Abstract

Executive summary: Understanding the nature of interactions between multiple stressors is essential for assessing their cumulative impacts on seagrasses and for guiding effective management. Current dredging thresholds are based largely on single stressors (light reduction or sediment burial), despite pressures typically co-occurring, and these thresholds may therefore need refinement to account for interactive effects. In addition, recovery times under single and multiple pressures remain a major knowledge gap, further reducing our ability to accurately predict impacts and implement effective management. This need is particularly pressing in regions facing ongoing dredging pressure. Cockburn Sound, which has already lost ~75% of its seagrass (Kendrick et al. 2002) and has further dredging proposed, requires robust threshold information to support Environmental Impact Assessment and Management (EIA&M). This report builds on the WAMSI-Westport 2.2 project “Seagrass buried in the dark: reduced light and moderate levels of burial have additive and lethal impacts on Posidonia sinuosa” (Said et al., 2025), an in situ experiment on the Kwinana Shelf in Cockburn Sound, addressing the cumulative impacts of, and recovery from, sediment burial and reduced light on the seagrass Posidonia sinuosa. To support the EIA for Westport and future developments, the in situ experiment included a 4-month impact phase followed by a 2.5-year recovery phase, consistent with the EPA (2021) zonation scheme (zone of influence, moderate impact, and high impact). This study simulated the cumulative impacts of reduced light (~2 mol photons m-2 d-1 compared to ambient) and sediment burial (0 cm, 4 cm, 8 cm) on P. sinuosa. Here, we present an additional 1.5-year dataset extending the seagrass recovery observations to 2.5 years.

Keywords

seagrass, Posidonia sinuosa, cumulative impacts, multiple stressors, sediment burial, light reduction, dredging impacts, seagrass recovery, environmental impact assessment, environmental management, dredging thresholds, marine ecology, coastal ecology, Cockburn Sound, Kwinana Shelf, seagrass decline, habitat restoration, ecosystem resilience

Document Type

Report

Date of Publication

12-2025

Type of File

PDF

Publisher

Western Australian Marine Science Institution

School

School of Science / Centre of Marine Ecosystems Research / Centre for People, Place and Planet

RAS ID

100635

Funding Information

Funding for the program has been provided by Westport (through the Department of Transport) and the science projects are being delivered by the Western Australian Marine Science Institution.

Content Type

Full object

First Page

1

Last Page

14

Recommended Citation

Said, N., Webster, C., Strydom, S., McMahon, K., (2025, December). Pressure-response relationships, building resilience and future proofing seagrass meadows [Appendix C].Prepared for the WAMSI Westport Marine Science Program. Western Australian Marine Science Institution, Perth, Western Australia. https://wamsi.org.au/app/uploads/2022/06/WWMSP_2.2.13_Appendix-C-Ongoing-recovery-measurements-in-Kwinana-Shelf-seagrass-meadow-in-CS-FINAL.pdf

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