A Dissolving Future for Coral Reefs

The latest findings suggest that ocean acidification by carbon dioxide may already be eroding reefs worldwide, and provide a crucial proof for a novel monitoring system tracking erosion on U.S. reefs.

One of NOAA’s Bioerosion Accretion Replicates freshly bolted onto a reef in PNG’s Milne Bay. Image Credit: Katharina Fabricius

To dive into the bubble-filled waters of Papua New Guinea’s Milne Bay is to enter a time machine to the oceans’ climate changed future. The bubbles — pure carbon dioxide (CO2) streaming up from volcanic activity below the seabed in southeastern Papua New Guinea (PNG) — acidify portions of its coral reefs, providing a glimpse at where coral ecosystems worldwide may be headed as carbon emissions acidify oceans over the next century.

The latest findings from this natural laboratory suggest that — barring a radical reduction in CO2 pollution from cars, power plants and other human sources — many reefs will literally dissolve away. “What we learned is that erosion is accelerated under high carbon dioxide conditions. It doesn’t paint a good picture for the persistence of reefs,” according to Katharina Fabricius, the Australian Institute of Marine Science coral reef ecologist who coauthored the work (and has led nine scientific expeditions to Milne Bay’s natural laboratory since 2002).

The findings from Milne Bay, published in November 2016 by a journal of the U.K.’s Royal Society, also verified a novel monitoring system to track the corrosive impact of carbon emissions on reefs launched by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 2014. That program delivered its first erosion data earlier this year, according to Ian Enochs, the scientist at NOAA and the University of Miami who designed the methodology and is the lead author on the Royal Society report.

Most research on coral reef health and climate change focuses on the living coral adorning a reef’s surface. As atmospheric CO2 dissolves into the ocean it makes the water more acidic, impairing coral and other shelled animals’ abilities to build their limestone exoskeletons. At the same time, global warming induced by atmospheric CO2 is raising sea temperatures and causing coral to expel nutrient-providing symbiotic algae  — a potentially fatal condition known as coral bleaching. Warm waters last year across a previously pristine 700-kilometer northern stretch of Australia’s Great Barrier Reef inflicted the worst coral die-off ever reported.

The latest work from PNG, however, tracks the fate of dead coral — the skeletal remains that gather over centuries and form the solid carbonate mass that defines a reef’s architecture and underpins all that grows upon it. The life of a coral reef — from plants to plankton and from fish and larger animals — depends on that three-dimensional structure. “It all kind of congregates around that framework,” says Enochs.

One of NOAA’s Bioerosion Accretion Replicates colonized by reef flora and fauna after two years. Image Credit: Katharina Fabricius

Laboratory studies by Enochs and other scientists strongly suggest that ocean acidification will impact reef ecosystems by slowing the accretion of new carbonate by living coral and accelerating the breakdown of dead coral by a variety of reef flora and fauna. The latter process has been primarily studied under artificial laboratory conditions, usually observing one or a few species of these “bio-eroders” in isolation and generally only at one stage of their lives. Enochs’ new methodology provides a standardized system to measure “accretion” and erosion under real-world conditions, where ecosystem-level feedbacks can nullify some agents of change and dangerously magnify others.

The method begins with blocks of clean solid coral, about the size of a pack of chewing gum, that are glued to a plastic base. These BARs — Bioerosion Accretion Replicates — are bolted to a reef and left to absorb its full range of natural influences for two-three years. Upon retrieval, BARs go through a battery of analyses. Incubations provide insight into what is living on them, while computerized tomography (CT) scans akin to those performed in hospitals reveal in high-resolution 3-D images how the BARs’ structure has evolved inside and out.

For the PNG study, BARs were installed at reef sites at varying distances from seeping CO2 and thus subject to different levels of acidity. Harvested nearly two years later, they revealed several surprises. One is that acidification accelerated erosion more than it slowed new coral growth. Even more unexpected was the leading agent of erosion: worms, which are stimulated by acidified water.

Sponges and a variety of microorganisms have proved the most efficient bioerosion agents in laboratory experiments on ocean acidification, but never worms according to Enochs. Yet the CT scans of Milne Bay BARs by Enochs’ lab showed that worms were the only bioeroders whose activity correlated with acidity. The closer BARs had been bolted to the CO2 vents, the more wormhole-ridden they became.

CT scan of a bioeroded BAR harvested from Milne Bay, showing surface growth in green and wormholes in red. Image Credit: Ian Enochs/NOAA

Annelids are ubiquitous residents of reefs worldwide, and they are well known natural eroders that chew and secrete dissolving acids to tunnel through solid coral. News that acidification stimulates them to bore faster and deeper is distinctly bad for reefs, according to Enochs. “Seeing that they are responding to ocean acidification has large implications that we didn’t previously know,” he says.

According to Fabricius, one such implication is a stronger likelihood that acidification will reduce reef biodiversity. The corals most susceptible to worm tunneling are likely those with fragile branches and fans, which form a 3-D web within which flora and fauna feed, mate and hide from predators. As they break down, reducing the complexity of the landscape, biodiversity also suffers, says Fabricius.

Bioerosion may explain why simpler boulder-shaped corals come to predominate the most acidified reefs at Milne Bay, which support a small fraction of the biodiversity thriving just a few hundred meters away. Swimming into these comparative moonscapes from neighboring rainforests of biodiversity is “depressing,” says Fabricius.

“Shocking” is how Enochs puts it. He recently encountered a similarly seep-affected reef in the Marianas Islands. “You go from a beautiful coral reef to a place you’re not at all interested to dive in. It looks like you’re diving on a bunch of rocks with seaweed. The ecosystem is completely altered,” says Enochs.

Habitat degradation could be further accelerated by climate change, warns Fabricius, as all oceans become more acidic and reefs weakened by bioerosion succumb to the stronger cyclones and other storms that global climate change is projected to deliver in decades ahead.

Sites where BARs will be used to monitor coral reef bioerosion. Image Credit: NOAA

These threats may already be changing reefs. Ocean pH (acidity) has decreased by 0.11 pH since the Industrial Revolution began, making seas an average of 30 percent more acidic, according to NOAA. As a result, bioerosion could already be contributing to coral loss in reefs worldwide. “Numerous surveys and models have shown that we’ve tipped the balance towards habitat loss,” notes Enochs. Acidification is likely playing a role in that, he bets, along with other components in the “complicated mess of problems” afflicting reefs, including overfishing and bleaching.

BARs installed as one component of NOAA’s National Coral Reef Monitoring Plan, launched in 2014, will help tease out how bioerosion affects reefs today. “This PNG study showed us the future conditions. We are now monitoring present day conditions and we’ll see how that changes through time,” says Enochs, head of erosion for NOAA’s monitoring plan.

There are now 600 BARs installed at remote sites across the Caribbean and Pacific, according to Enochs. They will flow back to Enochs’ lab as NOAA ships make their rounds, harvesting each site’s BARs and replacing them with fresh BARs every three years.

Peter Fairley is a freelance science and environmental journalist based in Victoria, Canada.

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