Southern recovery of ecosystem engineers

We tend to default to the colourful – to coral – when we think of oceanic reefs, but 200 years ago, thousands of kilometres of cold-water reefs dominated southern Australian coastal waters, from modern-day Noosa to Perth.

They were not the familiar kaleidoscope of tropical reefs, built atop a living coral skeleton – rather, they were raised on the backs of oysters. “They grow on top of each other to create a living structure like coral reefs, with a thin veneer of life on top of the dead shell,” explains James Cook University marine biologist Ian McLeod.

These oysters were ecosystem engineers. Not only did they provide food and habitats for biodiverse communities of fish, mussels, crabs, cuttlefish, prawns, worms and many other critters, they also cleaned the water, stabilised sediment, circulated nutrients and protected coastlines from erosion.

Indigenous Australians benefited from these rich ecosystems for millennia, says University of Adelaide researcher Dominic McAfee, who is working on the Oyster Reef Restoration Project in South Australia’s Gulf St Vincent.

“Around Australia, large oyster shell middens – used piles of shell – attest to the dietary and socio-economic importance of oysters, with shell remains dating to 10,000 years ago,” he says. “Huge coastal corroborees attended by hundreds of people across Indigenous nations were underpinned by gigantic oyster feasts, and occurred up to a century post-colonisation.”

During the colonial era, the easily exploited shellfish were massively overharvested to feed the growing whitefella population, and when land-based limestone ran out, settlers turned to dredging up the oyster reefs for lime. They tore up the foundations of compacted shells and hauled them into shore, turning productive ecosystems into building mortar. Land-clearing and agriculture further inland also triggered erosion, causing sediment to flow into the ocean and smother the shellfish.

By World War II, every Australian oyster reef had been wiped out except for a single example in Tasmania. The colonial oyster-dredge fisheries and lime burners removed every shell, alive and dead, explains McAfee. “This hard shell provides the foundations on which new reefs grow. If it’s removed, the oysters have nowhere to settle.”

In South Australia alone, 1500 kilometres of reef vanished, leaving a marine desert.

Today, shellfish reefs are among the most endangered marine habitats on the planet. In Australia, they’re functionally extinct – but perhaps not for long.

A landmark project was recently completed on the quiet western side of Gulf St Vincent: the largest human-made oyster reef in the southern hemisphere. Located a kilometre offshore near Ardrossan, Windara Reef comprises almost 160 individual reefs, using limestone boulders as a base and seeded with 50,000 hatchery-raised Australian flat oysters.

Windara is part of an ambitious project run by not-for-profit The Nature Conservancy Australia, which aims to rebuild 60 artificial reefs around Australia after successfully seeding over 100 in the US. The project has just received $20 million from the federal budget to fund 11 of these reefs and was awarded the prestigious 2020 Eureka Prize for Applied Environmental Research.

“Oysters are born survivors,” says McAfee. “Their form has changed little in 250 million years, through which they have survived numerous mass extinction and climate change events. If we provide the necessary support to revive these reefs, I believe they will flourish again.”

A critical part of the restoration project was determining where shellfish reefs used to occur. Since they were lost so long ago – outside of living memory – we’ve little knowledge of their location or appearance. But by trawling through historical records and even accounts from early explorers, Australian researchers pieced together a map of historical shellfish reefs, thus informing where new reefs should be seeded.

This intergenerational amnesia is a recurring issue in conservation. How can we conserve, restore or protect an ecosystem we don’t remember it, let alone understand it?

They tore up the foundations of compacted shells and hauled them into shore, turning productive ecosystems into building mortar.

Oyster reefs aren’t southern Australia’s only forgotten marine ecosystems. Another vast system of temperate reefs spans 8000 kilometres from New South Wales to Western Australia via Tasmania.

Called the Great Southern Reef (GSR), this unique ecosystem was only named and defined in 2016 – despite most Australians living less than 100km from it.

“Like the Great Barrier Reef, the Great Southern Reef isn’t a ‘reef’ per se, but a system of thousands of reefs connected by ocean currents,” explains University of Tasmania (UTas) marine scientist Craig Johnson.

It’s dominated by forests of golden kelp (Ecklonia radiata), the gleaming brown fronds of which provide critical habitat for thousands of species, including seaweeds, sponges, crustaceans, echinoderms, molluscs and more.

“Many of the species found here are found nowhere else on Earth,” says marine ecologist Adriana Vergés, from UNSW Sydney and the Sydney Institute of Marine Science. “This includes some of our most famous critters, such as the giant cuttlefish and the weedy and leafy seadragon.”

This unique array of temperate flora and fauna is due to the reef’s long geological isolation from the rest of the world. It evolved under relatively stable and benign conditions, with low nutrient levels, which makes it particularly susceptible to environmental changes.

