The Great Southern Reef is located on the edge of Australia’s continental shelf where coastal landforms extend below sea level in a mosaic of rocky reefs.

Marine environments such as the GSR have evolved over very long periods of time.

This can be explained by:

• Australia’s tectonic stability due to its location within the Indo-Australia plate

• low sediment loads from coastal rivers due to the dry climate

• a lack of continental glaciation

‘… thousands of kilometres of the present rocky coast of eastern and southwestern Australia largely retain the outline created more than 30 Ma.’

http://press-files.anu.edu.au/downloads/press/p194981/pdf/chapter-6.pdf

The rocky outcrops and boulders that form the Great Southern Reef are a result of the weathering and erosion of the continent. They may be extensions of rock platforms, remnants of coastal cliffs and headlands or isolated rocky outcrops.

Weathering and erosion create the rock surfaces that kelp colonises and forms crevices, channels and overhangs where non-kelp species grow. Loose sediment deposited on the seabed between the rocky outcrops is where seagrasses and non kelp species grow.

Coastal geology (rock type and structure) affects what the rocky reefs will be like. Fast eroding granite produces uneven surfaces and crevices that become habitat for a diversity of species such as eels and octopus while sandstone reefs have more smooth surfaces on which kelp attaches and overhangs where sponges will thrive.

On the sandstone around Sydney moderate waves erode cliffs at around 1 mm/year while along the Great Australian bight, soft limestone is eroded by high energy waves at around 25 mm/year.

Variations water depth contribute to species diversity underwater.

Marine organisms adapt to conditions in the water from sea level to the sea floor. Kelp forests thrive in the subtidal zone where there are fewer natural stresses than in the tidal zone. There are no big changes in temperature, water pressure and sunlight; organisms do not dry out and they extract essential nutrients such nitrogen and phosphorous from the water. As a result, kelp grows quickly here, can grow to great lengths, and produce high primary biomass. As the water gets deeper and the light fades kelp gives way to sponges, soft corals, and sea fans.

The following diagram shows the zonation of species on a rocky surface.