Westport has partnered with the Western Australian Marine Science Institution (WAMSI) to deliver the $13.5 million WAMSI-Westport Marine Science Program. This 3-year program is developing the latest data, information and modelling on the complex environmental systems and community values associated with Cockburn Sound.
Project 2.2: Seagrass buried in the dark: reduced light and moderate levels of burial have additive and lethal impacts on Posidonia sinuosa
Research theme: Benthic habitats and community
Researchers: N Said, C Webster, K McMahon, T Dunham, C Phelps, A Frouws, J Robinson, K Kilminster and S Strydom
Publication date: January 2025
Developing benchmarks for seagrass burial
There are a number of different stressors that can impact seagrass health, from light levels to sedimentation rates, as well water temperature.
How these stressors interact with each other is key to better environmental management practices.
Westport is developing a new container port in Cockburn Sound, which will include substantial construction and dredging.
This project investigated the synergistic effects of two key environmental variables that could be impacted by dredging activities: decreased light levels and increased sedimentation rates.
The hypothesis was that the two stressors combined would have an additive effect or greater impact on seagrass health. Experiments were done in-situ in seagrass meadows in Cockburn Sound, assessing the first 4 months of varying levels and combinations of impact, and the following 12 months of recovery, once the stressors were removed. A light reduction of 2 moles per square metre, over one day (2 mol m-2 d-1), was applied in conjunction with varying levels of burial sediment (0cm, 4cm, 8cm).
What the study found
- Low levels of burial (4 cm) applied over 4 months had no impact seagrass condition.
- Low levels of burial (4cm) did not result in additive impacts on seagrass already subject to reduced light levels.
- Moderate sediment burial (8 cm) had a lethal impact on seagrass, with significant leaf loss occurring after 1 month.
- Light reduction alone negatively impacted seagrass health after 2 months.
- Sediment burial of 8 cm, combined with light reduction had additive negative effects.
- Meadow density plays a role in resilience to pressures, with a higher leaf and shoot density beneficial for resistance and recovery from an impact. This suggests activities in areas of lower seagrass density need to more conservative management.
- Low levels of burial, of 4 cm or under, did not create lethal or sublethal conditions for seagrass, suggesting it is an appropriate threshold for sediment burial.
What the report recommended
This research demonstrated that as isolated and combined stressors, reduced light (2 mol m-2 d-1) and moderate burial (8 cm) have negative impacts on P. sinuosa seagrass.
How Westport will use this report
Westport will use this report to inform its dredging and environmental management strategies for the port development, with the intent of supporting the long-term health of seagrass and Cockburn Sound.
You can read the full report here.
