Researchers working to better predict the likely environmental impacts associated with dredging have found that branching corals are highly adept at cleaning their surfaces of depositing sediments compared to other coral structures.
The Western Australian Marine Science Institution Dredging Science Node study assessed the sediment rejection ability of eight common Indo-Pacific coral species from three different morphologies (coral structures) in a series of short-term exposure tests over a range of sedimentation levels and one longer-term exposure test at a high sediment concentration level.
Australian Institute of Marine Science (AIMS) scientists tested: branching corals, that are made up of thick upright and horizontal branches; foliose coral, that are more flattened and plate-like; and massive corals, that are characteristically ball or boulder-shaped.
The results, published in the Marine Pollution Bulletin, show that sediment accumulation rates on live corals and dead (enamel-covered) skeletons varied between morphologies, with branching species often more adept at self-cleaning.
Lead researcher, Dr Alan Duckworth explained corals have a range of different ways for shifting sediment primarily involving; mucus entrapment, hydrostatic inflation (the ability of corals to expand tissues, resulting in a shape which better sheds sediments), tentacle movement and ciliary action, which is small threadlike appendages producing strong swirls of water that draw nutrients toward the coral, while driving away waste products.
“These ‘active’ (energy-requiring) processes work in combination with ‘passive’ forces associated with gravity,” Dr Duckworth said. “Both the macroscale morphology (growth form, branch thickness and spacing) and microscale morphology (corallite size and shape) affect how sediments settle, collect and are cleared from the surface.
“We found that flow rates (0–17 cm s− 1) significantly affected the coral’s ability to shed sediment as did differences in particle sizes, with coarse silt rejected faster than fine silt, but only at very high (235 mg cm− 2) deposition rates.
“Noncarbonate siliciclastic sediment was rejected faster than carbonate sediments, and smothering for many days by millimetres of low organic content carbonate sediment resulted in bleaching, but no mortality.
“Estimating the sedimentation rate where the self-cleaning ability of corals is exceeded will improve our ability to make scientifically sound predictions of the likely extent, severity, and persistence of environmental impacts associated with dredging and can also be used with water quality monitoring during dredging to inform adaptive management,” Dr Duckworth said.
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.