Reef systems are the result of a fine balance between the formation of limestone (produced by coral and marine algae) and the destruction of the reef via mechanical and biological activities.
Coral reefs are found in shallow warm waters around the globe and require:
- Clear water to allow adequate light for photosynthesis
- Water temperatures between 22 degrees Celsius and 29 degrees Celsius
- Firm substrates for coral attachment
- Stable salinity (this is the reason why reefs do not occur near river outfalls
- Low levels of sedimentation (sediment reduces the amount of light and can smoother corals)
- Low nutrient levels (high nutrients allow algal coverage of corals)
Evolution of the Great Barrier Reef
65 million years ago the Australian continent was part of a landmass called Gondwanaland which was located in the cold southern waters. As the continent broke away and drifted north into tropical waters, a coral reef system started to grow on the continental shelf along the eastern coastline. The Great Barrier Reef is believed to be over 18 million years old in the north and 2 million years old in the south.
Through time, sea level changes have been caused by the warming and cooling of the earth. During the last Ice Age – 18,000 years ago, the sea level dropped to over 100 meters and created a landscape of grassy plains and limestone hills through the ocean basins. 12,000 year ago as the ice caps melted, all of the fresh water that was captured as ice returned to the ocean and has maintained a constant level for the past 8,000 years.
As the sea levels rose, the corals who had survived the Ice Age off the continental shelf, reproduced to colonise the limestone structures – old remanent reefs into the coral systems that we know today.
Today three different developmental stages of reef growth can be seen:
- Juvenile reefs are still growing to reach present sea levels
- Mature reefs have reached sea levels and are starting to fill in with sediment
- Senile reefs are filled with sediment, often having formed a coral cay.
Types of reefs
Fringing Reefs grow around continental islands.
Platform Reefs grow on the continental shelf. Their shape is the result of wind and rain erosion during the Ice Ages and the growth and erosion of reef systems under the water.
Ribbon Reefs grow along the edge of the continental shelf, with the most spectacular stretch being nearly 670 km long between cook town and the Torres Strait
Sand Cay is a small island of sand formed on top of coral reefs. The sand of coral cays consist of reef animal skeletons and other debris. As the ways curve around the reef, they deposit this sediment on the calm (leeward side) end of the reef. Initially the coral cays are exposed sand banks. Cays are dynamic, their position constantly changes with prevailing wind and weather conditions.
Mature Coral Cays are formed as more sediment is accumulated and stabilises the cay. Water flowing through the sediments may deposit limestone to form beach-rock. Similar to concrete, beach-rock further stabilises the cay. Plant seeds reach the cay by drifting in the ocean, or on the feathers of birds. The seeds germinate with the aid of guano (bird droppings) and colonise the cay. As the vegetation grows, more sea birds are drawn to the cay which increases the nutrient levels to support continued growth of the plants which stabilises the cay further.
For further Information, please view the following pages: