We have known for more than a decade that suspended sediments from dredging and other sources could impact coral fertilisation, but we didn’t know why. A WAMSI Dredging Science project is working to find the answers.

Lead researcher Gerard Ricardo from The University of Western Australia and the Australian Institute of Marine Science said the project team first designed an experiment to discover whether sediment was impacting the sperm or the eggs.

“We found that while the eggs were capable of fertilising in the presence of high sediment concentrations, far more sperm were needed to achieve adequate fertilisation success,” Gerard said. “This indicated that the sediment was ‘taking-out’ the sperm and preventing them from contacting the egg.”

“When we added sperm to the suspended sediments and noticed small flocs (flakes) appearing on the bottom of the containers, indicating the sediments were sticking to the sperm and both were sinking away from the floating eggs. We confirmed this hypothesis using microscopy to examine the flocs, as well as a sperm counts at the water surface which revealed a decrease in sperm numbers.

“Our results suggest that sediments may shrink the fertilisation window – a brief 1-2 hour period when sperm and eggs can fertilise before wind and waves dissipate them.

“The next step will be to determine what properties of the sediments cause the flocking and sinking of the sperm and subsequently which sediments present the greatest risk if dredging occurs during coral spawning events,” Gerard said.

The findings have been published in Scientific Reports: G. Ricardo, R, Jones, P. Clode, A. Humanes, A. Negri (Dec 2015) Suspended sediments limit coral sperm availability  doi:10.1038/srep18084

Fact File:

• Sediments can reduce the amount of sperm available to fertilise the egg
• Sperm can become tangled up in sediment flocs
• Multiple lines of evidence were used including fertilisation assays, flow cytometry and optical and electron microscopy to determine the mechanism that is responsible for the decrease in fertilisation with elevated suspended sediments.
• Sediment may further shrink an already brief (1-2 hour) ephemeral fertilisation window.

The work was carried out at the National Sea Simulator (AIMS) and the Centre for Microscopy, Characterisation and Analysis (UWA).

Naitonal Sea Simulator (SeaSim)

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