How can climate refugia help the Great Barrier Reef recover?

Ocean solutions

Two years ago, scientists studying the seas surrounding Australia made headlines by discovering the world’s longest sea animal in the depths offshore from Western Australia: a threadlike creature called a siphonophore, measuring 46 metres from tip to tail.

It was a stunning find, but buried in the excitement was the fact that ocean scientists were also making important discoveries about how Australia’s iconic marine parks will fare in an increasingly globally warmed future.

The findings are mixed. There is no doubt that climate change will be hard on the Great Barrier Reef and other ecological treasures. But some areas may fare better than others, says Chaojiao Sun, a physical oceanographer with CSIRO in Crawley, Western Australia. These areas, she says, may provide climate refugia in which organisms decimated elsewhere can survive and return in the more distant future.

In the ocean, climate refugia are places where the waters are cooler than in surrounding areas that may be becoming increasingly uninhabitable. Not only could this preserve species living in these areas from extinction, but these refugia could provide habitats from which the surviving organisms might someday emerge and replenish the rest of the reef.

These refugia could provide habitats from which the surviving organisms might someday emerge and replenish the rest of the reef.

Similar refugia exist on land, but there, the escape route from warming is usually by moving upward, or poleward. In the ocean, Sun told the 2022 Ocean Sciences Meeting, held online from 28 February to 4 March, the places where climate refugia might occur are those with upwellings of cold water from below. “Even though the water column is heating up,” she says, “below the surface layer the heating is slower. So when the upwelling happens, the deeper water is still much cooler.”

That makes it a priority to protect such areas from other threats, such as sediment runoff, pollution, invasive species, overfishing, dredging, and pressures from commercial shipping traffic. But that can only be done if scientists and refuge managers know what areas to protect. That way, Sun says, “We can reduce manageable stressors.”

Unfortunately, she says, when her team cataloged areas where the Great Barrier Reef experienced coral bleaching events in recent years and compared them to those in which bleaching hadn’t occurred, it became apparent that many of the best climate refugia candidates weren’t even within Great Barrier Reef Marine Park.

That oversight, she says, appears to come from the fact that the factors that produce such refugia are very site specific. Current scientific models, she says, aren’t sophisticated enough to take into account important local features, such as the interaction of complex reef topography with tides, which alter the flow of water around the reef and “pump” cool water up from below.

Figuring out the areas in which this is most likely to produce climate refugia, she says, may be critical to protecting the Great Barrier Reef and helping it to recover, when (or if) global warming is eventually abated.

It is also possible, she and her colleagues say, that protecting these refugia may give their denizens time to adapt to warmer temperatures, allowing them to survive even if climate control measures prove less than optimally effective. There’s certainly no harm in trying.

Protecting these refugia may give their denizens time to adapt to warmer temperatures.

Meanwhile, the team that discovered the 46-metre siphonophore was on the opposite side of Australia, in Ashmore Reef Marine Park in the Timor Sea between Broome, Western Australia, and the Indonesian island of Rote.

The Timor Sea is an arm of the Indian Ocean, and one of the sad facts of global warming is that sea-surface temperatures in the Indian Ocean are rising 50% faster than the global average, according to Emma Bonanno of the University of Maryland, US, who also reported at the Ocean Sciences Meeting. That means climate change in Australia’s western and northwestern waters is proceeding considerably faster than on its eastern shore – not a good thing.

Not that the news is entirely bleak. Findings from Ashmore Reef, says Amy Carmingnani of Curtin University, Perth (who also presented at the Ocean Sciences Meeting), show that coral are remarkably adapted to survive at potentially cooler depths, even without the benefit of upwellings from below.

Her results were limited and preliminary, but he and she found that corals (which are technically animals but have a symbiotic relationship with photosynthetic algae that live within their tissues) were able to photosynthesise at depths as low as 60+ metres, where the light intensity was only 1/200th that at the surface. “This is quite remarkable,” she said.

The corals did this, she added, by becoming more flattened (thereby exposing more area to what little light was available) and by increasing the distance between individual polyps, thereby reducing competition among them. They also produced more photosynthetic pigment and enhanced their skeletal reflective surfaces in order to magnify the value of what little sunlight was available by reflecting as much as possible into those pigments.

The bottom line, Carmingnani says, is that deep, low-light environments might provide another type of refugia in which corals can adapt…and survive.

Meanwhile, a more urgent need is to figure out how to predict marine heatwaves.

Marine heatwaves are events in which the sea-surface temperature warms up and stays warm, for days, weeks, or even months at a time. That makes them more damaging than the slow increase in average sea-surface temperatures, because just like heatwaves on land, they can be intense, and deadly.

In the ocean, marine heatwaves can bleach coral, kill kelp, and damage fisheries and marine ecosystems.

On land, heatwaves can stress ecosystems, exacerbate drought, fuel wildfires, and according to one recent study are the greatest natural hazard to humans in all of Australia. Bottom line: heatstroke is no joke.

In the ocean, marine heatwaves can bleach coral, kill kelp, and damage fisheries and marine ecosystems.

The most recent example, says Claire Spillman, a research scientist at the Bureau of Meteorology, Melbourne, was a 2021 heat wave that raised water temperatures offshore from Western Australia by 2.5°C to 3.0°C for two months in December and January. That heatwave only ended when a large tropical storm brought heavy wind and rain, churning up cooler water from below and returning sea-surface temperatures to some semblance of normality.

These Indian Ocean heatwaves, Spillman told the Ocean Sciences Meeting, originate far from Australia, or even from the Indian Ocean. Instead, they are linked to the El Niño/La Nña oscillation, which affects the surface temperatures of waters in the eastern Pacific, closer to South America.

In La Niña years, relatively cool waters offshore from South America produce a return flow of warm water through the channels between the islands of Indonesia. This water then moves down along the coast of Western Australia producing marine heatwaves like the one seen in 2021.

2021 was a La Niña year. So too is 2022. Fingers crossed for the next few weeks, as the 2022 season wanes. But the better scientists can learn to forecast such heatwaves, whenever they come, the better prepared people will be to deal with them.

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