Australia’s Great Reef in the south

It may be the cooler, little-known cousin of the Great Barrier Reef, but Australia’s Great Southern Reef is just as important and just as much in need of our attention.

This little-known habitat is in important need of our attention.

SURFERS PADDLE OVER it, anglers f ish off it, almost 70 per cent of Australians live within 50km of it and it contributes $10 billion a year to the Australian economy. Yet few of us have heard of it – indeed, until recently, it didn’t even have a name. The Great Southern Reef (GSR) covers an area of 71,000sq.km and runs for more than 8000km along Australia’s southern coastline, from northern New South Wales to Kalbarri in Western Australia and around Tasmania. Its northern cousin, the Great Barrier Reef (GBR), is made up of more than 2900 individual coral-dominated reefs, while the GSR is comprised of thousands of kelp-dominated rocky reefs. These range from intertidal rock pools to shallow reefs and deep-water environments dominated by sponge gardens.

Kelp is a large, f leshy type of seaweed that can form underwater forests. Like coral, it provides a diversity of habitats for a wide range of other organisms, including crustaceans, chordates, bryozoans, sponges, echinoderms and molluscs. It adds vertical structure while providing shelter, a point of attachment for suspension feeders such as anemones and bryozoans, and a rich source of food for grazers such as marblefish and herring cale.

All this adds up to an extraordinar y wealth of biodiversity that includes more than 1500 seaweed species – substantially more than is found along coastlines of comparable length elsewhere in the world. There are also high levels of endemism. For example, about three-quarters of the 565 red seaweed species the GSR supports are found nowhere else; and among some invertebrate groups, the proportion of endemics is even higher.

The GSR name was coined in a 2016 scientif ic paper written by a group of prominent scientists from lead ing mar ine-research institutions hoping to bring this valuable natural habitat to public attention as a single interconnected system. The authors hope that giving the reef a name will act as the f irst step towards educating people about the challenges faced by this fragile ecosystem. Those challenges are many and varied, arising from a combination of unprecedented population growth on the adjacent coast and global environmental change. They include pollution, pressure from infrastructure development, introduced species, overf ishing and, most destructively, a warming ocean.

Just as warm water can cause coral bleaching on the GBR, increasing water temperatures are having a signif icant impact on the GSR. Scientists have identif ied that Australia’s temperate seas are warming two to four times more rapidly than the global average, largely due to the inf luence of the East Australian Current off the east coast and the Leeuwin Current off the west coast, both of which transport warm water southward.

It’s the reef ’s keystone species group, kelp, that’s really feeling the heat. A recent study that examined kelp cover around the world found that in roughly a third of sites it is declining. Unfortunately, that includes much of the GSR. Several mechanisms are behind the loss. In WA, it’s marine heatwaves. In northern NSW, a ‘tropicalisation’ of the fish community

on the reef has seen voracious kelp-grazing rabbitfish move south as the water has warmed. And in South Australia, the kelp has succumbed to years of wastewater pollution. The most dramatic losses, however, have taken place off Tasmania’s east coast.

Tasmania was once home to vast forests of giant kelp, the world’s largest and fastest-growing kelp species. Swimming through a healthy giant kelp forest, in which individual plants can grow 30m from the sea f loor to the surface, is like f lying through a forest of huge swaying beanstalks. Lemon-drop-shaped gas -bladder f loats pull each plant up towards the sun, while at the base, the spindly tendrils of the root-like holdfast secure them to the sea bed.

The arrival of warm, low-nutrient water from the north has weakened the giant kelp forests, making them more susceptible to storms and eventually leading to their collapse across much of their former range. To make matters worse, the warming has also allowed long-spined sea urchins to invade Tasmanian waters. These herbivorous sea creatures are capable of reducing healthy kelp forests to what are known as barrens – areas of bare rock completely devoid of seaweed.

“I’VE ALWAYS HAD an intellectual and emotional connection to temperate reefs,” says Professor Craig Johnson, associate director of the University of Tasmania’s Institute for Marine and Antarctic Studies and one of the authors of the new paper. “They’re both very interesting and visually stunning environments in which to work.”

After studying temperate reefs in Canada for his PhD and then working on similar ecosystems off South Africa, Craig took up a University of Tasmania position in 1997. His return to the Apple Isle, where he was born and grew up, coincided with the warming of the surrounding water. “[And yet] most of the research community hadn’t woken up to the fact that the water conditions off south-eastern Australia had been changing so quickly,” Craig says.

He began documenting the impact of warming water on Tasmania’s marine ecosystems. Using a mix of historic and new aerial photography, he showed that between the 1940s and 2011, giant kelp forests had declined by a staggering 95 per cent. “It’s an iconic and really important coastal marine community that is now essentially gone from much of the east coast of Tasmania,” Craig explains. Giant kelp forests are now only found in areas that the warm southbound current doesn’t reach – off Tasmania’s south coast around the Actaeon Island and in a few sheltered pockets off the state’s west coast.

Craig hopes the publication of the new paper will focus attention on the GSR’s plight. “We haven’t been as effective as the coral reef community in promoting our temperate reefs,” he says. Although the GSR is much larger than its northern cousin and contributes more to the economy, it receives a fraction of the research funding.

