Dugongs at risk from toxic algae

Blue-green algae is threatening to smother the Western Australian seagrass beds that dugongs feed off.

OFF THE COAST OF BROOME, snaking trails are visible at low tide through glistening seagrass meadows. The pathways are a sign that a family of dugongs has swum through the area, pushing through the grassy seabed and grazing on a smorgasbord of tender shoots and roots.

Thriving seagrass meadows are vital for these hefty aquatic mammals, which can eat up to 40 kg of plant matter a day. Fiona Bishop, coordinator of the Broome Community Seagrass Monitoring Project, says dugongs (Dugong dugon) are regular visitors to Broome's Roebuck Bay, a Ramsar-listed wetland with extensive seagrass beds.

"We're lucky to have such low tides so we can walk out and see dugong trails everywhere, sometimes zigzagging back and forth. Two of the dugongs' favourite species of seagrass grow here, and they eat the whole plant, roots, flowers and seeds."

As well as 'mowing' the grass, the dugongs help disperse the seed widely when they expel their waste into the water

Toxic spread

But the health of seagrass is this year threatened by an outbreak of toxic blue-green algae, called Lyngbya majuscula, which scientists say they will study in coming weeks to establish whether Roebuck Bay's dugongs, famous migratory bird flocks and marine invertebrates are affected.

Andrew Storey, adjunct associate professor in the School of Animal Biology at UWA, is leading a $140,000 study of Broome's seagrass beds off the coast of Western Australia. While the algae is a naturally occurring plant, in certain conditions it can grow in matted profusion and literally smother the seagrass beds that dugongs feed on, he says.

Several bays around the Australian coast are plagued by Lyngbya outbreaks, with Moreton Bay in Queensland worst affected. Land-based pollutants like sewage leaching into the bay are thought to be the "triggering element" for these outbreaks.  

"Compared to outbreaks in Moreton Bay, the Roebuck Bay blooms are relatively small, but they are increasing with every Wet season," Andrew told Australian Geographic. "It's hard to pinpoint, but we think that nutrient enrichment from groundwater runoff in Broome may have caused it."

Biodiversity incubators

The worst outbreaks occurred in the most recent Wet season, which ended in April this year.  

Seagrass habitats are incubators of biodiversity; about 40 times as many animal species live in seagrass meadows than live on bare sand. Worms, sponges, shrimp, starfish, crabs, sea urchins and anemones thrive amid their shady fronds - unless the slime-like alga forms a light-smothering blanket.

Fiona says volunteers walking out at low tide are stunned by the beauty of seagrass: "it's glossy, green and like a wet golf course."

The combined research by scientists and volunteers, supported by Seagrass Watch - the world's largest scientific seagrass monitoring and assessment program - will hopefully build up a detailed picture of Broome's seagrass habitat. "It's an early warning system about the health of the entire bay," Fiona says.