On a remote coral reef near Papua New Guinea, endless streams of bubbles rise from cracks in the seabed into the shallow water, fed by an underground volcanic system. For scientists, this natural phenomenon has become a kind of crystal ball, revealing how our changing oceans will shape the marine life within them.
A team led by researchers from the Australian Institute of Marine Science (AIMS) has found that these volcanic bubbles – made up of almost pure carbon dioxide (CO2) – create a kind of localized change in the environment, due to the increased acidification of the water.
As the gas rises, it forms visible streams of bubbles, which dissolve into the surrounding seawater and change its chemistry. And because this is occurring near coral reefs in Papua New Guinea’s Milne Bay, the scientists are able to see what elevated CO2 in the water does to the ocean life exposed to it. Essentially, it’s a living, natural model that can’t be replicated in the lab.
Katharina Fabricius/Australian Institute of Marine Science
“These unique natural laboratories are like a time machine,” said senior author Dr Katharina Fabricius, a coral ecologist at AIMS. “The CO2 seeps have allowed us to study the reefs’ tolerance limits and make predictions. How will coral reefs cope if emissions align with Paris Agreement targets? And what happens if they don’t?”
Fabricius first encountered the volcanic bubbles streaming up through the coral gardens there in 2000, when she was conducting a diversity survey. Many years later, she returned with a team of scientists to analyze the gas – which was then identified as almost pure CO2. This was the start of a decade of research looking at how tropical marine ecosystems adapt – or fail to adjust – to increasingly acidic environments.
They established 37 stations across this gradient – from bubble-free areas that reflect today’s ocean chemistry to heavily bubbling patches that mimic conditions expected later this century. What makes the site so valuable is that nothing else changes – temperatures, currents, light and salinity stay the same – so the researchers could look a the effects of acidification on its own.
Katharina Fabricius/Australian Institute of Marine Science
At each station, they measured how friendly the water was to calcium carbonate formation – the material that corals and some algae use to build their skeletal structures. They also photographed the seafloor, counted juvenile corals, assessed habitat structure and collected algae to weigh and identify. All of this allowed them to build a continuous picture of how reef life changes as the water becomes less favourable to building and maintaining skeletons.
What they found wasn’t what some people predict – a sudden tipping point where life ceases to exist after a certain concentration of CO2 in the water is reached. Instead, they observed a steady, progressive reshaping of the reef’s coral community – and a change that was evident even when there were only slightly elevated levels of CO2 in surrounding water.
The first signs of change appeared with only small drops in pH, within the range already recorded on many reefs around the world. Diversity of both adult and juvenile hard corals dropped quickly, and the most sensitive species – the branching and plate-like corals, which form much of the shelter for fish and invertebrates – were hardest hit. These began to disappear after only modest declines in pH and were almost completely absent in the most acidified zones.
Meanwhile, one group of corals – the large, round stony Porites species – showed surprising resilience, but this in turn masked the actual decline in coral cover, giving the false impression that the community was holding up better than it was. When scientists excluded these corals, the scene was grim.
“These Papua New Guinea reefs are telling us that with every bit of increase in CO2, we will see fewer corals and more fleshy algae,” said first author Dr. Sam Noonan from AIMS. “Importantly, we also found far fewer baby corals, which means reefs won’t be able to grow and recover quickly. That has implications for all the species that depend on them, including humans. Many coastal communities depend on fish that start their lives using coral reefs for shelter and food.”
Katharina Fabricius/Australian Institute of Marine Science
The algae that help cement and build reef framework also declined rapidly as CO2 increased and eventually disappeared altogether. These algae normally help baby corals settle and grow, so their loss compounds the decline. Non-calcareous algae expanded as water became more acidic, with brown and red algae spread across the seafloor, enjoying the space and reduced competition. Sponges also increased in abundance. Overall, the reef shifted from a complex, coral-built environment toward a simpler, flatter, more algae-dominated space.
If reefs are the rainforests of the ocean, CO2 essentially turns a lush, diverse landscape into an open grassland – which is also bad news for an estimated 25% of the world’s fish that rely on corals for shelter, mating, rearing young and finding food.
“By studying organisms at 37 sites along a 500-meter (1,640-ft) gradient of CO2 exposure, we were able to see what happens as CO2 increases,” said Fabricius. “There was no sudden collapse or tipping point, instead, as the CO2 increased, we saw fleshy algae became dominant, replacing and smothering coral and calciferous algae.”
The research is important for predicting how reefs around the world will change as ocean acidification increases this century and beyond. While coral bleaching – which is due to water temperature rises – has been well documented, there’s less data on what acidification is doing to these vulnerable species and how those changes will then impact reef-dependent fish and invertebrates.
“We have observed coral reefs starting to change in response to CO2 gradients in the Great Barrier Reef,” Fabricius said. “The Papua New Guinea reefs tell us what will happen next. The more CO2 we emit into the atmosphere, the greater the changes will be to coral reefs and the coastal communities which depend on them. This is on top of the impact of global warming and sea level rise.
“Ocean acidification is a massive global problem, which has been understudied and underreported to date,” she added. “This research is a first of its kind, presenting unique field data and allowing us to assess how whole communities change in the real world.”
The research was published in the journal Communications Biology.
