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'Twilight Zone' Reefs Win a Conservation Spotlight

Oceans
Plates of leaf-like foliose corals are common on the Philippines' Benham Bank. Oceana / UPLB

By Allison Guy

In May 2016, technical divers descended 200 feet to Benham Bank, the shallowest portion of a huge underwater plateau off the Philippines' northeast tip. As they neared the bottom, an otherworldly landscape emerged from the dim cobalt blue.

Plates of coral grew one atop the other like china at a yard sale, dotted with sea fans and sprigs of algae. Coral columns encrusted in yellow, orange and pink coralline algae looked as though they'd been splashed with rainbow paint. Colonies thrived as far as the eye could see.


The divers, part of a scientific expedition to Benham, were shocked by the reef's health and size, said Gloria Estenzo Ramos, the head of Oceana in the Philippines. "Our country is at the center of the center of marine biodiversity, but there are almost no more reefs left that we can call pristine," she said. "At Benham Bank, we found a place that still has 100 percent coral cover."

Benham lies in the ocean's "twilight zone" between bright surface waters and the permanent night below. Only in the last decade has technology advanced enough to let scientists venture this deep. Recent discoveries reveal that these little-studied deep reefs are everywhere their sunlit cousins are—and that they're often bigger, healthier and home to species no longer common in the shallows.

Lovely, Dark and Deep

Almost 190 years ago, Charles Darwin dredged up tropical, reef-building corals from 420 feet deep. It's a strange place to find typically sun-loving organisms.

Tropical corals, the kind familiar to scuba divers, all need the sun to survive. Their tissues harbor tiny beneficial algae that convert sunlight into food. Life in the twilight zone's mesophotic depths—"meso" for half and "photic" for light—stretches this partnership to its limits. Deep-dwelling corals, like those at Benham, grow in plate and lettuce-leaf shapes to expand their surface area and capture as much sunlight as possible.

Corals aren't the only photosynthetic organisms in the mesophotic zone, a band between 100 and 490 feet deep. Halimeda algae, which look like strands of flat green beads, are common at these depths. For Celia Smith, a reef algae ecologist at the University of Hawai'i at Mānoa, light-dependent organisms are just one of mesophotic reefs' many mysteries.

"How does a photosynthetic organism function in extreme low light?" she asked. "How do they thrive? We're still at the very beginning of understanding how these ecosystems work."

Frisky Fish

On May 15, Philippine President Rodrigo Duterte put almost 190 square miles of Benham's reefs off-limits to all human activities except research. A further 1,160 square miles of the Philippine Rise, where Benham is situated, were closed to destructive forms of fishing. This decision safeguards Benham's delicate corals from bottom trawling and potential threats like mining, and will help preserve the healthy fish communities necessary for reef health.

Duterte's declaration was spurred by a campaign led by Oceana and joined by ally organizations. Oceana launched its conservation efforts soon after the May 2016 scientific expedition, where the team provided technological assistance. The campaign got a boost in December 2016, when the Convention on Biological Diversity declared Benham to be of "critical ecological importance."

Mesophotic reefs like Benham aren't just important habitats for corals and algae. Their intricate topography shelters fish and other animals as well. Some fish species are familiar from the shallows, while others, like a newly identified butterflyfish from the Philippines, are unique to the mesophotic zone.

A diverse collection of fish thrives on this 250-foot-deep reef in Hawaii.NOAA

These smaller fish attract bigger fish, like groupers, snappers and sharks, many of which are important for local food security and incomes. "When you protect these reefs, you get a huge benefit in terms of fisheries species build-up," said Tyler Smith, a coral reef ecologist at the University of the Virgin Islands in St. Thomas, who is not related to Celia Smith.

So far, more than 200 fish species have been documented at Benham. The spot is the world's only confirmed spawning ground for Pacific bluefin tuna, a lucrative species that's plummeted to just 3 percent of its pre-fishing abundance.

Safeguarding mesophotic spawning sites pays off, Tyler Smith explained. In the Virgin Islands, after 20 years of protection, endangered Nassau grouper are returning to mating spots once decimated by overfishing. "We're starting to see their numbers rebound, and you now see them not so uncommonly in shallow waters," he said. "They're slowly coming back to being a viable species."

Shelter From the Warm

Benham is just one of a vast network of mesophotic reefs that could benefit from protected status—and protection begins with simple knowledge of their existence.

When Smith and his colleagues surveyed the ocean southwest of St. Thomas in the U.S. Virgin Islands, they were surprised to find that a single mesophotic reef system was "more extensive and well-developed" than all the islands' shallow reefs combined, with potentially hundreds of millions of coral colonies. On top of this, he said, the deeper ecosystems harbored a species of boulder coral that's all but vanished from the Caribbean's decimated surface reefs.

St. Thomas isn't the only haven for rare coral. In Panama and the Galapagos, a species of fire coral is thought to vanish from the shallows during the unusual warmth of the El Niño weather phenomena, only to be re-seeded from deeper-living specimens.

