For most people, "coral reef" means something like the Great Barrier Reef: diverse multi-coloured ecosystems swarming with fish and the organisms they host.
But hidden from view and largely unknown, some coral reefs thrive deep in the oceans, in Australia and elsewhere. In fact, a wealth of communities exist deep underwater, living an almost unseen – and now potentially threatened – existence.
Deep in the darkness, coral reefs have formed on extinct volcanoes and rocky outcrops, better known as seamounts – undersea mountains, some more than a kilometre high.
Unlike other deep-sea environments, seamounts host rich and diverse life, thanks to the concentration of plankton and dissolved organic matter on their slopes. A wide range of sometimes bizarre organisms took advantage of the unique feeding opportunities and adapted to live in the cold, dark waters, at pressures 100 times greater than surface pressure.
On Australia's seamounts, one of the most common and ecologically important species is the stony coral Solenosmilia variabilis, which forms a vast matrix of living and dead coral colonies. At depths between 1000 and 1300 metres, scientists have found seamounts almost completely covered in this coral, supporting a wealth of associated fauna. The reef is several metres thick, growing at a patient 0.3 mm per year. It has probably been around since the last Glacial Maximum, more than 20,000 years ago.
Yet now, because of human-induced climate change, these delicate ecosystems may be imperilled and their poorly-known deep sea communities at risk of extinction.
Climate change under the sea
In recent years, researchers in Australia and elsewhere have reported declines in coral reefs’ ability to grow. The slow-down has been linked to the steady rise in carbon dioxide in the atmosphere, which in the ocean translates as acidification. This reduces the ability of corals and other marine organisms to deposit calcium carbonate on their skeletons, hindering their growth. Marine scientists estimate than in the last 250 years oceans have become 30% more acidic, and this worrying trend is projected to continue for the foreseeable future.
Climate change is also making water temperature change in different ways, warming parts of the ocean and cooling others. These temperature shifts have a direct impact on the physics and chemistry of deep-water ecosystems, and the biology of the animals living there. They may not be able to cope with the changes in their environment.
“Our analyses strongly suggest that Australia's deep sea coral reefs are at a high risk of extinction. They are basically the meat in a sandwich", says Ronald Thresher, from CSIRO’s Oceans and Atmosphere Flagship in Hobart. "They’re squeezed by high CO2 and falling carbonate levels from below, and warming surface temperatures pushing down from above”
“Without management intervention or a very substantial change in the world's production of CO2, we estimate that the reef will be squeezed out of existence within a few decades.”
Can anything be done to protect these underwater environments? In a recent workshop, deep-sea scientists from around the world met with marine reserve managers and environmental policy makers to discuss the current status of deep sea communities in the Huon Commonwealth Marine Reserve, how climate change may impact them and what can be done to help them.
According to a recent report[Link will open in a new window], a central problem affecting these communities is the availability of aragonite, a form of calcium carbonate that stony corals use to build their skeleton. Some of these, like colonial scleractinians, seem to be particularly sensitive to low carbonate levels. Scientists predict that these communities are doomed if current trends continue.
Blue-sky thinking on blue water solutions
The experts meeting in Hobart were open to blue-sky thinking: any possible solutions, no matter how impractical.
Some solutions were radical. Throwing lots of lime into the ocean would make more carbonate available. Or we could sacrifice some living corals for the sake of others, crushing portions of reefs to provide available carbonates. We could move colonies to more favourable environments.
We could develop genetically modify corals to be more resistant to unfavourable environmental conditions, or dump cement blocks with artificial branches attached to create homes for species associated with corals.
Other suggestions focussed on policy changes: expanding marine reserves, reducing commercial fishing and other human activities in the area, or designing protection strategies beyond Australian borders, in suitable deep water habitats.
Thresher thinks the best solution lies in computer modelling. “In the end, the best short term option is to use state-of-the-art ocean models to forecast where refugia for the reef might exist in future,” he says. Once these are identified, policy efforts can focus on protecting these sites. This, he explains, would help with establishing new coral colonies, possibly assisted with transplanted colonies.
In the absence of long-term monitoring, it is difficult to assess precisely what the impact of climate change on these corals might be. For now, the best way to get a quick answer is to rely on computer generated geophysical models. These use all the information currently available to come up with an educated guess on what coral can tolerate and how bad things will get if current environmental trends continue.
Looking to the future
But the road ahead is not clear. Thresher thinks Australia’s waters are unlikely to host significant refugia, and more research is needed to establish the current state of deep-sea ecosystems beyond Australian waters. “Ultimately, we are likely to need a regional and possibly even global approach to save the reefs,” he says.
In 2008 Thresher’s team laid out the world’s first deep-reef monitoring system. “Revisiting these stations will give us critical information on the health of the reef and how fast it may be changing”, says Thresher
Some current models suggest that the deep-sea corals of the Huon Commonwealth Marine Reserve could become non-viable within 50 years if present trends continue without some of these interventions being explored further and acted on.
This research was funded by the National Climate Change Adaptation Research Facility and managed by the Fisheries Research Development Corporation
More information
Thresher et al., 2015/Nature Climate Change[Link will open in a new window]