Case study 1 of TWO contrasting ecosystems at risk - the Great Barrier Reef

What this dot point is asking

NESA requires TWO contrasting case studies of ecosystems at risk. The Great Barrier Reef is the standard Australian marine case study. It pairs strongly with the Murray-Darling Basin (terrestrial freshwater) to give contrast in biophysical setting, stress profile, and management challenge. Strong responses are precise about location, scale, stresses, and management.

The ecosystem

The Great Barrier Reef stretches around 2,300 km along the Queensland coast, covering an area of 344,400 km2 (Marine Park boundary). It consists of around 3,000 individual reefs, 600 continental islands, and around 300 coral cays. Inscribed on the UNESCO World Heritage List in 1981 for outstanding universal value.

Biophysical setting

• Hydrosphere. Warm tropical water, 25-29 degrees C surface temperature. Salinity 32-35 ppt. Clear shallow water; light reaches the seafloor across most of the reef.
• Lithosphere. Continental shelf, mostly less than 50 m deep. The reef sits on a series of Pleistocene limestone platforms.
• Atmosphere. Tropical monsoon climate. Annual rainfall 1,200-4,000 mm along the catchment (Wet Tropics has the highest mean rainfall in Australia).
• Biosphere. Around 600 coral species, 1,500 fish species, 4,000 mollusc species, 215 bird species, 30 marine mammal species, six of the world's seven marine turtle species, plus seagrass beds, mangroves, and reef algae.

Biophysical interactions sustaining the reef

Reef-building corals are colonial animals (polyps) hosting symbiotic algae (zooxanthellae) in their tissues. The algae photosynthesise, supplying around 90 percent of the coral's energy. Corals in turn secrete calcium carbonate skeletons that build the reef structure over thousands of years.

The reef requires:

• Sea temperature within around 23-29 degrees C.
• Salinity around 32-35 ppt.
• Clear water (low sediment, high light penetration).
• Wave action moderate enough to bring nutrients but not so strong as to dislodge corals.
• pH around 8.0-8.3 to support calcium carbonate precipitation.

When any of these conditions are stressed, the symbiotic relationship breaks down.

The risk

Mass coral bleaching events

Bleaching occurs when corals expel their zooxanthellae under thermal stress. Without the algae, corals lose colour and most energy supply. Prolonged bleaching causes coral death.

Mass bleaching events on the Great Barrier Reef:

• 1998. First mass event, around 50 percent of reefs.
• 2002. Around 60 percent.
• 2016. 67 percent of northern reefs severely bleached; 29 percent of all corals dying.
• 2017. Bleached central reefs again with poor recovery.
• 2020. Most widespread to date, spanning all three regions.
• 2022. During a La Nina year, unusually.
• 2024. Largest spatial footprint on record; 81 percent of surveyed reefs experiencing some bleaching, around 24 percent severe.

Coral cover declined an estimated 50 percent between 1985 and 2012 (AIMS long-term monitoring), recovered somewhat by 2019-2022, then declined again in 2024.

Water quality stress

Catchment land use delivers around 14 Mt of sediment, 50,000 t of nitrogen, and 4,200 t of phosphorus annually. Sugar cane (north QLD), grazing (Burdekin), and bananas (Wet Tropics) are the dominant sources. The 2019 Townsville flood deposited a major mud plume on inshore reefs.

Sediment reduces light to corals. Nutrient enrichment favours algae over coral and fuels Crown-of-thorns starfish outbreaks.

Crown-of-thorns starfish

Acanthaster planci. A native species that has population outbreaks linked to nutrient enrichment. A single starfish can eat its body area in coral per day. Outbreaks have caused around 42 percent of recent coral loss according to AIMS.

Tropical cyclones

Strong cyclones cause direct damage. Yasi (2011, Cat 5) damaged 17 percent of the reef. Debbie (2017, Cat 4) caused widespread coral damage to central reefs. Climate change is projected to reduce cyclone frequency but increase intensity.

Fishing

Although 33 percent of the marine park is no-take zone, illegal fishing and historical overfishing have reduced large predatory fish populations on inshore reefs.

