Subtidal high energy over gravel

Long description

Subtidal gravels experiencing very high to moderate energy magnitude including muddy gravels, sandy gravels, and muddy sandy gravel (including cobbles and pebbles). Typically where gravels experience high wave energy in shallower waters, they will be tumbled over each other, becoming abraded through mechanical action. Gravel can be entrained in potholes, where abrasion is particularly pronounced[2]. 

Gravels can be of various Substrate compositions, such as terrigenous (fragments of decomposed rock) and carbonate (coarse shellgrits and coral rubble).

Biota of high current areas in deep water can have a variety of filter feeders attached to the gravel particles (sea pens, sea fans, sponges, bivalves) provided there is minimal particle movement. Typically high energy rubble beds and banks provide refuges for fauna (for example cowrie and cone shells on subtidal cobble beds in the lees of headlands or under coral rubble on high energy reef crests).

Special values

Carbonate gravels composed of shell or coral rubble are habitats for a variety of invertebrates (e.g. gastropod molluscs including cowries, cones and volutes, or burrowing bivalve molluscs).

Octocorallians or algae may attach to gravel fragments. These ecosystems may grade into reefal gardens (see type 98) or other biota e.g. sponge gardens (see type 62) and are likely to have values similar to these ecosystems.

Spanner crabs may be a significant fishery in rough grounds[1].

Diagnostic attributes:

Inundation ‘Subtidal’

Energy magnitude (wave) ‘Very high’, ‘High’, ‘Medium’

Sediment texture ‘Gravel’, ‘Gravel (muddy)’, ‘Gravel (sandy)’, ‘Gravel (muddy sandy)’

Qualifiers

None.

Distribution

K'gari (Fraser Island) and Breaksea Spit is the threshold of high wave energy in Queensland and generally north of K'gari and within the Great Barrier Reef lagoon wave energy is lower. High wave energy gravels predominantly occur south of K'gari but also in areas surrounding Agnes Water to Bustard Head. North of K'gari wave energy is lower. The extent of high current areas is unknown.

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

• Examples include gravelly substrates surrounding the Sponge Gardens at Little Woody Island are shellgrit beds with sparse sponge and other biota, where the tidal currents are rapid on steeply sloping subtidal banks sloping down into the deeper channels of the Great Sandy Strait.
• Subtidal gravels surrounding Breaksea Spit.
• Deep and very deep water swales between sand dunes west of K'gari may be subject to high current flows. These ecosystems resemble reefal gardens (Tim Stevens pers. comm.).

Comments

Only wave energy is mapped, but Energy magnitude from other sources including currents, riverine are also applicable. Substrate composition may be relevant, as carbonate substrates such as shellgrit and coral rubble may provide substrates for the settlement of different biota.

References

1. ^ Brown, IW, Queensland, Department of Primary Industries and Fisheries, Fisheries Research & Development Corporation (Australia), Victoria, Department of Primary Industries, New South Wales & Department of Primary Industries (2008), Reducing uncertainty in the assessment of the Australian spanner crab fishery, Dept. of Primary Industries and Fisheries, Brisbane.
2. ^ Woodroffe, CD (2002), Coasts: form, process and evolution, Cambridge University Press.

Last updated: 19 July 2019