Back-to-back floods impact seagrass meadows

The Great Sandy Strait and Hervey Bay regions have some of the largest and most ecologically important seagrass meadows in eastern Australia. These habitats support dugongs and green turtles as well as much of the region’s marine biodiversity.

Floodwater runoff can have severe impacts on the species that rely on these habitats. By carrying large amounts of sediment into the environment, floodwaters can reduce light and potentially smother seagrass meadows.

Floods in 1991 led to the loss of over 1000 km2 of seagrass in the Hervey Bay region, significantly impacting dugong populations. In early 2022, major back-to-back floods from the Mary River again carried significant sediment into marine waters, causing significant damage to seagrass meadows resulting in significant mortality of dugongs and sea turtles.

This threat prompted scientists to initiate ongoing large-scale surveys to understand the long-term impact of the floods on seagrass meadows.

Recent dugong surveys have also shown a large decline in both adult and calf dugong numbers in Hervey Bay.

Surveys show ongoing impacts on seagrass since floods

In response to the 2022 floods, scientists from JCU TropWATER and rangers from the Department of Environment and Science undertook surveys to understand seagrass loss and its impact on dugong and turtles.

To assess the extent of seagrass loss in Hervey Bay and the Great Sandy Strait after the floods, surveys were conducted in 2022, 2023, and 2024 to track recovery.

Over 2,300 km2 were surveyed by helicopter and by boat. Surveyed areas were divided into 353 sites based on where seagrass had previously been recorded.

2022 surveys: widespread seagrass declines

• Immediately after the 2022 floods, 80% of previously recorded seagrass meadows were gone. The remaining meadows showed substantial decline.

• The surveyed area in Hervey Bay revealed an area of just 744 km2 that retained some seagrass, representing just one third of the previously recorded meadow.

• The remaining seagrass showed significantly reduced plant cover and biomass, and nearly all seagrass was found in deeper waters. Only one shallow-water meadow remained intact.

2023 surveys: recovery begins

• Follow-up surveys in 2023 indicated that seagrass recovery is underway.

• There are concerns that the seagrass ‘seed bank’ was impacted during the floods and could slow regeneration.

• Results from the 2023 surveys are available here.

Recent dugong surveys have also shown a large decline in both adult and calf dugong numbers in Hervey Bay. Read more about the dugong surveys here: https://www.tropwater.com/projects/large-scale-monitoring-of-dugong-populations-across-northern-australia-

Data to form the foundation of future restoration strategies

Seagrasses form the foundation ecosystem for much of the region's marine biodiversity. Meadows need long-term monitoring and supported restoration efforts so that valuable species are not lost. These surveys will help provide critical data insights, including:

• Baseline post-flood data for seagrass and dugongs in 2022. This has since been compared to 2023 data and will serve as a point of comparison for future surveys to reveal changes over time.

• Valuable insights into seagrass ecosystems and the effects of disruption on dugong and sea turtle populations from the 2022 and 2023 surveys. This information will be important for ongoing monitoring under predicted increases in extreme weather events due to climate change.

• Knowledge of regional-scale seagrass meadow collapse. This will drive research into meadow restoration and propagation techniques by TropWATER scientists.

Project details

Surveys were funded by the Department of Environment and Science (DES).