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: Appendix A: Ephemeral seagrass meadow assessment to improve understanding of resilience of ephemeral seagrasses on Kwinana Shelf for Environmental Impact Assessment and Management
Research theme: Benthic habitats and community
Researchers: K McMahon, N Said, C Webster, H Williams and S Styrdom.
Publication date: April 2025
Understanding drivers of seagrass decline around Perth water
Seagrasses like Zostera (Heterozostera) and Halophila, are observed on Kwinana Shelf and form ephemeral or seasonal meadows.
The seagrass mapping work undertaken through the WAMSI-Westport Marine Science Program found seasonal seagrass habitats were likely to form near to the development footprint of the new port in the Kwinana Shelf.
This research project was undertaken to understand the biological thresholds of these ephemeral seagrasses, to understand potential impacts from port development, and their ability to recover.
Three sites across Cockburn Sound and Owen Anchorage were assessed to understand the different species of seagrass evident, the seasonal behaviour, and the impacts of different pressures such a light.
What the study found
- Seagrass species Halophila ovalis forms ephemeral meadows in Cockburn Sound and Owen Anchorage with meadows disappearing between February and April. Flowering was observed at all meadows in December, but only one of the three meadows produced a viable seed bank. Therefore, the recovery potential of these meadows is likely to vary between sites.
- The species Zostera nigricaulis, which has been recorded in Cockburn Sound and Owen Anchorage previously, was not evident during this study. As it’s a species impacted by warming waters, this could be due to impacts from ocean warming.
- Light thresholds for the species, Syringodium isoetifolium were evaluated at summer and winter temperatures, and found the species had higher light requirements in summer compared to winter, and higher requirements compared to larger and slower-growing persistent species such as Posidonia sinuosa. Positively, the light threshold was also lower than previous international seagrass research had found.
What the report recommended
The report found that ephemeral seagrass meadows are more likely to be resilient to dredging pressures during cooler months, when the meadows are naturally declining, when temperatures are lower, and the risk from light reduction is of lesser concern.
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.
