive coral reef die-off in the area in September 2010, the team suspected that a dead zone instead of warm or acidic ocean water could be the culprit.

After measuring water quality and going through thick bacterial slime and dead marine animal bodies on the ocean floor, the researchers found extremely low oxygen levels in greater depths while there was still high oxygen in shallow waters where corals were healthy — a symptom of a dead zone.

Underreported Worldwide

The findings, Altieri said, could apply to coral reefs globally, as dead zones may be commonly occurring in the tropics but have gone largely undocumented since scientists didn’t look for them. Another problem is that research institutions from tropical areas tend to suffer from poor funding compared with those in temperate regions.

Dead zones may be underreported “by an order of magnitude,” warned co-author Nancy Knowlton, estimating that there are likely 10 for every 1 dead zone in the tropics.

But the team saw that some coral species can fare better with low oxygen than others. Stephanocoenia intersepta, for instance, usually manages to survive in the Almirante Bay dead zone while others have died out. This species, though, is not a branching coral and does not offer an ideal habitat for others.

Warming temperatures also play a role, since coastal environments become more vulnerable to hypoxia amid warming climate.

Altieri insists on delving deeper into the tropics, proposing that at least 370 dead zones in the tropics remain undocumented.

The findings were discussed in the journal PNAS.

Great Suffering

The Great Barrier Reef, while still recovering from the massive coral bleaching that ravaged 400 miles of its northern regions in 2016, is still facing an “elevated and imminent risk” of another widespread bleaching this year.

Authorities are closely watching the accumulated heat stress it is demonstrating just like in the same period last year, when it experienced the worst bleaching incident.

Experts have proposed different measures to mitigate bleaching, including addressing overfishing and pollution.

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