2.4.2 Seagrasses

Seagrasses are the only flowering plants to grow submerged in the ocean.257 The Region is home to 15 seagrass species, around 20 per cent of the world’s total.258 One deep‑water species, Halophila tricostata, is endemic to the area;147 and another rarer species, Halophila capricorni, was first identified in deep-water coral environments in 1995.259 Most seagrass species in the Region are found in water shallower than 15 metres. A range of disturbances, both natural and human-caused, can affect seagrass growth and survival (Section 2.3.4).

The species composition of seagrass meadows is important in determining the ecosystem services they provide, and their susceptibility to disturbances.133,140 Seagrasses are the primary food source for dugongs and a major source for green turtles and several herbivorous fishes.260 Dugongs prefer colonising seagrasses, such as species of Halophila, Halodule and Cymodocea,260 though they will eat parts of virtually all seagrass species encountered.141 The types of seagrasses present in a meadow affect both the quantity and nutritional quality of available food resources.261 Dugongs and green turtles are both able to expand their diet opportunistically when meadows are seriously depleted. However, reliance on less preferred foods or more ephemeral colonising species can affect animal health 262,263 and leave these herbivores at greater risk from disturbances that affect short- and long-term seagrass availability (Sections 2.3.4 and 6.3.2).141,260

Recovery from disturbances occurs at different rates in different seagrass species

Recovery from disturbances occurs at different rates in different seagrass species.145 Seagrasses are broadly categorised as having either persistent or colonising traits based on their ability to resist or rapidly recolonise following disturbances.145 Persistent species are generally slower growing and invest more in biomass, enabling them to compete effectively for resources, whereas colonising species tend to have low biomass and high turnover rates. Species with a mixture of those traits are characterised as opportunistic. On a global scale, the seagrasses of the Great Barrier Reef would be mostly categorised as colonising (Halophila and Halodule) or opportunistic (Syringodium and Cymodocea). More robust and persistent types (such as Thalassia and Enhalus) are found in sheltered reef-top areas and in bays.

A close-up underwater photograph focused on a pair of young leaves of the seagrass Halophila ovalis growing in carbonate sand.
Colonising seagrasses such as Halophila can rapidly return to areas after disturbances. © Dieter Tracey 2010 

Monitored sites across the Region shifted towards a greater proportion of colonising species, reflecting declining meadow condition, between 2016 and 2020.145 Since 2019, there has been recovery of species that are foundational to the meadows, resulting in a decreasing trend in the proportion of colonising species in the northern regions. This trend has not been evident in the southern Fitzroy and Burnett Mary regions, where meadow condition remains poor and a higher proportion of colonising species remains. 

Seagrasses continue to be affected by increasing sea temperatures and localised decreases in water quality. Recent changes in the proportion of colonising and successional seagrass species in northern regions are indicative of seagrass recovery following past impacts from flooding. Seagrasses in the southern regions of Fitzroy and Burnett Mary are in poor condition, primarily due to the impacts of recent floods, and are vulnerable to future disturbances. 

  • 133. York, P.H., Macreadie, P.I. and Rasheed, M.A. 2018, Blue Carbon stocks of Great Barrier Reef deep-water seagrasses, Biology Letters 14(12): 20180529.
  • 140. Hayes, M.A., McClure, E.C., York, P.H., Jinks, K.I., Rasheed, M.A., et al. 2020, The differential importance of deep and shallow seagrass to nekton assemblages of the Great Barrier Reef, Diversity 12(8): 292.
  • 141. Marsh, H., O'Shea, T.J. and Reynolds III, J.E. 2011, Ecology and conservation of the Sirenia: dugongs and manatees, Cambridge University Press, Cambridge.
  • 145. McKenzie, L.J., Collier, C.J., Langlois, L.A. and Yoshida, R.L. 2023, Marine Monitoring Program: annual report for inshore seagrass monitoring 2021–22. Report for the Great Barrier Reef Marine Park Authority, Great Barrier Reef Marine Park Authority, Townsville.
  • 147. Coles, R., McKenzie, L., De’ath, G., Roelofs, A. and Lee Long, W. 2009, Spatial distribution of deepwater seagrass in the inter-reef lagoon of the Great Barrier Reef World Heritage Area, Marine Ecology Progress Series 392: 57-68.
  • 257. Les, D.H., Cleland, M.A. and Waycott, M. 1997, Phylogenetic studies in Alismatidae, II: evolution of marine angiosperms (seagrasses) and hydrophily, Systematic Botany 22(3): 443-463.
  • 258. International Union for Conservation of Nature 2023, The IUCN Red List of Threatened Species, <https://www.iucnredlist.org/>.
  • 259. Larkum, A.W.D. 1995, Halophila capricorni (Hydrocharitaceae): a new species of seagrass from the Coral Sea, Aquatic Botany 51(3-4): 319-328.
  • 260. Erftemeijer, P. and Djunarlim, M. 1993, Stomach content analysis of a Dugong (Dugong dugon) from South Sulawesi, Indonesia, Australian Journal of Marine and Freshwater Research 44(1): 229-233.
  • 261. Thibault, M., Letourneur, Y., Cleguer, C., Bonneville, C., Briand, M.J., et al. 2024, C and N stable isotopes enlighten the trophic behaviour of the dugong (Dugong dugon), Scientific Reports 14(1): 896.
  • 262. Bastos, K.V., Machado, L.P., Joyeux, J., Ferreira, J.S., Militão, F.P., et al. 2022, Coastal degradation impacts on green turtle's (Chelonia mydas) diet in southeastern Brazil: Nutritional richness and health, Science of the Total Environment 823: 153593.
  • 263. Flint, M., Brand, A., Bell, I.P. and Madden Hof, C.A. 2019, Monitoring the health of green turtles in northern Queensland post catastrophic events, Science of the Total Environment 660: 586-592.