How healthy are coral reefs? How are they responding to climate change?

After more than 10 years of monitoring the Great Barrier Reef, geoscientists at the University of Sydney have developed a technique that allows them to answer these questions using satellites in space - and it all relies on sand.

It turns out that sand aprons, deposits of sand along the shore of a lagoon, that are always found with coral reefs, can give a reliable estimate of how coral reefs are growing as well as their overall health.

How corals grow and recede, and how healthy they are, is dependent on a complex combination of factors – such as waves and storm surges, sedimentation rates, seawater chemistry, land-based runoff and even fish populations. This means predicting the health of a single coral reef group, and how it will behave in response to climate change, is an intricate puzzle.

However, the hard work paid off, and the researchers found that sand aprons – formed as waves and currents from reef crests carry sediment that becomes trapped in the reef lagoons – can be used to estimate carbonate productivity over time. This is a process which indicates the health of a coral ecosystem - the higher the production rate, the healthier the reef.

Ana Vila-Concejo, who is associate professor at the University of Sydney associate professor and deputy director of One Tree Island Research Station in the Great Barrier Reef, led the study. She said: “The traditional way of collecting such data is very work intensive.

"It requires actively measuring the chemistry of water or taking thousands upon thousands of photos to calculate how much each creature in the ecosystem is contributing to carbonate sediment productivity.”

The field team worked from a 12-metre catamaran to visit 21 reefs in the southern Great Barrier Reef, collecting over 100,000 records of reef composition.

The team later worked with satellite imagery to measure the sand aprons and estimate their volume for each reef. They then matched that data with carbonate production measurements taken over more than a decade to try and understand sand apron evolution, and how it correlated with productivity.

Vila-Concejo said: “If we can understand the evolution of sand aprons in each reef over time, we can use the data to manage coral reefs and prepare for climate change.”

The study was published in the journal Geology.