A long-snouted boarfish swims among the giant kelp that once spread 250km along Tasmania's coast. Photo: Mick Baron / Mercury

By Karl Mathieson
16 October 2016

(Mercury) – It was one of the world’s great marine ecosystems. Stretching hundreds of kilometres along the eastern coastline of Australia, it provided shelter to a multitude of fish, algae and crustaceans and for many divers was considered a must-see spectacular. But this year a massive underwater heatwave smashed it into oblivion.

This is not the Great Barrier Reef, but its southern equivalent; an underwater jungle that in the middle of last century ran the length of Tasmania’s East Coast. The trees, often more than 30m tall, were Macrocystis pyrifera – giant kelp – the world’s largest seaweed.

Two weeks ago, Mick Baron, a dive operator and marine biologist, went to search for a patch on the inner coves of Munroe Bight at the extreme southern end of the East Coast. It was the last patch remaining of the great eastern forest that once choked the unbroken 250km stretch of coast from Eddystone Point to the Tasman Peninsula. But when Baron arrived an almighty storm had ripped the trees from the rocks below.

Baron has dived on the East Coast since the 1970s. Back then it was impossible to get a boat through the thick mats of canopy it left floating on the surface. “All those years ago it was everywhere. I mean, it was common as muck,” he says. “Now it’s just gone.” It has been an extraordinarily rapid decline. But unlike deforestation on land, these forests are neither being cut nor burnt. They are being starved.

The Australian arm of the huge gyre that moves water around the Pacific is the East Australian Current (well known to Finding Nemo fans as ‘the EAC dude’). Traditionally, it pushed warm water south along the coast of the mainland before turning east towards South America long before it hit Tasmania.

But in recent decades, something has gone awry. The warming global climate has discombobulated this once-reliable system. Huge eddies of “hot”, nutrient-poor water keep spinning down towards the Tasmanian coast.

Because of this, eastern Tasmania has some of the fastest-warming ocean water on Earth, rising at two to three times faster than the global average. Over the past two decades, says Baron, the ocean has been getting “more and more rapidly mad”. Tiger sharks, marlin and Queensland grouper have all been recorded in places where they have no right to be. This February, one tuna fisherman caught 500 southern bluefin before the normal season even began.

The current is bringing change, but it doesn’t carry the nitrogen the forests need to fuel their prodigious growth rate.

“You get a double-whammy for the giant kelp. They get stressed because of heat and they get nutrient-starved and that combination is lethal,” says Dr Thomas Wernberg from the University of Western Australia. [more]

TasWeekend: Tassie's disappearing underwater forests



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