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 Environment, Science and Innovation (DESI) 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- event state and condition, persistence of the event, seed bank reserves, and availability of fragments. The continual resuspension of fine sediments due to tidal flows and wave action prolongs the events impact in subtidal areas.
• Extensive seagrass surveys had not been completed in the Great Sandy Marine Park since 1998. Therefore, to understand the impacts to and recovery of seagrass post- 2022 flooding events, extensive surveys of both marine parks was required.

Survey and research trip updates

• Surveys were completed using drop camera recordings and aerial surveys between October and November 2022, and re-surveyed between August and October 2023 to collect further information on the recovery of seagrass following the 2022 flooding events.
• The Great Sandy marine park was also surveyed in May 2022 funded by the DESI Aquatic Ecosystem Health unit.
• Projects to monitor the health and resilience of seagrass are also being undertaken.
• Light availability within the Great Sandy Strait is being monitored, an herbivory exclusion study is looking at how marine life feeding on seagrass impacts recovery, and seagrass seed bank availability is being examined.
• Results from these projects are expected to be available mid-late 2024.

Summary of 2022 survey findings

• Great Sandy Marine Park seagrass ecosystems, particularly intertidal meadows consisting of Zostera muelleri, Halodule uninervis and Halophila ovalis,are well below the extent and abundance reported in 1973, 1998 and 2002.
• The area of seagrass observed in May 2022 surveys was reduced to around a third the size of the distribution mapped in 1998 when the seagrass had not been impacted by flooding.
• The deep-water seagrasses in the middle of Hervey Bay appear to have been better able to recover from the impacts of the flood plume than intertidal communities.
• The May 2022 distribution and mean seagrass percent cover is comparable to that of the post-flood monitoring undertaken in 1992 following a similar series of sequential events (two floods and a cyclone). By the October 2022 survey, the deep-water seagrasses were already showing good signs of recovery with considerable expansion of the meadow and an increase in abundance measured as above ground biomass and percentage cover of the seabed. Intertidal seagrass communities in both Hervey Bay and Great Sandy Strait remained extremely low in abundance compared to historical broad scale surveys with very little recovery detected from May to October.
• It is difficult to determine the impact of the flood in these areas in the absence of recent broad scale assessments and Seagrass Watch data from some intertidal monitoring transects in the region show that seagrass abundance was low prior to the flood at those locations.
• We expect to see further seasonal recovery in Hervey Bay and the Great Sandy Strait toward the end of 2023 but the likely extent of this is unknown. Previous seagrass losses in the region have taken at least three years to recover.

Read the full report: Post-flood seagrass monitoring in the Great Sandy Marine Park 2022.