Known as the ‘other’ CO2 problem, ocean acidification stunts the growth of corals and shelled marine organisms, slowing future recovery from mass bleaching events.
In the late 1990s, California-based marine biologist Dr Victoria Fabry noticed something odd about the tiny planktonic animals called pteropods that she was keeping in jars of seawater. The calcium carbonate shells of the creatures had begun to look thinner, and after a few days, had dissolved away completely.
Dr Fabry had accidentally stumbled on what we now know as ocean acidification – a worst-case scenario for corals and shelled marine organisms.
The cause of the acidification in the jars turned out to be the carbon dioxide (CO2) that was being produced by Fabry’s pteropods in respiration. The CO2 was accumulating in the water as a weak solution of carbonic acid, which meant the availability of calcium carbonate – used by many marine organisms to build shells or skeletons – had become dangerously low.
As CO2 levels rise in seawater, they make the chemical formation of calcium carbonate very difficult, affecting everything from tiny Antarctic pteropods to massive boulder corals on the Great Barrier Reef.
The waste CO2 being pumped out by human civilisation isn’t only responsible for causing climate change as it builds up in the atmosphere – in the centuries since the Industrial Revolution, it has also been quietly dissolving into Earth’s oceans, and very gradually increasing their levels of acidity.
In this way, the oceans have helped mitigate the effects of greenhouse gases, absorbing about 25% of the excess CO2 produced in the past two centuries. But this buffering effect comes at a price.
“By absorbing around a quarter of the total human production of CO2, the ocean has substantively slowed climate change. But it also has less desirable consequences, since the dissolved CO2 affects seawater chemistry, with a succession of potentially adverse impacts,” notes a 2014 United Nations report.
The problem with pH
While the damage caused to coral reefs by mass bleaching events is more dramatic and obvious, as CO2 further builds in the atmosphere, ocean acidification will hinder the recovery of these coral reefs and slow their growth. Both mass bleaching and ocean acidification are different faces of the same problem caused by the pollution of greenhouse gases.
“In general, the most pressing concern at the moment is the increased ocean temperature, and that's having widespread effects on coral reefs, causing large amounts of coral mortality,” says Associate Professor Mia Hoogenboom at James Cook University’s (JCU) ARC Centre of Excellence for Coral Reef Studies.
Quantifying the effects of ocean acidification on a whole-reef scale is not easy, she says, but it will become a significant problem in the future.
For millions of years, pure water has had a neutral pH of around 7, while ocean water has consistently held a slightly alkaline pH of 8.2. But over the past few centuries, this has fallen to 8.1, representing a 26% increase in acidity. The fear now is that the acidity could rise to 170% of pre-industrial levels by 2100, as CO2 continues to accumulate.
“Experiments show that as you reduce the pH of water to a level predicted for the future, it will get increasingly hard for corals to produce their skeletons,” says JCU’s Professor Philip Munday. “As the saturation state of aragonite – one of the major forms of calcium carbonate – declines, it becomes much harder for corals.”
By the end of this century, some models predict that CO2 levels in the atmosphere could be higher than at any point in the past 23 million years, and the consequences could be catastrophic if we follow that pathway.
Examining coral cores – in a similar way to looking at tree rings – suggests that the decline in the rate of calcification of corals between 1990 and 2005 is unlike anything seen in the past 400 years, and has likely been caused by both warming and acidification.
The diversity of life on the Great Barrier Reef is under threat. Photo: Dr Mia Hoogenboom
The battle to survive
Experts predict that within a century, the Great Barrier Reef may have a weakened reef structure, which is more easily damaged by storms and has low levels of new coral growth.
Ecosystems around shallow volcanic vents in the Mediterranean Sea and naturally occurring volcanic seeps in the waters of Papua New Guinea (PNG) offer clues as to how reefs might look in worst-case future scenarios.
Near the Mediterranean vents, the waters are rich in CO2 and carbonic acid, and have a pH of around 7.5. They play host to plenty of algae and seagrass, but very few animals with calcium carbonate skeletons.
And several hundred metres from the CO2 seeps at Milne Bay in PNG, there’s a wide diversity of branching and other corals and the wildlife that associate with them. But as the CO2 concentration increases towards the seeps, there are fewer branching corals, and the brown boulder corals begin to dominate, finally giving way to nothing but seagrass adjacent to the most vigorous seeps, where the pH is very low.
Ocean acidification is likely to benefit seagrasses, which often compete for space with corals. It also has an effect on a wide range of other organisms. Some fish, for example, may alter their behaviour and exhibit impaired responses to predators as CO2 concentrations rise.
In lab experiments, the researchers found a “20% decrease in the proportion of squid that attacked their prey after exposure to elevated CO2 levels”, Spady says. “They were also slower to attack, attacked from further away, and often chose more conspicuous body pattern displays.”
But there’s still reason for hope.
Another study from JCU researchers has shown that some baby corals are better able to acclimate to higher levels of CO2 than we’d realised, and fish may be able to alter their physiology to adapt.
Researchers led by Professor Munday have also shown that the amount of CO2 present in marine ecosystems varies throughout the day, and fish may be able to cope – as long they continue to experience some daily periods with relatively low concentrations.
In addition to this, it’s been discovered that some fish have a surprising ability to adapt quickly to higher temperatures and more acidic conditions over several generations.
“We've seen really massive improvements when we look across several generations,” says Professor Munday.
“If you look within just one generation, it looks really bad. But if you look over a couple of generations, for example, where they've been exposed to higher temperatures, they're doing much better.”
Only more research will tell us the scale of the problem we face with ocean acidification, and how rapidly we’re going to have to contend with it.
But with the array of conservation efforts currently underway, the future is still very much an open book.
Feature image: Ethan Daniels/Shutterstock.com