Seagrass depletion a concern for marine resilience

Posted 19 October 2016

Seagrass depletion a concern for marine resilienceThe stinking piles of seaweed plaguing southern Western Australia's beaches could become a thing of the past as a result of climate change, researchers have found, but they warn the shift will also bring more worrying trends.

Researchers from the Edith Cowan University Centre of Marine Ecosystems have found changes associated with ocean warming, particularly tropicalisation, are occurring more rapidly on Australia's west coast than in other parts of the world.

Furthermore, those changes are tipped to have significant flow-on effects not only for seaweed, but for marine life, carbon dioxide levels and beach stability.

“Tropicalisation occurs when you have warming of water associated with global warming so you have more tropical temperatures in temperate environments,” said Lead Researcher Associate Professor Glenn Hyndes.

“As a consequence of that, you can have tropical species moving down into what are currently temperate regions. That can have an influence on a whole range of ecosystem processes.”

Hyndes and his colleagues predict sea surface temperatures off the coast of WA will be two degrees higher in 100 years, prompting some tropical species to move more than 500km south along the coast.

Because those tropical species – including fish, turtles and dugongs – are herbivorous grazers, they're likely to decimate seagrass beds, Hyndes said.

“That means you're losing a habitat for a range of different fauna. In particular, in the surf-zone at certain times of the year you have large numbers of juvenile fish species occupying the wrack [seaweed] so we're likely to see a major reduction in the amount of biomass present in surf-zones and on the beach,” he said.

“Then you're also losing the ability for [seaweed] degradation processes to release nutrients back into the coastal system.”

Seagrass meadows are also important for their ability to absorb atmospheric carbon dioxide at a rate 40 times higher than tropical rainforests.

Less seagrass could also mean more beach erosion, Hyndes said.

“The material washed up onto the beach, particular during the winter months, is a form of sediment stabilisation that prevents the beach from being washed away in storms,” he said.

The findings, published in the journal Biosciences, are among world-first research on the impact of tropicalisation on temperate seagrass meadows.

Hyndes said the Western Australian researchers were well placed to see the transformation occurring more rapidly than in other parts of the world.

“We've got the Leeuwin Current on the west coast, which brings warm water down and a few years ago [in 2011], we had a heat wave event where sea temperatures went up five degrees above average during the summer,” he said.

“So we're already seeing some major increases in ocean temperatures. That's partly the result of the Leeuwin Current.

“But put those global changes in sea temperatures on top of the existing Leeuwin Current effect and you have some substantial increases in sea surface temperatures.”