How are coral reefs built – and destroyed?

Coral reefs are like houses. They’re built from coral bricks that come in all shapes and sizes cemented together by red coralline algae.

Both the bricks and the cement are made of calcium carbonate, which is a great construction material, except in the face of climate change.

As our oceans absorb the increased amount of carbon dioxide (CO2) humans are producing into the atmosphere, seawater becomes more acidic. And acidic seawater dissolves the calcium carbonate.

Photo credit: Hayden Martin

To know how coral reefs will fare in the future we need to understand how both the bricks and the cement respond to this acidification of the oceans. Much research has been done on the coral bricks, but there has been little work on how ocean acidification will affect the cement produced by the coralline algae.

To provide some answers, scientists are working at an ANU research station on One Tree Island in the Great Barrier Reef to determine the relative importance of corals and coralline algae in reef construction. This requires precise measurements of the rates at which both the corals and coralline algae calcify and build the reef, and the rates at which they dissolve as CO2 and temperature vary. The researchers are using the large natural day/night changes in temperature and CO2 to understand how ocean acidification and increasing temperature will impact the reef in future.

One Tree Island. Photo credit Hayden Martin

The ANU team employs Perspex domes to isolate patches of the reef and measure changes in oxygen, acidity and the amount of carbon in the water. This allows rates of calcification, dissolution, photosynthesis, and respiration to be quantified within the domes, revealing the response of corals, coralline algae and other common reef dwellers to changing ocean conditions.

The best sampling sites for this work are accessed at low tide, so the work is ruled by the tides. The team is up day and night to catch every low tide, and make as many measurements as possible in their short time on One Tree Island. The most spectacular times are sunrise and sunset, but the dark night sky 100 km off the coast is a favourite too. In fact, there is no bad time to be on the reef!

ANU student Patrick Goodarzi sampling the reef chemistry at sunrise.

The setting might be straight out of a holiday brochure, but the scientists are doing serious research to better understand how climate change is affecting this ecosystem. Because while a coral reef might be built like a house, it can also collapse like one too.