Dredging Science: Understanding how low light levels can affect coral health

Research has confirmed that some coral and important reef building algae can sustain being exposed to low light conditions for up to 10 consecutive days before their health is critically affected.

The findings have important implications for the environmental impact assessment and management of dredging operations, which cause sediment plumes.

The results of the Western Australian Marine Science Institution’s Dredging Science Node research, conducted in specially developed tank systems at the AIMS National Sea Simulator (SeaSim), have been published in Scientific Reports.

Lead researcher Pia Bessell-Browne from The University of Western Australia Oceans Institute, Centre for Microscopy, Characterisation and Analysis and Australian Institute of Marine Science said corals and important reef building algae are particularly susceptible to light limitation resulting from elevated sediment particles in the water column which can be generated by naturally occurring events, such as cyclones, as well as coastal development activities such as dredging.


Experimental tank set up at the AIMS National Sea Simulator used to determine the impacts of 6 light levels on both coral and algal health. (SeaSim)


“This study has determined light levels that need to be maintained in order to reduce negative impacts on the health of coral and reef building algae,” Ms Bessell-Browne explained. “When corals do not receive adequate light, the algae that live within their tissues and provide them with up to 90 per cent of their daily energy requirements leave, meaning the corals are without their main food source and can starve if this condition is maintained over extended time frames.”

A range of low light levels were investigated and their impacts on several species of corals, along with juvenile corals and a species of reef building algae were determined.


Photographs of representative A. millepora and P. acuta fragments after 30 d of exposure to the six daily light integral (DLI) irradiance treatments of ~0, 0.02, 0.1, 0.4, 1.1 and 4.3 mol photons m−2 d−1 (Bessel-Browne et al., (2007))


“The results demonstrate that both corals and algae are sensitive to exposure to low light conditions for more than 10 consecutive days,” Ms Bessell-Browne said. “Juvenile and adult corals have similar low light tolerance, while the reef building algae is more sensitive to the low light conditions than the corals.”

The thresholds determined by these research findings are expected to contribute to the environmental impact assessment and management of dredge programs where these coral and algae species occur in the field.

Bessell-Browne P, Negri A.P., Fisher R, Clode P.L., Jones R (2017) Impacts of light limitation on corals and crustose coralline algae Scientific Reports doi:10.1038/s41598-017-11783-z


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

Branching corals are better at rejecting dredging sediment

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.

Duckworth A, Giofre N, Jones R (2017) Coral morphology and sedimentation Marine Pollution Bulletin doi.org/10.1016/j.marpolbul.2017.08.036

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

SNAPPING BACK: Saltwater crocodiles back from the brink

For many people, a primeval fear of large carnivores clouds their ability to see how top-order predators struggle to survive on a planet dominated by an ever-intrusive human population. Lions, tigers, wolves, bears, sharks and crocodiles have all suffered extensive population losses due to the proximity of their habitats to cities and towns. Today, Australia has sophisticated nature management capabilities supported by appropriate legislation and enforcement. However these capabilities were not present when the saltwater crocodile was hunted to the brink of extinction between 1950 and 1970. It was only after a national ban on hunting crocodiles was put in place around 1970 that crocodiles were able to begin a slow process of recovery which continues to this day. FULL STORY

This article appears in the Spring 2017 issue of LANDSCOPE, published by the Department of Biodiversity, Conservation and Attractions. For more information, and to order a copy of the magazine or purchase a subscription, visit shop.dpaw.wa.gov.au.



Kimberley Marine Research Program

Critical sawfish nursery habitats identified in Fitzroy River

An eight-year study into the movement of critically endangered sawfish in the isolated freshwater reaches of northwestern Australia’s Fitzroy River has identified the habitats that are important to their survival.

A group of Murdoch University researchers has found that deep pools and shallow environments, like glides, are important habitats for the Freshwater (Largetooth) Sawfish (Pristis pristis) and that restriction of flow or altering of river pathways could jeopardise these environments.

Little is known about the movements of these sawfish which rely on the intermittently flowing rivers and estuaries of the Fitzroy River as a globally significant  nursery. Locally they have been recorded up to 400 kilometres from the coast.

With increasing pressure from fishing and the impact to their migration by instream barriers the researchers, supported by the Western Australian Marine Science Institution, Chevron Australia, Western Australian Government’s State Natural Resource Management Program and people of the Kimberley, including the Nyikina-Mangala Rangers, have been working to identify the habitats and conditions that need to be considered in conservation and management decisions for the region.

Lead researchers Jeff Whitty and Associate Professor David Morgan explained the study, published in Endangered Species Research, was conducted over the 2008 to 2015 dry seasons (May or June to November), when the Fitzroy River is transformed into a series of isolated reaches in which the sawfish are trapped.

Two freshwater reaches of the Fitzroy River were monitored that were located between 120 kilometres and 150 kilometres upstream from the river mouth.

The researchers monitored the movements of 32 juveniles sawfish (952 to 2510 mm in length) using acoustic telemetry, with sound-emitting transmitters tracked by a series of loggers over an eight-year period.



“This study demonstrated for the first time that juvenile Freshwater Sawfish are least active by day, when they occupy deeper runs and pools near large woody debris,” Mr Whitty said. “They are most active during night-time and twilight hours in shallow water such as glides, pool edges, and shallow runs, when their prey are also more abundant in shallow waters.

“More observations need to be done, however, on why the juvenile sawfish move to the deeper pools in the day,” Mr Whitty said. “It appears unlikely that they are moving to deeper water only to conserve energy in cooler water because, at least during the early dry season, there’s little difference between surface and bottom water temperatures.”

Some of these questions are being addressed by PhD researcher, Karissa Lear, a Forrest Foundation Research Scholar, who is using accelerometers (the same technology that is in FitBits, smart phones, and other smart devices) to study sawfish behaviour.


PhD student Karissa Lear with a sawfish pup temporarily held at Kimberley Training Institute in Broome to examine metabolic rate. (David Morgan, Murdoch University)


Ms Lear is also examining the metabolic rate of sawfish while they are wearing their accelerometers, which tells how much energy each specific behaviour costs.

“These studies will allow us to put accelerometers on wild sawfish in the Fitzroy, look at what they are doing in their natural environment, estimate how much energy they are using based on their behaviour, and importantly, see how their behavioural patterns, feeding effort, and energy use change throughout the dry season as temperatures increase,” she said.



Sawfish Project