Intertidal low energy over intermediate substrate

Long description

Intertidal low to very low wave energy over either:

1. coffee rock, typically part of a coastal sand mass
2. beach rock, typically part of a sandy or gravelly beach with a high carbonate component.

These ecosystems are less dynamic than high energy intermediate ecosystems in terms of exposure and burial. Energy is typically low to very low, however these ecosystems can be periodically exposed and buried by sand deposition associated with wave action (e.g. storm events, cyclones) or current action (e.g. tidal flow or riverine flow following heavy rainfall, which can both undercut cliffs).

Coffee rock is a soft, dark-brown coloured friable rock consisting of sand grains weakly cemented together by organic matter (mostly plant-based). Coffee rock formed when humic acids from decaying plant matter are washed down through sandy soils, meeting shallow aluminium-rich groundwater where it precipitates to form an indurated sand layer in the B horizon of podzol soils. These occur on quartz sand systems in the sub-tropics and tropics[1]. On low energy shorelines they may have become exposed by riverine action against the sandy dune escarpment, outcropping on shorelines and in the shallow subtidal area. Freshwater flows and riverine currents can break up the soft rock into boulders by undercutting the cliff face with coffee rock blocks dropping or sliding off the cliff face[4].

As it forms a hardened layer, coffee rock can be associated with groundwater discharge via freshwater seepage or springs draining through the sand dunes and then flowing sideways where it meets the low permeability rock (see Coastal sand masses (high dunes) conceptual model). There is seldom any habitat forming biota but green algae such as Ulva sp. and other freshwater tolerant filamentous algae will grow in pools in the coffee rock, and crabs occupy the crevices between the rock (Moyra McRae pers. comm.).

Special values

Fishing hot spots on ledges, often nestled beneath unvegetated sand cliffs or associated with an estuary entrance (exposed by an erosional process).

Shorebird feeding habitat for resident species such as the beach stone curlew and sooty oyster catcher[2].

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'

Consolidation 'Intermediate'

Energy magnitude (wave) 'Low', 'Very low'

Qualifiers

The Naturalness qualifier is relevant.

Coffee rock is very soft and it is possible to scrape the surface or cut and remove it. Coffee rock is commonly encountered during beach driving where it becomes an obstacle to drivers, who in some instances have cut through coffee rock to gain beach access (e.g. historically on Ngkala Rocks, K’gari (Fraser Island)). Coffee rocks can be damaged by vandalism or graffiti. The period and trend qualifiers are also relevant as it can be alternately exposed and buried by sand (Moyra McRae pers. comm.).

Shellgrit, coral rubble and other carbonate gravel adjacent to beach rock can be harvested, which can potentially change the chemistry of these areas.

Distribution

Coffee rock can be found up and down the Queensland coastline as a result of coastal processes and sea level change, in association with coastal sand masses. Beach rock can be found on carbonate rich beaches, such as coral-dominated ecosystems (e.g. fringing and platform reefs) and shelly intertidal areas.

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

• In the Central Queensland mapping area, low energy coffee rock typically occurs on a sandy beach along the western shore of K'gari and through the Great Sandy Strait. It is a remnant of a time when the K'gari sandmass extended further seaward and the coffee rock that we can see today was further inland and a part of the sandmass soil layers[3]. Coffee rock ecosystems of the Great Sandy Strait were validated by ReefCheck. Subtidally, some of these coffee rock reefs were soft coral ecosystems (see Type 99).

Comments

Other relevant attributes could include Substrate composition, Energy source, Energy magnitude and Lithology. The Terrain roughness and Terrain morphology are indicative of crevices and other spaces as potential habitat for macroalgae and other biota.

Additional Information

• K'gari (Fraser Island) - Department of the Environment, Tourism, Science and Innovation
• Checking maps for Great Sandy reefs - Reef Check Australia

References

1. ^ Cox, M, Harbison, J, Ezzy, T, Preda, M, Brooke, B, Lee, R, Lester, J, Oberhardt, M & Laycock, J (2000), 'Coffee rock: an overview of its character and occurrence in the Pumicestone region', PASSCON 2000. Science informing catchment management : 22 and 23 November, 2000, Queensland University of Technology : extended abstracts / Pumicestone Passage and Deception Bay Catchment Conference, pp. 23-24, QUT, ed. Malcolm E. Cox.
2. ^ Driscoll, P (1995), Survey of wader and waterbird communities along the central Queensland coast, Queensland Ornithological Society Inc.
3. ^ Queensland Government (2018), 'K'gari (Fraser Island), Great Sandy National Park - Nature, culture and history', Department of Environment and Science. [online] Available at: https://parks.des.qld.gov.au/parks/fraser/culture.html.
4. ^ Woodroffe, CD (2002), Coasts: form, process and evolution, Cambridge University Press.

Last updated: 11 July 2019