Algae blooms could hold key to controlling crown-of-thorns starfish on Great Barrier Reef
In short:
Scientists believe summer algae blooms off the north Queensland coast could accelerate the population growth of crown-of-thorns starfish.
Mackay is experiencing its first bloom of the season and scientists are concerned it could lead to more of the coral-eating starfish spawning.
What's next?
Researchers hope to find starfish larvae eating the algae outside lab conditions.
It looks like a slick of oil floating on the ocean. Then the smell hits you.
Dubbed "sea sawdust", the blue-green algae bacteria Trichodesmium has turned the normally blue Coral Sea off the Mackay coastline shades of brown, green and pink.
The bacteria, which is slightly toxic to humans, normally sits at the bottom of the ocean, but blooms and rises to the surface to die and rot if the conditions are still and warm.
A bacteria bloom turned the water at Shoal Point, Mackay, shades of pink and green. (Supplied: Cassandra Ramsay)
Surfer Dwayne Close said the "slick substance" normally appeared when the surf died down for a few days.
"It's a bit hard to describe and has that bit of a pongy smell. It doesn't smell too good sometimes, but you get used to it," he said.
Dwayne Close checks out the conditions at Lamberts Beach. (ABC Tropical North: Jenae Madden)
Most sea creatures won't touch the stringy mass, but Queensland scientists have discovered the crown-of-thorns starfish (COTS) larvae eat the bacteria, the only animals to do so.
They say that understanding the sea sawdust better could be the clue to help control the starfish population, which has devastated part of the Great Barrier Reef.
A crown-of-thorns starfish feeds on coral on the Great Barrier Reef. (Supplied: Australian Institute of Marine Science)
Crown-of-thorns thrive on bacteria
Benjamin Mos, a marine scientist at the University of Queensland, said crowns-of-thorns starfish had an enormous appetite for hard coral and could occur in plague proportions.
The juvenile starfish normally eat small plants, but when scientists fed them a diet of sea sawdust, they thrived.
"The larvae can access these cyanobacteria [blue-green bacteria] as food and actually use that to fuel their development and growth," he said.
Two-week-old crown-of-thorns starfish larvae under a florescence microscope. The pink dots are microalgae food. (Supplied: Corinne Lawson, SCU)
Dr Mos had suspected the increased numbers of crown-of-thorns were related to additional food sources for the larvae.
He's concerned about the long-term impact on the reef if the starfish continue to spawn in greater numbers.
"[They] can grow up to a metre across, and in an outbreak they can be anywhere to 100 animals in an area the size of a football pitch.
"So if you have 100 animals, and each of those animals can eat a square metre of coral every two or three days, it doesn't take much math to work out that they can literally eat away coral reefs very, very quickly."
Current control methods are limited to attacking adult crown-of-thorns and injecting them with vinegar or coarse, liquid salts.
"We're definitely concerned about outbreaks [of sea sawdust] as being another one of those impacts, which just adds to the negative impacts on colonies," Dr Mos said.
Benjamin Mos says crown-of-thorns starfish can be as big as 1 metre. (ABC News: Melissa Martin)
His research colleague, Symon Dworjanyn from Southern Cross University, said the findings could contribute to cracking the "puzzle" of growing crown-of-thorn starfish populations.
"If we can figure out how to reduce the impact of COTS, we might give coral reefs a little more time," Professor Dworjanyn said.
The scientists now hope to find larvae eating the algae in the wild, outside lab conditions.
They believe if they can incorporate the prevalence of algal blooms into their "population models", they will be one step closer to preventing mass hard-coral destruction.
The water at Half Tide Beach, Hay Point, was a vivid shade of green and pink with an algae bloom. (Supplied: Kate Cole)
Cause not well understood
Lamberts Beach lifeguard Nick Jackson said his team was keeping an eye on the latest algal bloom.
"They're quite thick blankets of algae coming through to the shore," he said.
"We've seen them come through a couple of times a year, just obviously depends on the weather, nutrients in the water, all those sorts of things."
A bacteria bloom turned the water at Shoal Point, Mackay, shades of pink and green. (Supplied: Cassandra Ramsay)
Barbara Robson, a coastal biogeochemical modeller at the Australian Institute of Marine Science (AIMS), said the cause of the "slightly toxic" blooms was not well understood.
"They occur naturally, especially in sort of lower nutrient waters, because they're able to use nitrogen from the atmosphere, whereas most algae can only use nitrogen that's already in the water," Dr Robson said.
She said it was possible they were influenced by coastal run-off because they needed phosphorus they couldn't make on their own.
"There's some signs that then we might be starting to see more blooms and we're still working out why that might be.
"They're possibly increasing with climate change due to some changes in oceanographic conditions."