THIS is a story about sea levels and Australia’s coastal marine ecosystems. Usually, you could expect a few words on the threats faced by the Great Barrier Reef. Not this time. On the other side of the country there’s another outcrop of coral that deserves close attention, with concerns it could soon start losing its race to the top.
Ningaloo Coast in Western Australia’s north west is perhaps best known for the whale sharks that venture into its waters every autumn. But to many, the region’s most valuable jewel is the 250 kilometres of coral that hugs the sands.
Tens of thousands of people visit the area every year, bringing in the tourist dollars as they snorkel, camp, and soak up the natural beauty. Without the coral, there’d not only be a relative shortage of wildlife to ogle – the very coast itself would be threatened by the pounding Indian Ocean.
“Ningaloo has enormous cultural, recreational and ecological value,” says CSIRO marine ecologist Damian Thomson.
“The reef is home to over 500 species of fish and 200 species of coral and is an important nursery habitat for iconic species such as turtles.”
Looking up
Working with an international team of researchers as part of the Ningaloo Outlook research program, Thomson keeps a close eye on the reef’s health as it struggles to deal with the challenges of a changing climate. He’s previously looked at the rate at which new colonies of coral establish themselves on the reef’s seafloor, but this time he’s more interested in its vertical rise.
Corals rely heavily on the nutritional boost provided by the algae that share their tiny apartments. As oceans warm, their photosynthesising house mates leave town, resulting in a much whiter, more ‘bleached’ looking coral.
Those algae could also suffer if rising sea levels put sufficient amounts of sunlight out of reach. Since the vertical rise of coral’s calcium carbonate skyscrapers roughly matches the annual 2 to 3 millimetres added every year to sea levels, we don’t tend to give that problem a lot of thought. But this ideal accretion rate relies on striking a delicate balance.
“Reef growth is the rate at which calcium carbonate (CaCO3) is deposited minus the rate at which CaCO3 is eroded,” Thomson explains.
There are a number of factors that affect that equation, not least the amount and types of coral that already make up the reef’s real estate. Water quality, sedimentation, increased fishing pressure and ocean temperature can also upset the rate at which coral reefs can spread out and up.
Ningaloo offers a perfect test subject as far as reef studies go. While many other reefs in the region and around the world battle it out with pollution and the voracious appetite of predators such as the crown-of-thorns starfish (Acanthaster planci), Ningaloo is going strong. At least for now.
Together with CSIRO colleague Mick Haywood, Thomson and his team examined Ningaloo along with more than 200 reefs in the western Atlantic and Indian Ocean tropics. The results of their investigation have recently been published in the journal, Nature.
You might be pleased to know that current levels of accretion among the reefs are more or less in pace with recent rising sea levels. But don’t breathe a sigh of relief yet – that slow chase could soon get wild.
Rising waters
Under relatively modest projections for greenhouse gas concentrations, most of those reefs will soon struggle to keep up with the rising waters without serious sustained growth. In fact, only about 3 percent of tropical reefs in the Indian Ocean will be up to the task of keeping up with sea level rise unless they overcome their ecological challenges.
And if we consider less than modest projections? Expect those reefs to be swamped by around half a metre of ocean by 2100.
“Despite Ningaloo Reef being one of the best-placed reefs in the Indian Ocean to keep pace with sea-level rise, our results suggest Ningaloo Reef will also be unable to keep up with predicted sea-level rise,” says Thomson.
And never mind the loss of breeding grounds, colourful marine life, and tourist dollars. A loss of reef creates a whole other threat to those living on Western Australia’s north west coast.
“As a result, water depths above the reef may increase through this century, which is of concern because the reef plays a key role in limiting coastal wave energy exposure.”
The solid infrastructure of coral reefs like Ningaloo soaks up the intense power of waves so the sands and coastal rocks don’t have to. Without them shielding the shoreline from tropical storms, we might expect some significant changes to the coast thanks to the gradual grind of weathering and erosion.
Right now all eyes are currently on the Great Barrier Reef in the east, and for good reason. Ocean acidity, temperature changes, predation and silt are just some of the threats it needs to deal with. Rising sea levels will be the icing on the cake, and Queensland’s coastal residents have good cause to be concerned.
By comparison, Ningaloo is in a strong position. It could have a few secrets to share that might even help us save other reefs, if we pay attention.