Underwater garden beds provide hope for seagrass restorationUnderwater garden beds provide hope for seagrass restoration

Researchers and commercial divers have created underwater garden beds in Cockburn Sound, using sediment dredged from a nearby area, as part of a project aimed at improving the survival rates of transplanted seaweed.

“We want to see if manipulating and adding sediments helps grow seaweed and if there are better ways to develop new underwater meadows.”

Dr Giulia Ferretto, UWA School of Biological Sciences

About 80 percent of the Sound’s seagrass beds have been cleared since the 1970s due to excessive nutrient discharge and have struggled to recover.

Capping existing seabed sediment, which contains residual nutrient pollution, is seen as a potential solution.

The project, which involves researchers from the University of Western Australia and Murdoch University, is part of the Western Australian Institution’s Marine Science Program at Westport.

Photo: Diver capping sediment. Photo by Dr. Giulia Ferretto

Seagrass researcher Dr Giulia Ferretto, from UWA’s School of Biological Sciences, said the project had been a major undertaking and involved several months of planning to organize a boat, large bags of sediment from the operation of dredging of Cockburn Cement and various that could establish the gardens.

“We used a small boat to take 15 rings of garden bed to the site in Cockburn Sound, then commercial divers fixed them on the seabed,” Dr Ferretto said.

“The boat also brought tons of dredge spoil in 700 kilo bags and a hoist helped lower the contents into the water.”

Dr. Ferretto said capping the existing nutrient-rich sediment with new dredged material may be a solution to a problem that has hindered restoration attempts.

“We will compare the health and survival of seaweed sprigs planted in existing sediments with those planted in the dredged material,” he said.

“We mixed a controlled amount of organic seaweed roots into some of the dredged sediment to see if natural ‘compost’ also makes a difference.

“We want to see if manipulating and adding sediments helps grow seaweed and if there are better ways to develop new underwater meadows.”

Dr. Ferretto said it was like gardening, but more complicated underwater.

“Once the beds were in place, various raked the sediment and then dug small trenches to plant the seaweed twigs,” he said.

Henry Evans, a research assistant at Murdoch University, said sea trees were a vital part of the Cockburn Sound ecosystem and helped stabilize sediments and provide food and habitat for marine wildlife.

“If the addition of dredged spoil is shown to improve sediment conditions for seaweed, then we will be able to increase the survival of future restoration efforts, as well as expand the area suitable for seaweed restoration in Cockburn Sound,” Evans said.

Seaweed shoots were planted in September and scientific discoveries will check them at regular intervals.