Effective management of dredging operations requires understanding the effect of the activities on the surrounding environment including important habitats like coral. New research conducted by researchers from the Australian Institute of Marine Science (AIMS) and The University of Western Australia (UWA) has supported decision-making in this field by defining a series of thresholds that can be used to reliably predict the impact of water quality levels on coral health.

The study, conducted as part of the Western Australia Marine Science Institution Dredging Science Node, used a comprehensive water quality and coral monitoring dataset collected by industry during a past dredging project that occurred off Western Australia.

“Essentially, we’ve used this data to understand how turbid the water can get before coral health begins to be affected,” AIMS Ecological Risk Assessment Modeller and lead author Dr Rebecca Fisher said.

The researchers examined water quality measurements, such as light levels, turbidity and sedimentation, and compared them to observed changes in the health of hundreds of individually marked corals that were surveyed each fortnight. This led to an improved understanding of how corals in shallow, reef environments respond to changing water quality which then allowed the team to develop water quality ’trigger-points‘– or thresholds – at which the corals were negatively affected. Cutting-edge modelling techniques were applied to account for uncertainty in the assessment of coral health in order to increase the reliability of the water quality thresholds.

Figure 3 from Fisher et al –  Example coral mortality and water quality data used for analyses, showing time series of proportional live coral cover measurements for representative colonies of each taxa (black circles) and mean daily turbidity (NTU; red lines). Solid black lines represent GAM smooths (see Appendix S2 for details) with grey bands indicating 95% confidence bounds, fitted using a beta distribution to the proportion of live coral cover. Right hand panels show images of each colony at the start of dredging and immediately preceding the start of bleaching (day 203, used as the end point in the present analysis, Figure 2)

 

“The dataset used was particularly valuable because it contained measurements of water quality and coral health close to the dredging, in areas where it was expected and approved by the regulators that corals would be impacted through to areas further away where there was no effect,” Dr Fisher explained. “It’s important to understand how corals respond when exposed to a full range of conditions, including those that are more extreme. This provided the information needed to build a pressure:response curve: without observing those upper limits, you just have no way of finding thresholds.

“This work will be useful to industry to improve confidence in managing dredging operations to protect the environment. It will assist managers and regulators assessing future proposals to provide confidence about the potential impacts to coral reef environments, based on the predicted water quality conditions,” Dr Fisher said.

Links:

• Fisher R, Walshe T, Bessell-Browne P, Jones R. (2017) Accounting for environmental uncertainty in the management of dredging impacts using probabilistic dose–response relationships and thresholds. J Appl Ecol. doi.org/10.1111/1365-2664.12936
• Read more about AIMS dredging research.

 

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