Like so many other ecosystems, the reef currently faces a range of threats – one above all others.

“Humans,” says Thomas Wernberg, a University of Western Australia researcher who co-authored the study that first defined the reef. “The GSR is a massive marine ecosystem and it is generally still in good condition in most places. However, many unfortunate consequences of humans are a threat.”

These include coastal development, pollution, increasing ocean acidification – and climate change.

The waters of southeastern Australia are warming several times faster than the global average, according to scientists from the CSIRO and the Institute for Marine and Antarctic Studies at UTas. This is partially due to the intensification of the East Australian Current, which transports warm water from the equator to the poles.

Warming puts incredible stress on the reef ecosystem in several different ways. Severe heatwave events, for example, can cause mass die-offs of kelp like the devastating losses seen along the coast of WA over the past 20 years.

The overall trend of warming waters also causes a larger trend of kelp die-offs over time, which is occurring in eastern Tasmania with the iconic giant kelp.

Additionally, Vergés explains that “as waters continue to warm and marine heatwaves become more intense and frequent, we are seeing many species start to expand their distribution towards the poles, with some also contracting their distribution at the warm edge”.

For example, sea urchins from NSW have dramatically expanded their range, surging down to Tasmania where they have overgrazed kelp and other seaweed. This has caused “some areas to ‘flip’ from rich, productive biodiverse kelp-beds to poorly productive sea urchin ‘barrens’ with no seaweeds and low production,” says Johnson.

Other species have also followed the warming waters south, such as sub-tropical fishes on both the eastern and western coasts of Tasmania, contributing to the pressure on kelp beds.

Eventually, however, the trespassing species will have nowhere to go – shallow reef habitats vanish south of Tasmania.

“It’s a little bit like the species at the southern edge of the GSR are on a cliff-edge, destined to fall into the abyss, as there’s nothing but deep oceans between Australia and Antarctica,” Vergés says. “This is very different to what happens in the north pole, where there are continuous land masses and reefs all the way from high latitude systems to the pole.”

Vergés says that conservation efforts need “to acknowledge these inevitable changes and adapt”, both by better understanding the science driving the impacts and by implementing better management plans.

“Restoring coastal habitats can be major part of the solution,” she says. “For example, mangrove restoration can help us protect our coastlines from increasing extreme events, seagrass and kelp restoration can help draw down carbon, and oysters can improve water quality and enhance the resilience of ecosystems by reducing impacts of pollution.”

This unique array of temperate flora and fauna is due to the reef’s long geological isolation from the rest of the world.

Even though it’s tricky to reconstruct the marine ecosystem as it was before the massive changes of the past 200 years, most researchers consider shellfish reefs to be a lost component of the larger GSR system.

Historically, according to McAfee, these reefs “would have acted as nursery habitats for many of the fish species that would eventually mature and ‘graduate’ to the deeper water GSR system”.

They may also a play a vital role in the health of the coastal ecosystem as a whole, says Sean Connell, a colleague of McAfee at the University of Adelaide.

“They create the diversity of habitats and interconnected highways between habitats needed for fish and other marine life to breed, mature and thrive,” he explains.

Restoring oyster reefs will thus benefit the entire ecosystem.

The Nature Conservancy’s reef-building project will not only enhance water quality and biodiversity, it may also help the environment adapt to changing conditions. Research in marine protected areas has shown that more intact ecosystems are more resistant to invasion by warm-water species.

“By restoring oyster reefs and the associated ecological communities supported by these habitats, the overall marine environment should become a lot more resilient,” Vergés says.

It helps that oyster reefs are seriously tough and adaptable. Connell’s lab, for example, observed that oysters may still be able to grow and survive in the forecast conditions of warming and acidifying oceans.

“From our studies of other shell-building creatures, we know that shell-builders can rearrange their atomic-scale building of shells to construct tougher and stronger shells,” he explains. “So we would not be surprised if future oysters will also be adapted to future conditions.”

Of course, oysters aren’t immune to warming oceans. Disease susceptibility will increase in shellfish, and the optimum areas for them to grow will change.

Even so, if The Nature Conservancy’s project succeeds in reseeding 60 reefs, it would be an incredible achievement.

“It would still be a mere veneer of reef habitat relative to what was [here] 200 years ago,” says McAfee. “But you have to start somewhere, and even small, healthy reefs will provide broad social and ecological benefit.”

Encouragingly, Connell notes that in terms of logistics, it’s easier to restore native oyster reefs than marine ecosystems such as seagrasses and kelps.

“My marine laboratory is working on restoration methods that will drive down costs and allow for kilometre-scale restoration of oyster reefs,” he says. “It seems that oyster restoration will be one of Australia’s most successful restoration projects on land or sea.”