Abstract

Executive Summary:

• This study was conducted to generate knowledge to inform the environmental impact and prediction process for seagrass species that form ephemeral meadows in Cockburn Sound and Owen Anchorage.

• Local light thresholds for the opportunistic species, Syringodium isoetifolium were generated at summer and winter temperatures, with higher light requirements in summer compared to winter, and compared to larger, more slow-growing persistent species such as Posidonia sinuosa. The half-saturating irradiance (Ik) was 115 μmol m-2 s-1 at 23 °C.

• Based on the dynamics at three sites from December 2023 to April 2024, 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 and assessment of flowering or seed banks should occur to aid prediction of recovery potential and timescales.

• One colonising species that has been recorded in the past, Zostera nigricaulis, was rare within Cockburn Sound and Owen Anchorage, and there was not enough material available to conduct light threshold experiments or meadow dynamics, including recovery potential assessment. It was identified in another component of this WAMSI Westport Marine Project 2.2 Pressure response relationships, building resilience and future proofing seagrass meadows, that Z. nigricaulis is currently growing in conditions in summer that are above its thermal optima and is of high concern with warming oceans due to climate change. The low abundance could be due to impacts from ocean warming that have already manifested.

Keywords

seagrass ecology, seagrass meadows, ephemeral meadows, marine ecology, coastal ecosystems, environmental impact assessment, ecological prediction, light thresholds, irradiance requirements, temperature effects, seagrass growth, Syringodium isoetifolium, Halophila ovalis, Zostera nigricaulis, Posidonia sinuosa, Cockburn Sound, Owen Anchorage, seagrass recovery

Document Type

Report

Date of Publication

1-2025

Type of File

PDF

Publisher

Western Australian Marine Science Institution

School

School of Science / Centre for People, Place and Planet

RAS ID

100637

Funding Information

The $13.5 million WAMSI Westport Marine Science Program was funded by the Western Australian Government, Department of Transport. WAMSI partners provided significant in-kind funding to the program to increase the value to >$22 million.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Content Type

Full object

First Page

1

Last Page

22

Recommended Citation

McMahon, K., Said, N., Webster, C., Williams, H., & Strydom, S. (2025, January). Ephemeral seagrass meadow assessment to improve understanding of resilience of ephemeral seagrasses on Kwinana Shelf for Environmental Impact Assessment and Management. [Appendix A]Prepared for the WAMSI Westport Marine Science Program. Western Australian Marine Science Institution, Perth, Western Australia. https://wamsi.org.au/app/uploads/2025/02/WWMSP_2.2-Ephemeral_seagrass_assessment_FINAL.pdf

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