Scientists now debate whether mesophotic reefs can shelter shallower species from climate change, like a Svalbard Seed Vault for corals. So far, the answer has been "it depends."

During the recent nightmare bleaching that wiped out one-third of Australia's Great Barrier Reef, some mesophotic corals appeared to fare better. But in other parts of the world, mesophotic reefs are more imperiled than shallow one. That's because twilight corals aren't acclimatized to the dramatic temperature swings that can batter reefs up-top. How well mesophotic reefs fare depends on local ocean conditions, like deep, cool currents, which can mean the difference between life and death.

Deep Thinking

Colorful creatures like this hawkfish enliven the "twilight zone." Oceana / Karl Hurwood

There's no debating that twilight zone reefs have one advantage on their side: distance from us. Living at a remove from humans helps insulate them from coral-killers like bottom dredging, coastal development and agricultural runoff. It's also tougher to fish at mesophotic depths, said Tyler Smith, so these reefs often host bigger and more abundant fish.

Tyler Smith's big fear is that fishing gear and other technologies will advance to the stage that mesophotic reefs will lose their built-in safeguards. This makes legal protections, like those on Benham, all the more vital.

We might not fully understand these reefs, said Celia Smith, the algae ecologist, but that's no excuse to ignore or damage them. "It's the precautionary principle," she said. "These reefs could have a significant role in the future."


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Last week, the Peruvian Palm Oil Producers' Association (JUNPALMA) promised to enter into an agreement for sustainable and deforestation-free palm oil production. The promise was secured by the U.S. based National Wildlife Federation (NWF) in collaboration with the local government, growers and the independent conservation organization Sociedad Peruana de Ecodesarrollo.

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The rallying cry to build it again and to build it better than before is inspiring after a natural disaster, but it may not be the best course of action, according to new research published in the journal Science.

"Faced with global warming, rising sea levels, and the climate-related extremes they intensify, the question is no longer whether some communities will retreat—moving people and assets out of harm's way—but why, where, when, and how they will retreat," the study begins.

The researchers suggest that it is time to rethink retreat, which is often seen as a last resort and a sign of weakness. Instead, it should be seen as the smart option and an opportunity to build new communities.

"We propose a reconceptualization of retreat as a suite of adaptation options that are both strategic and managed," the paper states. "Strategy integrates retreat into long-term development goals and identifies why retreat should occur and, in doing so, influences where and when."

The billions of dollars spent to rebuild the Jersey Shore and to create dunes to protect from future storms after Superstorm Sandy in 2012 may be a waste if sea level rise inundates the entire coastline.

"There's a definite rhetoric of, 'We're going to build it back better. We're going to win. We're going to beat this. Something technological is going to come and it's going to save us,'" said A.R. Siders, an assistant professor with the disaster research center at the University of Delaware and lead author of the paper, to the New York Times. "It's like, let's step back and think for a minute. You're in a fight with the ocean. You're fighting to hold the ocean in place. Maybe that's not the battle we want to pick."

Rethinking retreat could make it a strategic, efficient, and equitable way to adapt to the climate crisis, the study says.

Dr. Siders pointed out that it has happened before. She noted that in the 1970s, the small town of Soldiers Grove, Wisconsin moved itself out of the flood plain after one too many floods. The community found and reoriented the business district to take advantage of highway traffic and powered it entirely with solar energy, as the New York Times reported.

That's an important lesson now that rising sea levels pose a catastrophic risk around the world. Nearly 75 percent of the world's cities are along shorelines. In the U.S. alone coastline communities make up nearly 40 percent of the population— more than 123 million people, which is why Siders and her research team are so forthright about the urgency and the complexities of their findings, according to Harvard Magazine.

Some of those complexities include, coordinating moves across city, state or even international lines; cultural and social considerations like the importance of burial grounds or ancestral lands; reparations for losses or damage to historic practices; long-term social and psychological consequences; financial incentives that often contradict environmental imperatives; and the critical importance of managing retreat in a way that protects vulnerable and poor populations and that doesn't exacerbate past injustices, as Harvard Magazine reported.

If communities could practice strategic retreats, the study says, doing so would not only reduce the need for people to choose among bad options, but also improve their circumstances.

"It's a lot to think about," said Siders to Harvard Magazine. "And there are going to be hard choices. It will hurt—I mean, we have to get from here to some new future state, and that transition is going to be hard.…But the longer we put off making these decisions, the worse it will get, and the harder the decisions will become."

To help the transition, the paper recommends improved access to climate-hazard maps so communities can make informed choices about risk. And, the maps need to be improved and updated regularly, the paper said as the New York Times reported.


"It's not that everywhere should retreat," said Dr. Siders to the New York Times. "It's that retreat should be an option. It should be a real viable option on the table that some places will need to use."

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