Subtidal boulders

Long description

Subtidal boulders of any Substrate composition, where boulders are defined as exceeding 25.6 centimetres in diameter (see Substrate grain size attribute). Examples of boulders include those of terrigenous origin (i.e. fragmented rock or coffee rock) and of carbonate origin (e.g. large coral rubble). Boulders can be either adjacent to consolidated intertidal or subtidal ecosystems, or subtidal extensions of intertidal boulders, or exist in isolated patches.

Boulders result from a wide variety of erosion processes on land and in the sea, and many factors influence how they break down from parent rock or pavement etc. For example, boulders may occur beneath high wave energy ecosystems, where platforms have been subject to wave action and broken down into fragments on the sea floor. On coral platforms, boulders may be thrown across the reef by cyclone action. Subtidal boulders may also occur within estuaries and rivers where riverine flow has eroded down subtidal geological formations. More friable substrates such as coffee rock may break off steep-sided cliffs to fall into subtidal areas[3]. The Substrate composition and its Lithology will determine the size and shape of the boulders, influencing how they break away from parent material and the structural complexity of the terrain (i.e. the Terrain roughness).

There is no typical biota occurring on subtidal boulders, although as for consolidated ecosystem types, boulders are likely to have different biota according to their Substrate composition and Lithology, and within the context of surrounding ecosystems. The cracks between the boulders and spaces below are also important habitat for the attachment of biota, and these and other structurally complex areas act as refuges for fish, to protect them from predators, and fulfil a nursery role[1]. Depending on conditions for biota settlement, boulders may have coral growth, in which case they are mapped as subtidal coral. Many boulder ecosystem are yet to be surveyed for biota. Coffee rock boulders often provide good fish habitat surrounded by unconsolidated ecosystems where there is little structure to hide. Potentially boulders may have unmapped Structural macrobiota, e.g. corals, algae or oysters.

This ecosystem type also includes those where naturalness is modified, i.e. boulder rock walls (adjacent subtidal extent).

Special values

Subtidal boulders provide structural complexity as a refuge for fish to hide in, as nursery areas and as staging posts between habitats, thus they are good fish habitat[1].

Modified structures such as boulder rock walls do not support as diverse a fish fauna as natural boulder ecosystems[2].

Diagnostic attributes:

Inundation ‘Subtidal’

Sediment texture ‘Boulders’

Qualifiers

The Naturalness qualifier is relevant to coffee rock boulders as they are friable and vulnerable to mechanical disturbance (e.g. anchor damage, trawling etc.).

Structures such as boulder rock walls modify the Energy magnitude of waves in the surrounding vicinity, and may reconfigure the unconsolidated surrounding shorelines (e.g. changing Grain size/Sediment texture and Benthic depth).

Distribution

Boulders are found adjacent to consolidated (reef) areas along the coastline, in estuaries and on offshore coral reefs. Geology maps provide a good understanding of the underlying Lithology extending out beyond the land onto the sea floor.

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

• Subtidal boulders occur along the Woongarra coastline near Bundaberg, along Rodd’s Peninsula, from Agnes Water to 1770, along the eastern shoreline of Curtis Island, and Worthington Creek (estuarine subtidal boulders). Within the Great Sandy Strait coffee rock reefs break off into boulder fragments surrounding the intact coffee rock.
• Modified boulder rock walls are mapped throughout Central Queensland close to areas of human habitation, and surrounding boat harbours and marine infrastructure, for example in Gladstone Harbour, Rockhampton, Bundaberg Port, Urangan Boat Harbour, Burrum Heads boat ramp complex, Tin Can Bay and Carlo Point.

Comments

Relevant additional attributes include Substrate composition, Energy source and Energy magnitude, Lithology, Terrain morphology, Terrain roughness.

Additional Information

Rocky shore (coastal marine habitats) - Queensland Government

References

1. ^ a b Bradley, M, Baker, R & Sheaves, M (July 2017), 'Hidden Components in Tropical Seascapes: Deep-Estuary Habitats Support Unique Fish Assemblages', Estuaries and Coasts. [online], vol. 40, no. 4, pp. 1195-1206. Available at: http://link.springer.com/10.1007/s12237-016-0192-z [Accessed 25 March 2019].
2. ^ Henderson, CJ, Gilby, BL, Schlacher, TA, Connolly, RM, Sheaves, M, Flint, N, Borland, HP, Olds, AD & Handling editor: Henn Ojaveer (4 February 2019), 'Contrasting effects of mangroves and armoured shorelines on fish assemblages in tropical estuarine seascapes', ICES Journal of Marine Science. [online] Available at: https://academic.oup.com/icesjms/advance-article/doi/10.1093/icesjms/fsz007/5306605 [Accessed 13 June 2019].
3. ^ Woodroffe, CD (2002), Coasts: form, process and evolution, Cambridge University Press.

Last updated: 12 July 2019