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Subtidal encrusting algae

Short description

Subtidal encrusting algae growing in a sheet-like form or forming concretions as gravel on unconsolidated substrates.

Disclaimer: Ecosystem type descriptions are based on biophysical attributes identified in Central Queensland through expert advice and supported by scientific literature. Not all ecosystem types are mapped based on current inventory, and many of the ecosystems described here may also occur in other parts of Queensland.

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Classification categories

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Long description

Subtidal encrusting algae growing in a sheet-like form attached to the substrate on either consolidated (e.g. crustose coralline algae - CCA) or unconsolidated substrate, or forming their own free-living concretions as gravel on unconsolidated substrates (Rhodoliths).

Crustose coralline algae are crustose plants that grow as hard crust, appearing as a coloured layer on the substrate rather than with a typical plant-like growth form. These algae grow on coral reef substrates and CCA can produce calcium carbonate (limestone) which helps to cement reef frameworks especially on reef crests[1].

Rhodoliths are loose-lying non-jointed coralline red algae (non-geniculate coralline red algae, NCA) which are vast carbonate factories that occur in unconsolidated substrates. They form pebbles or gravel composed of encrusting carbonate (known in Europe as ‘maerl’) across unconsolidated substrates and can occur in shallow water or deep water. Taxa include Hydrolithon, Lithophyllum, Lithothamnion, Mesophyllum and Spongites. Rhodoliths can co-occur with macroalgae and seagrasses where they appear as understorey[2].

Special values

Crustose coralline algae is an important precursor coloniser of substrate which induces coral settlement.

This type and type 54 contribute to calcium deposition on the sea floor, to consolidated and unconsolidated substrates, respectively. Rhodoliths are carbonate fixers which provide a consolidated substrate for the attachment of other biota, and may be very long lived over thousands of years.

Diagnostic attributes

Inundation 'Subtidal'

Structural macrobiota 'Algae – encrusting (including CCA)'


Rhodolith beds in particular are vulnerable to trawling and to dredging, in which case the Naturalness qualifier would apply to Sediment texture[2].


Crustose coralline algae is commonly found on coral reef substrates and also on rocky shores. Rhodolith beds are known to occur in mesophotic ecosystems (greater than 75 metres) but their extent is poorly known in Queensland.  See the live map (see Additional Information) for reports of rhodolith beds.

The following relates to distribution of this ecosystem type within the Central Queensland mapping area:

  • Rhodoliths are mapped in deep waters off Fraser Island (see Additional Information). Rhodolith beds were reported by Butler (pers. comm.) adjacent to Woody Island in the Great Sandy Strait.


Relevant additional attributes include Energy magnitude (wave) for certain algal communities such as those on offshore reef crests (e.g. the genera Porolithon, Neogoniolithon, Lithophyllum) [1]. Rhodoliths move and may be propelled by wave and current energy sources. Sediment texture is relevant to rhodolith beds.

Additional Information

Rhodolith distribution in Australia - Dr Adela Harvey

Saltmarshes, seagrasses and algae - Department of Agriculture and Fisheries


  1. ^ a b Diaz-Pulido, G & McCook, L (2008), State of the Great Barrier Reef report.. [online], Great Barrier Reef Marine Park Authority, Townsville, Qld.. Available at:
  2. ^ a b Harvey, AS, Harvey, RM & Merton, E (2016), 'The distribution, significance and vulnerability of Australian rhodolith beds: a review', Marine and Freshwater Research, CSIRO.

Last updated: 22 July 2019

This page should be cited as:

Department of Environment, Science and Innovation, Queensland (2019) Subtidal encrusting algae, WetlandInfo website, accessed 18 March 2024. Available at:

Queensland Government
WetlandInfo   —   Department of Environment, Science and Innovation