A thin layer of sediment impacts coral settlement behaviour

Further research into dredging pressures on coral has revealed that even a very fine layer of sediment can prevent coral larvae from settling on a surface and building on the coral community.

Successful recruitment, where coral larvae attach and establish themselves as part of the adult community, is important for sustaining and recovery of coral reefs.

The research published in the Science of The Total Environment for the Western Australian Marine Science Institution’s Dredging Science Node found that coral larvae avoid sediment-covered surfaces, but will settle nearby on clean substrates including grooves and downward facing surfaces.


A conceptual diagram showing possible coral settlement behaviour and cause–effect pathways in response to sediment stressors, such as suspended sediments, deposited sediment and reduced light. (Ricardo et al)


The study also looked at how sediment affects coral behaviour with regard to calcareous red algae (CRA), which are rock-hard calcareous algae that contribute significantly to reef calcification and cementation, and induce larval settlement of many benthic organisms including corals.

Study author, Gerard Ricardo, from the Australian Institute of Marine Science, looked at coral settlement on sediment-covered and sediment-free surfaces under controlled conditions at the National Sea Simulator facility in Townsville.


VIDEO: A time-lapse of Acropora millepora larvae settling on calcareous red algae (CRA) recorded over 2.5 hours.


VIDEO: A time-lapse of Acropora millepora larvae settling on sediment-covered and sediment free calcareous red algae (CRA).


“We found a very fine layer of sediment prevents larvae from settling in two ways,” Gerard Ricardo said. “First by creating a physical barrier over the CRA, and second by deteriorating the quality of the CRA. This means that sediment deposited during dredging, and even following some natural deposition events, may influence where larvae settle on a reef.”

“These results have implications for reef recovery and resilience because of the overall reduction in optimal-substrate space, and the flow-on effects for newly-metamorphosed corals that settle in sub-optimal locations,” Mr Ricardo said


Ricardo G.F., Jones R.J., Nordborg M, Negri A.P. (2017) Settlement patterns of the coral Acropora millepora on sediment-laden surfaces. Science of the Total Environment doi.org/10.1016/j.scitotenv.2017.07.153

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside Energy, Chevron Australia and BHP Billiton 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 Australia, Woodside Energy and Rio Tinto Iron Ore. The commercial entities had no role in data analysis, decision to publish, or preparation of the manuscript.


Dredging Science