Understanding environmental stressors for the Great Barrier Reef

Assessing present-day reef health against a potentially already disturbed baseline

With the future of Australia’s Great Barrier Reef under threat, a newly funded three-year project will expand our understanding of the drivers and context of recent mass coral bleaching on the reef.

With the future of Australia’s Great Barrier Reef under threat, a newly funded three-year project will expand our understanding of the drivers and context of recent mass coral bleaching on the reef.

 The reef suffered a mass coral bleaching event due to unusually warm water temperatures in February 2020–this was the third such event in just five years. A sharp decline in coral cover across the reef over the decades has caused concern for the reef’s ability to withstand the cumulative impacts of ocean warming, acidification and regional threats, including more severe tropical cyclones, crown-of-thorns starfish predation, and reduced water quality. But we do not fully understand how reef biota and systems react to environmental change over the long term and there are lessons we can learn from the past.

A multi-disciplinary team led by Associate Professor Helen McGregor at the University of Wollongong and including Wollongong Australian Research Council DECRA Fellow Tara Clark are investigating the impact of environmental change on the reef. Their work has already made substantial advances in our understanding of threats to the reef.

“I study the climate of the past to better understand the present‐day climate. This addresses key questions on the origins of natural climate variations and improves our ability to forecast climate change,” A/Prof McGregor says.

In a project recently funded by the Australian Research Council, A/Prof McGregor, Dr Clark and a diverse team of national and international researchers will be investigating how the reef has changed over time by reconstructing past climate, rainfall, water quality, coral bleaching and reef ecology. This data will help researchers understand which environmental stressors and paleoclimate variations are most critical for reef health.

Defining paleoecologic change

The project has three main objectives. First, the team is aiming to define paleoecologic change by investigating how reef communities and erosion rates have changed through time. This research will add to previous landmark studies by co-Investigators Associate Prof. Jody Webster (University of Sydney), Prof. Yusuke Yokoyama (University of Tokyo), Dr Thomas Felis (University of Bremen) and McGregor, such as those that drilled cores along the length of the reef to determine its history. These studies showed that the reef as we know it developed in response to sea-level fluctuations and geomorphic change over the past 10,000 years.

“We will use a range of imaging techniques, such as x-raying and CT scanning, similar to methods used on broken bones, to quantify long-term ‘bioerosion’ of the reef by organisms as a measure of reef stress during the early Holocene around 10,000-7,000 years ago when the most recent phase of reef growth began, and for periods leading up to and after major reef growth hiatuses since then,” A/Prof McGregor explains.

“There is now compelling experimental and field evidence that coral bioerosion rates are a stress indicator which may accelerate in the future as coral reef systems become stressed due to increasing SST, ocean acidification, sediment and nutrients.

“We will ensure reproducibility and spatio-temporal extent of our down-core results by investigating cores from multiple regions, multiple reefs within each region, and multiple zones within a given reef. We will expand on recent work to collect numerous fossil reef cores from One Tree and Heron Islands in the southern GBR, where the University of Sydney and University of Queensland respectively, which maintain world class research stations.”

UOW DECRA Fellow Clark’s work on ecological change over the past few hundred years will build the bridge between long term and short-term change.

Testing causal links

Objective two of the project is to test the causal links between reef death and changes in sea level, monsoon rainfall and El Niño-Southern Oscillation (ENSO) variability.

Several hiatuses in reef growth occurred during the mid-Holocene (6,000– 3,000 years ago), an interval that had higher sea levels and different seasonal rainfall distribution, and reduced ENSO amplitude compared to today. The mid-Holocene is considered a testbed for exploring the links between the growth hiatuses and climate variability. A/Prof McGregor’s team will investigate seasonal scale climate variations for this period.

“We will cross-check that any stress bands are bleaching events, and will investigate how frequently they occur, and coral recovery as the coral geochemistry returns to pre-bleaching values,” A/Prof McGregor says.

Understanding the history of sea surface temperatures

Objective three is to determine the multi-century context, physical processes and large-scale forcing mechanisms driving the high sea surface temperatures that lead to coral bleaching.

A landmark study in 2002 by Hendy and including Partner Investigator Janice Lough at the Australian Institute of Marine Science, and a subsequent study at Flinders Reef, reconstructed decadal-scale sea surface temperatures, showing that temperatures in the early 1700s were at least as warm as the late 20th century. A/Prof McGregor, co-Investigator Dr Ben Henley (Monash University) and Lough are aiming to expand on the Hendy study to answer key questions such as: What was the nature of the warm period in the early 1700s? Was it limited to one season? Have we crossed a threshold whereby current warming is beyond the norms of the past several centuries?

“We propose to use state-of-the-art climate model simulations and revisit previous studies to reconstruct and detail the temperature history. We will focus on cores from Britomart Reef, used in the Hendy study.

“The coral core at Britomart Reef extends to the 1600s and annual density bands provide a clear internal chronology, which will be cross-checked against luminescent lines from occasional large river floods that bring fresh water to the reef.

“We also propose to reconstruct sea surface temperatures from three coral cores from Lizard Island in the north, in a region that saw the most extensive bleaching in 2016.”

Data-driven decision making

An overall objective of this project is the identification of thresholds and triggers for ecosystem change over different spatiotemporal scales so that environmental regulators, government funding bodies and reef managers can recognise the most pressing threats to the reef.

“The issue of shifting baselines needs to be addressed. Present-day reef health is assessed against a potentially already disturbed baseline; by giving new information on reef dynamics in times prior to human disturbance managers will then have the hard evidence to review policy and practice to halt and reverse factors responsible for the decline in reef state and bolster the resilience of coral communities,” A/Prof McGregor says.