Intertidal high energy over boulders

Long description

Intertidal very high to medium energy over boulders with geology of mineral rock or chemical origin (terrigenous). Boulders are defined as exceeding 25.6cm in diameter, of any Substrate composition, and can include terrigenous origin (i.e. fragmented rock or coffee rock) or carbonate origin (e.g. larger coral rubble). They can be either adjacent to consolidated intertidal or subtidal ecosystems, or exist in isolated patches.

Very high and high oceanic wave energy on these shorelines creates a splash zone above the usual tidal limits with a characteristic biota of barnacles and periwinkles. Tidal inundation also creates strong faunal zonation from highest to lowest tidal extremes - periwinkle, barnacles, tubeworms and finally ascidians and/or macroalgae. There is also energy-based zonation along the shore with a typically windward environment (high energy with south-easterly aspect) and typically leeward environment (lower energy with northerly aspect), with different species composition in each area (i.e. microhabitats).

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. Intertidal boulders may also occur within estuaries and rivers where riverine flow has eroded down geological formations. More friable substrates such as coffee rock may break off steep-sided cliffs to fall into intertidal areas[6]. 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 intertidal 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, providing moist refuges for molluscs, crustaceans, ascidians, anemones and echinoderms[1]. These and other structurally complex areas act as refuges for fish, to protect them from predators, and fulfil a nursery role[2].

Special values

More variation in microhabitats than on consolidated substrates, notably variation in attributes of Terrain roughness, slope and morphology and associated hydrodynamic changes in Energy source (i.e. waves) and magnitude. Higher faunal diversity is likely to be associated with these crevices and pools of variable energy. Eddies forming in the lee of headlands (where high energy boulders are likely to occur) tend to concentrate larval settlement, resulting in higher benthic diversities in the lee of topographic features[4]. Tidal pools form above the water mark and provide submerged habitat for typically subtidal species[1].

Diagnostic attributes

Inundation 'Intertidal – Lower low', 'Intertidal – Mid low', 'Intertidal – Upper low', 'Intertidal – Low undifferentiated', 'Intertidal – Lower medium', 'Intertidal – Upper-medium', 'Intertidal – Medium undifferentiated', 'Intertidal – High', 'Intertidal – Undifferentiated', 'Intertidal – High undifferentiated'

Energy magnitude (wave) 'Very high', 'High', 'Medium'

Energy magnitude 'Unknown'

Sediment texture 'Boulders'

Qualifiers

Period and Trend qualifiers are likely to change slowly in boulder ecosystems, however high wave energy impacts during extreme events may move boulders. The Naturalness qualifier is relevant to boulder breakwaters constructed to lessen wave energy for example around river entrances etc.

Distribution

K'gari (Fraser Island) and the Great Barrier Reef form a barrier to wave energy at a regional to national scale[5]. The northern-most limits of high energy rocky shorelines along the Queensland coast are K'gari and Double Island Point. K'gari and Double Island Point also mark a delineation in terms of subtropical and temperate species compositions of molluscs and barnacles[5][3]. Many boulder ecosystems are yet to be surveyed for biota.

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

• Limited extent in Central Queensland, includes Woongarra and the Agnes Water, 1770 and Bustard Head area, and in association with most large rocky headlands such as Double Island Point, Waddy Point and Indian Head and Curtis Island eastern shoreline. High energy outliers inside of the Great Barrier Reef lagoon (i.e. the north of K'gari) include the Round Hill Head and Bustard Head area, extending west to Rodd’s Peninsula, Wreck and Flat rocks on the sandy beaches of Deepwater National Park, and occasional headlands on Curtis Island. These outliers support a different suite of species to communities south of K'gari.
• Depending on conditions for biota settlement, boulders may have coral growth in lower energy areas (see Type 35) (as along the Woongarra coastline and on Rodds Peninsula at Ethel Rocks) in which case they are mapped as intertidal coral. Corals are not likely to occur intertidally on the high energy boulder shorelines from K'gari south[5][3].

Comments

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

Additional Information

Rocky shore - Queensland Government

Intertidal rocky shores - Queensland Museum

Intertidal rocky shores - Museum of Tropical Queensland

Indigenous Fish Traps and Weirs of Queensland - Rowland and Ulm 2011

References

1. ^ a b Bennett, I & Dakin, WJ (1992), Australian seashores, Collins/Angus & Robertson, Pymble, N.S.W..
2. ^ 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].
3. ^ a b Endean, R, Kenny, R & Stephenson, W (1956), 'The Ecology and Distribution of Intertidal Organisms on the Rocky Shores of the Queensland Mainland', Marine and Freshwater Research. [online], vol. 7, no. 1, p. 88. Available at: http://www.publish.csiro.au/?paper=MF9560088 [Accessed 30 April 2019].
4. ^ Mace, A & Morgan, S (2006), 'Larval accumulation in the lee of a small headland: implications for the design of marine reserves', Marine Ecology Progress Series. [online], vol. 318, pp. 19-29. Available at: http://www.int-res.com/abstracts/meps/v318/p19-29/ [Accessed 4 June 2019].
5. ^ a b c Poloczanska, ES, Smith, S, Fauconnet, L, Healy, J, Tibbetts, IR, Burrows, MT & Richardson, AJ (2011), 'Little change in the distribution of rocky shore faunal communities on the Australian east coast after 50years of rapid warming', Journal of experimental marine biology and ecology, vol. 400, no. 1, pp. 145-154, Elsevier.
6. ^ Woodroffe, CD (2002), Coasts: form, process and evolution, Cambridge University Press.

Last updated: 12 July 2019