Climate change as the integrating driver

Ocean temperature has risen around 1 degree C in the GBR region since pre-industrial. Marine heatwaves are around five times more frequent than 1900. Ocean acidification has lowered pH by 0.1 units, reducing coral calcification rates by an estimated 11 percent since 1990.

Management

Governance

• Great Barrier Reef Marine Park Authority (GBRMPA). Federal agency, established 1975. Manages the Marine Park.
• Queensland Government. Manages adjacent terrestrial protected areas and catchments.
• World Heritage Listing. Since 1981. UNESCO has periodically considered "in danger" listing.

Marine Park zoning (2004)

The current zoning plan divides the marine park into multiple use zones:

• No-take zones (green zones). 33 percent of the marine park; no fishing or extraction.
• Habitat protection zones. Restrict bottom trawling.
• Conservation park zones. Limit certain commercial fishing.
• General use zones. Most fishing and tourism activity.

Fish biomass on no-take reefs is roughly twice that of fished reefs, providing reservoirs that re-seed surrounding areas.

Reef 2050 Long-Term Sustainability Plan

Released 2015, refreshed 2021. Coordinates Australian and Queensland Government actions. $3 billion in committed funding through 2030 across 35 actions including:

• Reef Trust Partnership ($443 million via Great Barrier Reef Foundation, 2018) for catchment management, COTS control, and reef restoration.
• Sustainable industries (zero-net sediment construction, regulated grazing).
• Climate action (linked to national Climate Change Act 2022).
• Indigenous heritage and Traditional Owner partnerships.

Catchment management

Reef 2050 Water Quality Improvement Plan targets 60 percent nitrogen reduction and 25 percent sediment reduction by 2025 (against 2009 baseline). Many catchments are not on track. The Great Barrier Reef Foundation's reef rescue programs work with sugar cane and grazing operators.

COTS control

The Crown-of-thorns Starfish Control Program (around $80 million committed) deploys teams of divers killing starfish with injection at high-value reefs. Has reduced COTS damage on protected reefs.

Climate mitigation

The most important management response in principle but the slowest in practical effect. Australia's 2022 Climate Change Act locks in 43 percent emissions reduction by 2030 and net zero by 2050. Without globally coordinated reductions, sea temperature will continue to rise and bleaching events will continue.

Indigenous co-management

Over 70 Sea Country Indigenous Land Use Agreements are in place. Traditional Owner partnerships in monitoring (Indigenous Rangers), reef restoration, and policy.

Active reef intervention

Australian Institute of Marine Science (AIMS) operates research programs in:

• Coral seeding. Producing larvae in laboratories and releasing onto degraded reefs.
• Selective breeding. Selecting heat-tolerant coral genotypes.
• Coral cooling. Marine cloud brightening experiments.
• Larval restoration. Boosting larval supply on small reefs.

These are research-scale at present; whether they can scale to the full reef remains uncertain.

Effectiveness assessment

Local stresses (sediment, nutrients, COTS, fishing) are being managed effectively where funded. Catchment management progress is slower than targets. The most-fished reef areas have recovered fish biomass in no-take zones. COTS control protects priority reefs.

Climate change remains the dominant unresolved threat. Even strong national mitigation under the Climate Change Act will not slow ocean warming on the timescale that coral needs to adapt. Without globally coordinated reductions, the IPCC AR6 projections indicate around 70-90 percent of coral reefs globally will be lost at 1.5 degrees C of warming, and over 99 percent at 2 degrees C.

UNESCO has repeatedly considered World Heritage in Danger listing for the GBR. The 2024 reactive monitoring report registered concern about the 2024 bleaching extent.

Why this case study works for the exam

The data are dense and public (AIMS, GBRMPA, ARC Centre of Excellence for Coral Reef Studies). The stresses span natural (cyclones, COTS as native species) and human (catchment runoff, climate change). The management response includes multiple instruments at multiple scales (federal, state, Indigenous, scientific, international). Pair it with the Murray-Darling Basin to give marine-terrestrial contrast.