Seagrass meadows

Geographe Bay supports the most extensive seagrass meadows in temperate Western Australia1 and around 9,500 hectares in waters < 10 m are in good condition2.

The most common species is Posidonia sinuosa which is listed as vulnerable on the IUCN redlist. In Geographe Bay the P. sinuosa meadows are healthy with reasonably consistent density over the last seven years, and a higher density than found in other places in WA.

The seagrass meadows of Geographe Bay are not only large but also highly diverse, with 10 species from the five genera Amphibolis, Posidonia, Halophila, Heterozostera and Thalassodendron being identified.3 4

Threats to seagrass meadows

  • Seagrasses are sensitive ecosystems that are impacted by changes to water quality. High levels of nutrients entering these ecosystems causes rapid growth of epiphytes (plants that attach to other plants) that smother the blades of seagrass. This blocks the ability of seagrass to photosynthesise. Sediment run off into coastal waters can also reduce light penetration, smother seabed habitat and introduce nutrients and toxins. Seagrasses communities are particularly susceptible to changes in water quality.
  • Warming of waters due to heat waves or climate change can also cause the die-off of seagrass. Warmer water above a certain threshold increases mortality (e.g. Thomson et al 2015).

Calls for action

Boat anchors and propellers can leave “scars” in a seagrass bed—killing sections of the seagrass and fragmenting the habitat.

Case Studies


  1. DEC. (2006) Indicative management plan for the proposed Geographe Bay/Leeuwin-Naturaliste/Hardy Inlet Marine Park.
  2. McMahon (2017). Keep Watch Seagrass Monitoring, 2017. Report to GeoCatch. Centre for Marine Ecosystems Research, Edith Cowan University 31 pages. 
  3. Elscot, S. V. & Bancroft, K. P. (1999) A review of existing ecological information for the proposed Geographe Bay-Capes-Hardy Inlet marine conservation reserve. Literature Review MRI/CF/GBC-19/1999. December 1998. Marine Conservation Branch, Department of Conservation and Land Management, Fremantle, Western Australia, 6160. (Unpublished report). Perth.
  4. Walker, D., McMahon, K., Cosgrove, J. & Whittemore, R. (1994) Geographe Bay: Seagrass, algal and water quality studies, Summer 1993-1994. Perth.