Seagrass

Seagrass.

Photo credit: © James Udy

Seagrasses are marine plants with the same basic structure as terrestrial (land) plants. They have tiny flowers and strap-like or oval leaves. They form meadows in estuaries and shallow coastal waters with sandy or muddy bottoms. Most closely related to lilies, they are quite different from seaweeds, which are algae. All seagrasses, dead and alive, are protected under the Fisheries Act 1994.

Read more about the ecology and value of seagrass.

Seagrass in the Great Sandy Marine Park

The Great Sandy Marine Park includes at least 2500km² of seagrass habitat. Dugongs and green turtles feed directly on seagrass but many more species live in seagrass beds. Small fish, seahorses, prawns and shellfish use seagrass meadows to shelter from predators, direct sunlight and extreme temperature changes.

Seagrass in the Great Sandy region is very seasonal. While abundance can vary throughout the year, the marine park consists of extensive areas of seagrass that grows on sand and mud in intertidal and subtidal areas to a depth of 32m.

Some of the seagrass habitats in the marine park include:

• the extensive meadows on the flats on the southern side of the Burrum River mouth
• the deepwater seagrass meadows of Hervey and Platypus Bays
• the dynamic intertidal and subtidal meadows along the coastline of the Northern Great Sandy Strait, Point Vernon to Woongarra, and in Hervey Bay
• the numerous dynamic intertidal and subtidal meadows in Great Sandy Strait, including at Poona, Boonooroo, Kauri Creek and Tin Can Bay.

Seagrass management project

The Department of the Environment, Tourism, Science and Innovation (DETSI) received funding from the joint Australian-Queensland government funded Disaster Recovery Funding Arrangements Environmental Recovery Program to investigate the impacts of the 2021/2022 severe weather events on seagrass.

Using this funding the Queensland Parks and Wildlife Service contracted James Cook University to conduct surveys and research on the impacts to and resilience of seagrass in Great Sandy Marine Park.

This important surveying and research help government understand the impacts to seagrass from the 2022 flooding events and its ability to recovery.

Background

• Both Great Sandy and Moreton Bay marine parks were heavily impacted by the 2022 flood events with a significant volume of freshwater, sediment, and nutrients entering the marine parks.
• Seagrasses are prone to the impacts of flood events due to freshwater stress, sediment smothering, increased sediment in the water and algal growth reducing light availability, toxicants reducing plant growth, and physical damage from flood debris.
• The rate of seagrass recovery is dependent on its pre-disturbed state and condition (e.g., quantity of seedbank reserves, availability of fragments), and the persistence of the event (e.g., extent and duration of sediment resuspension via tidal flows and wave action).
• Extensive seagrass surveys had not been completed in Hervey Bay since 1998. To understand the impacts to, and recovery of, seagrass following the 2022 flooding event, extensive surveys were completed in May 2022 (i.e. three months post-flood, funded by DESI Aquatic Ecosystem Health Unit), October-November 2022 (i.e. nine months post-flood) and August-October 2023 (i.e. 21 months post-flood).

Summary of 2022 survey findings

• Compared to the pre-flood (1998) extent (2,307km²), the area of seagrass declined by 68 percent (744km²) three months post-flood but demonstrated signs of recovery nine and 21 months post-flood, containing 58 percent (1,343km²) and 74 percent (1,700km²) of the pre-flood extent, respectively.
• Seagrass recovery, however, mostly occurred in deep subtidal meadows, which increased significantly after 21 months, but most intertidal areas remained lower than pre-flood recordings, suggesting that deep-water seagrasses are more resilient to the impacts of the flood plume than intertidal communities.
• This increase in seagrass cover and extent in subtidal meadows was mostly due to the colonisation of Halophila decipiens around the periphery of the large deep-water Halophila spinulosa (mixed with Halophila ovalis and H. decipiens) meadow in the middle of the bay.
• Total seagrass extent in the Great Sandy Strait (GSS) more than tripled post-flood (i.e. extent increasing from 12km² after three months to 53km² after 21 months) and was identical to the pre-flood extent (53km²).
• Despite the full recovery of seagrass extent in the GSS, the distribution of meadows changed with very little seagrass being recorded in remnant pre-flood meadows between the Mary River and Moonboom Islands.
• Overall seagrass cover in the GSS remained low compared to pre-flood surveys, but significant increases were found at Moon Point, Urangan, Poona and Tin Can Bay after 21 months post-flood.
• Intertidal meadows of the GSS were, and continue to be, dominated by Zostera muelleri and Halodule uninervis.
• Species such as Cymodocea serrulata and Syringodiumisoetifolium remain undetected since pre-flood surveys.
• Subtidal light monitoring in Hervey Bay indicated levels above minimum requirements for Halophila species between March and August 2023, which likely contributed to the further recovery of this species.
• Light levels for other species (such as H. uninervis and Z. muelleri) in subtidal areas were well below growth requirements and could be impeding the recovery of this species.
• Megaherbivore exclusion studies at Moon Point indicated herbivory was not driving seagrass declines or significantly contributing to impeding recovery.
• Seed bank assessment found low numbers of H. uninervis and Z. muelleri seeds in Hervey Bay compared to other areas of Queensland. Low numbers indicate the lack of a large persistent seed bank and long-term resilience.
• Continued recovery is expected with favourable growing conditions and the absence of major flooding.

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) .