2. Biodiversity

Seagrass meadows are among the most extensive and important habitats in the Region, providing stability to the seafloor,¹³¹ cycling nutrients,¹³² and sequestering and storing carbon.^133,134 They are considered key ‘blue carbon’ ecosystems.^117,127 Approximately 3464 square kilometres of inshore seagrass meadows have been mapped in the World Heritage Area, and approximately 32,000 square kilometres occur in deeper waters.¹³⁰ The total estimated area represents nearly half of Australia’s, and between 13 and 22 per cent of the world’s, seagrass meadows — making the Region’s seagrass resources globally significant.^133,135 

Seagrass meadows provide shelter and anchoring structures for small fish, prawns, and various other invertebrates.^136,137 These provide food for both resident and migratory fish species, thereby facilitating connectivity and productivity of fisheries.^138,139 Deep-water meadows tend to support larger-bodied fish and invertebrates but are not as productive as inshore meadows for fisheries resources.¹⁴⁰ Seagrasses are also a food source for dugongs and green turtles, which are important to First Nations communities.¹⁴¹ These larger grazers play critical roles in structuring and connecting seagrass meadows.^142,143

The Region’s seagrass ecosystems are a complex mosaic of different meadow types with multiple species.¹⁴⁴ They are found in estuaries, sheltered intertidal and subtidal coastal areas, in deep water, and on coral reefs.^144,145 All are subject to frequent disturbance from wave and air exposure, for example, and naturally fluctuate in extent and condition.⁴²

Deep-water meadows, which grow below 15 metres and down to 76 metres,¹⁴⁶ are poorly documented compared to shallower meadows, and their extent is largely inferred from models of habitat suitability.¹⁴⁷ They are usually sparse and have a high capacity for recovery as long as seed reserves remain intact.¹⁴⁸

The primary threat to seagrass meadows is poor inshore water quality caused by land‑based runoff or wind-driven resuspension of sediments (Section 6.5.2). Poor water quality reduces the amount of light available to seagrasses for photosynthesis, either directly (suspended sediments absorb and reflect light) or indirectly (excess nutrients promote the growth of algae in the water, on the sea floor, and on the seagrass itself) (Section 3.2.7).¹⁴⁵ Pesticides from land-based run-off, thermal stress from rising sea temperatures,⁴² anchor damage, and acute disturbance events, such as cyclones, can also damage these habitats. In turn, seagrass habitat depletion affects dependent species, such as dugongs (Section 2.4.16) and marine turtles (Section 2.4.10).

Long-term seagrass condition is measured through seagrass abundance and resilience, which are summarised as a seagrass condition index.¹⁴⁹ Reef-wide seagrass abundance reached a low point in 2019–2020 due to cumulative pressures and legacy impacts of cyclones, floods and heat stress, but condition has since improved. Increase in seagrass abundance is one of the factors underlying the recovery in seagrass condition. In approximately 60 per cent of the monitoring sites, abundance either improved or remained stable over the past 3 monitoring years (2020–21, 2021–22 and 2022–23). Since 2019, seagrass losses in the northern regions have subsided. The greatest improvement was seen in the Burdekin Natural Resource Management (NRM) region as it recovered from the effects of heavy rainfall and above-average discharge in early 2019. However, declines in abundances were detected in all 3 southern regions during 2020–21 and although these declines have abated in the Mackay Whitsunday and Fitzroy NRM regions, they have continued in the Burnett Mary NRM region (Figure 2.4).¹⁴⁹ Data from monitoring programs will assist in determining the effects of the high temperatures, cyclones and associated flooding that occurred during summer 2023–24.

A new resilience metric has been developed and included as part of the seagrass condition index since the 2020–21 reporting period ¹⁵⁰ (Section 8.3.2). In the past 5 years, resilience has generally improved. However, in the southernmost regions, measured resilience continued to decline over the past 3 monitoring years (2020–21, 2021–22 and 2022–23), and these meadows remain vulnerable to further disturbances. Across the Region, the recent impact of marine heatwaves is likely impairing recovery in exposed areas, even when meadows exhibit a substantial level of resilient traits.¹⁴⁹

Seagrasses are affected by increasing temperatures and localised poor water quality. Recent shifts away from colonising species in northern regions are indicative of post-disturbance recovery. Due to impacts from flooding, seagrasses in the southern Fitzroy and Burnett Mary regions have not shown the same recovery. Information on condition of deep-water seagrass meadows is limited. Acknowledging regional variation, overall condition across the Region is assessed as good (borderline poor). In some parts of the Region, summer 2023–24 brought multiple stress events, the effects of which are not yet known.