Our temperate reefs’ lack of prof ile is partly down to the fact that they can be relatively diff icult to access. Most are exposed to large swells, and, even if you manage to get below the surface, the visibility can be poor. “The water’s also colder, so you just have to be a whole lot f itter and tougher,” Craig adds. “You can’t put hundreds of divers and snorkellers into this system with the same ease and safety that you can on the Great Barrier Reef.”

That is not to say that the GSR isn’t pulling its economic weight. Conservative estimates suggest fishing and tourism associated with it generate about twice the economic return of the GBR, which is worth about $5.5 billion to the Australian economy. Its most valuable commercial f isheries are rock lobster and abalone, which together contribute about $500 million annually to the nation’s coffers; Tasmania’s wild-caught abalone industry alone is worth $100 million a year. However, the i ncur sion of long - spined sea urch ins into Tasmanian waters is a threat to that industry on the state’s east coast.

Like the urchins, black-lipped abalone feed on seaweeds, but are now being outcompeted by the spiky intruders. As a result, Tasmania’s east coast abalone productivity has halved. “It’s a problem when the urchins form full-on barrens because they strip everything away,” says Dean Lisson, chief executive of the Tasmanian Abalone Council and executive chairman of Abalone Council Australia.

Swimming through a healthy giant kelp forest is like flying through a forest of huge, swaying beanstalks.

For the past five years, the Tasmanian industry has reduced catch quotas in an attempt to halt the decline, an emergency response that appeared to be working. “The catch rates had bottomed out and were just starting to climb again, and we were thinking, ‘you beauty’,” Dean says. But then, during the 2016 summer, water temperatures off Tasmania’s east coast rose to 4oC above average and stayed there for more than 100 days – the worst marine heatwave in temperate Australian waters on record. “The heatwave definitely had a direct mortality effect on our abalone. Some of them just rolled over and died on the spot,” Dean says. “They can handle a bit of temperature shock, but the prolonged nature of the event had a direct impact on our stock.”

As with the last remaining stands of giant kelp, Tasmania’s most productive abalone grounds are now located off Actaeon Island and in the state’s south-east. “It seems to be fairly stable down there. The warm water doesn’t appear to get down that far,” Dean says. “Those reefs look much healthier than Eaglehawk Neck at the moment.”

Mick Baron, co-owner of the Eaglehawk Dive Centre, first came in the 1970s to Eaglehawk Neck, a small town in a constriction of the Tasman Peninsula, making regular weekend visits with a friend from university, where he was studying marine science. Over the years, he’s watched with growing alarm the changes taking place in the surrounding seas. “It’s a pretty dramatic story,” he says. “In the early days, giant kelp was everywhere, it was diff icult to get your boat through. You could stop right on top of the canopy and it would just hold you in place. Then, in the mid-’80s, it started to disappear and nobody really twigged why; there was no real conversation about it.”

When Mick set up the dive centre in 1991 there were giant kelp forests at every site he visited around the Tasman Peninsula. Today the picture is signif icantly bleaker.

“Over the past three years, the few remaining stands were located off Munro Bight. There were a couple of nice beds in there until mid-2016,” Mick says. “But during the marine heatwave last year, you could tell that the giant kelp was struggling, getting weaker and weaker through January. By the end of March, it was pretty much all gone. Heading into winter, there were a couple of small beds left and then a huge storm with the biggest waves I’ve ever seen came through in June and just tore every plant out.”

CRAIG JOHNSON SAYS there are two critical aspects to the future management of the GSR: preventing the loss of the kelp and restoring reefs where the kelp has gone. During the past three years, his research team has painstakingly transplanted 500 common kelp plants – the most abundant kelp species on the GSR – onto 28 artif icial patch reefs covering more than a hectare near Maria Island. “Although the focus of that experiment wasn’t reef restoration, the team learnt a lot about that process while conducting it,” Craig says.

By controlling the size and density of the kelp cover, the researchers were able to determine the f low-on effects of a thinning of the forests. The results showed that both of these variables had a strong inf luence on the structure of the communities that formed within the newly established forests, while also highlighting the kelp’s central importance to the health of temperate reefs. “The only thing we transplanted was the kelp and what really surprised us was how quickly the rest of the community seemed to follow,” Craig explains. First came the understorey of turf ing algae, then larval fish, followed by the adults. Finally, the various benthic invertebrates arrived. “It was just incredible how quickly the rest of the community established around the kelp. In every respect, our expectations were far exceeded by the response.”

The next step is to use sophisticated modelling to develop tools to better inform decision-making about the GSR’s future management. “During one dive on our kelp-transplant experiment, a group of spider crabs the size of a football f ield wandered through, totally unexpected. While we know about these seasonal aggregations, we have no idea of the impact of spider crabs on the ecology of the system,” Craig says. “The Great Southern Reef is really important to the nation, and we need to look after it, and to acknowledge it in the same way as we do our coral reefs. Otherwise, we may lose parts of it before we realise what we’ve lost.”

Over the years, he’s watched with growing alarm the changes taking place in the surrounding seas.