By the year 2100, rising carbon dioxide levels are expected to alter ocean chemistry so severely that coral reef communities in Australia and worldwide will recover more slowly, lose ecological complexity, and become increasingly dominated by fleshy algae.

A new international study released in Communications Biology has examined rare coral reef environments in Papua New Guinea to better understand how ocean acidification may affect coral ecosystems as the climate continues to warm.

As the ocean absorbs carbon dioxide from the air, seawater becomes increasingly acidic, which can slowly erode the limestone that forms coral skeletons. However, experiments in tanks or computer simulations offer only limited insight into how full reef systems might respond over time.

To address this gap, a team led by the Australian Institute of Marine Science (AIMS) investigated entire reef communities located near several of Papua New Guinea’s shallow submarine volcanoes. These sites are naturally exposed to higher CO₂ levels because gas escapes from the sea floor, creating conditions similar to those expected in future oceans.

Dr. Katharina Fabricius, a coral researcher at AIMS in Townsville and senior author of the study, explained that the work has identified species capable of surviving long-term exposure to elevated CO₂.

“These unique natural laboratories are like a time machine,” said Dr. Fabricius.

“The CO₂ seeps have allowed us to study the reefs’ tolerance limits and make predictions. How will coral reefs cope if emissions are in line with the Paris Agreement level emissions? How will they respond to higher CO₂ emissions scenarios?”

Origins of a Unique Research Site

In 2000, Dr. Fabricius came across bubbles of gas emerging through coral reefs while surveying species in Milne Bay, about 500 km east of Port Moresby. In 2009, as ocean acidification emerged as an issue, she thought back to that experience, had samples of the gas analyzed, and discovered it was nearly pure CO₂.

The scene was set for the creation of a unique living laboratory and a decade-long research program to study how tropical marine ecosystems may adapt and how organisms acclimatize after generations of exposure to high CO₂.

Dr. Sam Noonan, also from AIMS and first author on the paper, said: “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. 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.”

Oceans are slightly alkaline with a pH of 8.0, but their acidity has already increased by 30%. As CO₂ emissions rise, the ocean pH is predicted to decline further down to a pH of 7.8 by the year 2100.

A Gradual but Profound Ecological Shift

“By studying organisms at 37 sites along a 500-metre gradient of CO₂ exposure, we were able to see what happens as CO₂ increases. 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,” Dr. Fabricius said.

The reefs are hard to reach, requiring a flight into Papua New Guinea, a second to Milne Bay Province, then six hours in a boat.

“The coral reefs in Milne Bay are amazing, and the local people are so welcoming. It was a real privilege to work at their reefs with these volcanic CO₂ seeps, which are globally unique,” Dr. Fabricius continued.

“Ocean acidification is a massive global problem, which has been understudied and underreported to date. 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.

“We have observed coral reefs starting to change in response to CO₂ gradients in the Great Barrier Reef. 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 that depend on them. This is on top of the impact of global warming and sea level rise.”