Seagrass

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Seagrass mixed Halophila species

Photo credit: © James Cook University

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Seagrass Zostera muelleri

Photo credit: © James Cook University

Seagrass meadows form one of the most widespread marine habitats occurring in estuarine and coastal waters across every continent except Antarctica.¹

These unique marine plants share a similar structure to terrestrial (land) plants, featuring small flowers and strap-like or oval leaves. Unlike seaweeds, which are algae, seagrasses are more closely related to lilies. Seagrasses establish in estuaries and shallow coastal areas with sandy or muddy seabeds, forming vital underwater meadows. To thrive, seagrasses require sunlight, clear water and nutrients.

Under the Fisheries Act 1994, all seagrasses—whether living or dead—are protected to ensure their conservation and the health of marine ecosystems. Seagrasses within the Great Sandy Marine Park are also protected under a Marine Park Zoning Plan. This protection is achieved through specific zoning measures, such as green and yellow zones, which prohibit certain fishing activities like trawling. Additionally, special management strategies are in place, including designated no anchoring areas to prevent physical damage to sensitive habitats caused by anchors. Designated go-slow areas also help protect seagrass beds by minimising potential harm from vessel motors.

Read more about the ecology and value of seagrass.

Seagrasses of the Great Sandy Marine Park

There are over 70 species of seagrass worldwide, half of which are found along Australia’s coastline. The Great Sandy Marine Park hosts seven of these species, representing three of the four seagrass families found in subtropical coastal waters.²

• Halophila ovalis
• Halophila decipiens
• Halophila spinulosa
• Zostera muelleri
• Cymodocea serrulata
• Halodule uninervis
• Syringodium isoetifolium

The Great Sandy Marine Park is home to over 2,300 km² of seagrass habitat, providing a vital food source for dugongs and green turtles. Additionally, seagrass meadows support a diverse range of marine life, offering shelter and protection from predators, sunlight, and temperature extremes for species such as small fish, seahorses, prawns and shellfish.

Seagrass in the Great Sandy region is highly seasonal, with its cover and extent fluctuating throughout the year. Despite this variability, the marine park features extensive seagrass meadows growing on sandy and muddy seabeds in both intertidal and subtidal zones, reaching depths of up to 32 metres.

Seagrass management project

The department secured funding from the joint Australian-Queensland government funded Disaster Recovery Funding Arrangements Environmental Recovery Program to examine the effects of the 2021/2022 severe weather events on seagrass.

To support this initiative, Queensland Parks and Wildlife Service and Partnerships engaged James Cook University to conduct surveys and research into the impacts and resilience of seagrass in Great Sandy Marine Park. The research aimed to better understand the effects of the 2022 flooding events on seagrass and its capacity for recovery.

Background

• The 2022 flood events had a significant impact on the Great Sandy Marine Park, with large volumes of freshwater, sediment and nutrients entering the ecosystem.
• Seagrasses are particularly vulnerable to flood events due to factors such as freshwater stress, sediment smothering, increased sediment and nutrient levels reducing water clarity, algal growth limiting light availability, toxicants hindering plant growth and physical damage caused by flood debris.
• The recovery of seagrass depends on its condition prior to the disturbance (e.g., size of seedbank reserves and availability of fragments) and the duration and intensity of the event (e.g., sediment resuspension caused by tidal flows and wave action).
• Comprehensive seagrass surveys had not been conducted in the Great Sandy Marine Park since 1998.
• To assess the impacts of the 2022 flooding event and monitor seagrass recovery, surveys were carried out at intervals of three months (May 2022), nine months (October–November 2022), 20 months (August–October 2023), and 33 months (August–November 2024) post-flood. Preparations are underway for another round of surveys 45 months (October-November 2025) post-flood.

Summary of project findings

• The flooding of the Mary River in early 2022 resulted in persistent flood plumes, significantly impacting the extent and density of seagrass meadows within the Great Sandy Marine Park.
• Seagrass meadows in Hervey Bay and the Great Sandy Strait have shown significant recovery, particularly in intertidal meadows, which have increased in density and area, but remain below pre-flood levels.
• Deep-water seagrasses in Hervey Bay remained relatively stable, with some areas experiencing slight increases or declines in abundance. The largest decline was observed in the shallow subtidal meadow offshore from Burrum Heads.
• Continued annual monitoring and broadscale surveys every five years are recommended to track long-term recovery trends.

Project reports

2024 August-November Monitoring Report
Read the full report: Post-flood seagrass monitoring in the Great Sandy Marine Park – 2024 (PDF, 8.2MB) .

2023 August-October Monitoring Report
Read the full report: Post-flood seagrass monitoring in the Great Sandy Marine Park – 2023 (PDF, 13.7MB) .

2022 October-November Monitoring Report
Read the full report: Post-flood seagrass monitoring in the Great Sandy Marine Park – 2022 (PDF, 10.8MB) .

2022 May Monitoring Report
Read the full report: Post-flood seagrass monitoring in Hervey Bay May 2022 (PDF, 5.4MB) .

More information

References / further reading

1. Green, E.P. and Short, F.T. (2003) World atlas of seagrasses, University of California Press.
2. Waycott, M., McMahon, K., Mellors, J., Calladine, A. and Kleine, D. (2004) A guide to tropical seagrasses of the Indo-West Pacific, James Cook University, Townsville.