Groundbreaking research into the sensitivity of seagrasses off the northwest coast has uncovered unique behaviour that could lead to a re-think in the way the region is managed.

The seagrasses off Western Australia are the most extensive and diverse of any region in the world with 26 species in 11 genera, accounting for more than 35 per cent of all species currently described globally.

The submerged flowering plants play a vital role in supporting biodiversity, filtering harmful chemicals and nutrients, and sequestering CO2 from the atmosphere. Tropical seagrasses are also a critical food source for fauna such as dugong and green turtles, but little is known about populations off the subtropical northwest .

A Western Australian Marine Science Institution (WAMSI) Dredging Science Node project has brought together researchers from Edith Cowan University (ECU), The University of Western Australia (UWA) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to measure the effects of light reduction and sediment burial to determine the capacity for northwest seagrasses to withstand change.

Tolerance levels have previously been determined for seagrasses in the southwest but they are very different to the species off the far northwest coast according to ECU Professor Paul Lavery.

“In the southwest, the dominant species of seagrasses have large storage organs and carbon reserves, and produce large non-dormant seeds,” Professor Lavery said. “Because of their considerable carbon reserves, when they are placed under stress by dredging operations, they draw on those carbon reserves and can survive for several months.

James McLouglin and Roisin McCallum establish a field experiment at Thevenard Is. to determine the mechanisms and rate of seagrass recover

“The seagrass species up north are much smaller, producing small dormant seeds that lay waiting in sediments. They appear to be much more sensitive to changes in light and sediment cover. However we need to be cautious,” he said. “While they may show a rapid response to dredging-induced changes, we don’t really understand yet if they can recover quickly from those impacts. It’s possible that a few months after complete loss, the meadow returns from seed.”

The researchers are conducting a combination of field studies and controlled laboratory experiments.

“We’re working in the Pilbara areas around Exmouth Gulf and Thevenard Island (about 20km off Onslow),” Professor Lavery said. “We’ve been going up every few months to measure characteristics of the meadows, from when they grow, to when they die off and how much biomass there is. This is information we just don’t have for seagrasses in the north.”

Research sites in the Pilbara areas around Exmouth Gulf and Thevenard Island

The research program is focused on gaining information that is useful and relevant in a systematic way. An initial recommendation from the research is that pre-development surveys and ongoing monitoring programs for seagrass should consider the time of year. In the month of June, for example, there appears to be no seagrass meadows. It’s not until September that they start to grow back.

“This most basic and fundamental piece of information we didn’t understand before,” Professor Lavery said. “This in turn will save money for companies as they often conduct costly surveys when seagrass is not naturally present.”

We’ve now conducted field studies in several locations to see if the same sort of patterns exist in each location and so far we’re finding that different places have different patterns, which makes things more complicated and is going to make advising government and industry more challenging.”

Off Thevenard Island the researchers removed seagrasses from both shallow and deep water meadows to observe how the system recovers.

“We wanted to see if the meadows can recover from seed or by material drifting in from elsewhere,” Professor Lavery said. “So far we aren’t seeing any recovery by seed. There seems to be a need to have vegetative material available for it to grow back. So it’s back to the lab now to find out the capacity for the seagrasses to withstand change.”

 

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.

Dredging Science