How healthy are our coral reefs? Researchers have built a tool to help find out

Researchers use over 30 years of data to build new framework to assess reef health

Researchers at the University of Sydney have harnessed over 30 years of data to build a new method to help more accurately measure the impact of climate change-induced events on the Great Barrier Reef.

Their framework quantifies reef degradation over time and provides other researchers a 'health check’ tool to assess reef health. It can be used to measure the impact of severe disturbances, including coral bleaching, cyclones and floods, on reefs.

Led by PhD candidate Kate Whitton from the University's School of Geosciences and its Marine Studies Institute, the framework draws on data dating back to 1995 to evaluate how key parameters, particularly coral cover and algal buildup, have changed over time. High coral cover indicates a reef able to sustain healthy marine life populations and high levels of algae point to poor reef health.

The comparative benchmark was developed for three study reefs within the Great Barrier Reef: Lizard Reef, Low Isles Reef and One Tree Reef (where the University has a research station).

The research underpinning the framework was supervised by the University of Sydney's Professor Ana Vila-Concejo, Dr Ana Paula da Silva, Professor Maria Byrne and the University of Wollongong's Associate Professor Sarah Hamylton. It is published in Marine Environmental Research.

Assessing reef health from absolute coral cover alone is difficult, as healthy cover can vary widely among reef systems. Instead, the research team compared each reef against its own historical data, which began being collected in the late 1980s by the Australian Institute of Marine Science.

"A reef with 30 percent coral cover might be considered healthy in one location but indicate substantial decline in another," Ms Whitton said.

"Variation can depend on factors like exposure to ocean currents and proximity to shore.

"By comparing reefs against their own historical data, we can assess a reef's condition in a way that accounts for its idiosyncrasies. This allows us to compare reefs more consistently and better understand where they sit along their recovery trajectory."

The research created three ecological categories for reefs - healthy, coping and struggling - along with changes in coral cover, algal dominance and reef composition. It found reefs generally transitioned into a 'coping' state when coral cover declined below approximately 65 per cent of their historical maximum.

Further declines below 35 percent were associated with a 'struggling state', characterised by increased algal buildup and substantial reductions in reef-building corals.

The study found complex coral forms are more impacted than simple, round corals when a reef is disturbed, resulting in a flattening of reefs as they approach ‘coping and ‘struggling’ states.

A flat reef offers fewer nooks and crevices for fish to shelter and feed in. Complex corals are also better at absorbing energy from incoming waves. When they are destroyed, the ensuing, flatter reef provides less protection from storm waves, making it a hostile environment for organisms to thrive in and for corals to regrow.

Ms Whitton says the framework will help scientists assess when a reef is imperilled and recommend timely interventions to aid recovery. Abstaining from fishing and dredging and restoring more corals are among the measures that can hasten the replenishment of struggling reefs.

"With coral bleaching becoming more frequent and cyclones more severe, this new framework will better help us assess reefs after damaging events and determine how we can help them recover."

Although focused on three study reefs, the findings can be applied to other reefs across the Great Barrier.

"Our three study reefs have quite distinct geological structures covering the range of reef structures, or morphologies, on the Great Barrier Reef. This means our framework can be applied across most areas of the wider reef."