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 Moreton Bay Marine Park

Seagrass meadows form one of the most widespread marine habitats. They occur in the coastal waters around all of the world's continents, except Antarctica. ¹ There are approximately 60 species of seagrass worldwide, of which seven are found in Moreton Bay Marine Park.

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

These species represent all three seagrass families. Some seagrass species in the marine park are not found any further south on the east coast. ²

The distribution of seagrass in Moreton Bay Marine Park is primarily determined by the amount of sunlight that reaches the sea floor which depends on the turbidity (or murkiness) of the water. In the western bay where the water is more turbid, seagrass is restricted to shallower areas where only six percent of the surface light reaches the sea floor, which is just enough for this species to survive. ³

Seagrass plays a critical role in the health of the Moreton Bay Marine Park. ⁴ It provides habitat for many marine park species and is the primary food source of dugong and some marine turtle species.

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 Science Under Sail Australia to conduct surveys and research on the impacts to and resilience of seagrass in Moreton Bay 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 Moreton Bay since 2011. Therefore, to understand the impacts to and recovery of seagrass post- 2022 flooding events, extensive surveys were required.

Survey and research trip updates

• Surveys were completed using drop camera recordings between October 2022 and January 2023, and re-surveyed between August 2023 and January 2024 to collect further information on the recovery of seagrass following the 2022 flooding events.
• Projects to monitor the health and resilience of seagrass are also being undertaken.
• In the Moreton Bay Marine Park  seagrass biomass and seed bank availability will be examined.
• Results from these projects are expected to be available mid-late 2024.

Summary of 2022 survey findings

• Moreton Bay Marine Park seagrass ecosystems are well below the extent and abundance previously reported as a result of the wet 2021-22 austral summer, particularly in deeper areas.
• The area of seagrass has reduced by 30 percent since the previous surveys completed in 2020/21 pre-flooding.
• Seagrass was absent from many areas where it had been previously observed, including the central sections of the Pumicestone Passage (e.g. Tripcony Bight) and many areas of Southern Bay and the Gold Coast where large deposits of sediment likely occurred.
• The largest shift was observed at the Eastern Banks suggesting reductions in water clarity were greater in the eastern Bay than in the western Bay.
• The average and maximum observed depth of all seagrass species declined, as a likely consequence of reduced benthic light.
• The species most affected was Halophila spinulosa, with distribution and % cover both declining. The decline in H. spinulosa was greatest in the deeper habitats (deep subtidal (>2m)), with the maximum depth this species was observed at post flood decreasing significantly.
• Cymodocea serrulata and Syringodium isoetifolium were only rarely observed, with their distribution restricted to a few areas with relatively good water clarity.
• Seagrass density decreased significantly in all depth categories, with some regional and species-specific variations.
• The greatest decrease in % cover was observed in deep subtidal areas (>2m).
• Recovering seagrass meadows had been recorded in northern Bramble Bay pre-2022 flood, but this is the first known documentation of extensive meadows (up to 80% cover of Halophila ovalis) in the middle of Bramble Bay.
• Further seasonal recovery is expected toward the end of 2023.

More information

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
3. Udy, J. and Levy, D. (2002) Deep seagrass and coral habitats found in Eastern Moreton Bay, University of Queensland, Brisbane
4. Dennison, W.C. and Abal, E.G. (1999) "Moreton Bay study: A scientific basis for the Healthy Waterways campaign", SEQ Regional Water Quality Management Strategy, Brisbane