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Coral Reefs

Coral reefs are underwater structures created by a thin layer of living coral polyps secreting calcium carbonate to build a limestone skeleton over many generations.

 

MILLENNIUM ECOSYSTEM ASSESSMENT REPORTING CATEGORY

Coastal Ecosystems

 

What ecosystem functions do coral reefs perform?

Coral colonies at south Peel Island in Moreton Bay.

Soft corals growing in the shallow waters of Moreton Bay.

Although coral reefs account for less than 0.1% of the world’s surface, as shown in Table 1, relative to other ecosystems coral reefs perform numerous essential ecosystem functions that can contribute to the provision of ecosystem services. Corals construct complex structures and are extensively utilised for food and supporting habitat for marine life. Coral reefs provide an essential home for an estimated 25% of marine organisms (e.g. fish, molluscs and crustaceans) and therefore have high genetic diversity. This includes many plant and animal species with high dependence on each other for shelter, nutrients and for pharmacological purposes. Humans have inextricable physical, emotional and economic relationships with reefs, hence reefs contribute to the landscape opportunity of the region.

Reefs are highly connected ecosystems, spreading their offspring over a broad geographic area via ocean currents. Even coral communities here in SEQ are still closely connected to the Great Barrier Reef (GBR), as some SEQ reefs receive coral larvae from the southern GBR during mass spawning events. Mass spawning events occur when many reef-building corals simultaneously release thousands of eggs and sperm into the water column. This is a similar reproductive approach to flowers allowing wind to carry their seeds to new locations, hence the high score for pollination, however it is well recognised that these two processes are not synonymous. Spawning helps maintain genetic diversity throughout coral populations and establish new reef locations. Similar reproductive strategies are used by many marine species, including fish and other invertebrates. This process highlights the importance of maintaining connectivity between different reefs, even though they may seem like distinct entities.

Coral reefs are incredibly competitive habitats, space is a premium and therefore biological control mechanisms are used to battle for space - competition is stiff for new reef arrivals. Corals also contain stinging cells in their tentacles which can immobilise and/or discourage species from inhabiting reefs, consuming coral or other species, or even enabling corals to consume other species. There are also biological controls that regulate reef growth (e.g. bio-eroders like sea urchins).

Research from the GBR reveals corals contain chemical dimethyl sulphide. When released into the atmosphere, chemical dimethyl sulphide helps clouds to form, which could have a large impact on the local climate. As well, the physical structure of coral reefs provides local micro-climates and habitat important to specific species. Reefs also moderate extreme ocean weather conditions by acting as coastal barriers.

  

Table 1:The relative magnitude (to other Ecosystem Reporting Categories) coral reefs perform each ecosystem function.

Ecosystem Function Category Ecosystem Function 0
1
2
3
4
5
Regulating Functions
Gas Regulation            
Climate Regulation            
Disturbance Regulation            
Water Regulation            
Soil Retention            
Nutrient Regulation            
Waste Treatment and Assimilation            
Pollination            
Biological Control            
Barrier Effect of Vegetation            
Supporting Functions
Supporting Habitats            
Soil Formation            
Provisioning Functions
Food            
Raw Materials            
Water Supply            
Genetic Resources            
Provision of Shade and Shelter            
Pharmacological Resources            
Cultural Functions
Landscape Opportunity            

 

What types of coral reefs are in SEQ?

When most of us think of reefs, images of the coral encrusted GBR spring to mind. However, reefs can actually be any hard, raised underwater structure that provides a space for other plants and animals to grow. Coral reefs did exist in SEQ thousands of years ago, when sea level was higher and environmental conditions allowed. Today, some of the limestone structures from these historic reefs remain, but corals in SEQ are limited from accreting (building) reef structure by factors such as light, temperature, water chemistry and/or water clarity.

Reefs in SEQ form on rocky outcrops and artificial structures like old boats where corals and other sessile (stationary) invertebrates grow. Corals continue to grow on some of the limestone reefs of the past, even though they are no longer helping to build more limestone structures. SEQ reefs are diverse and incredibly unique, with a blend of both tropical and temperate species living together in this transitional marine zone right on the doorstep of highly developed urban areas. In the south-west of Moreton Bay, reefs are dominated by hard coral species such as Favia sp. and soft corals such as Alclonium sp.. In the north-east hard corals such as Acropora sp. and soft corals such as  Xenia sp. and Sarcophyton sp. dominate.

 

Soft coral, like the Dedronephthya pictured here, reach out into the current to feed on drifting plankton.

Tubastrea sp (also known as sun coral) are commonly found under ledges or in caves. They are a hard coral, but do not help to build reef structure.

Under stress, corals may expel their zooxanthellae, which leads to a lighter or completely white appearance known as 'bleaching'.

Links to other publications and websites

Reef Check Australia
GBR Marine Park Authority
Sustainable Subtropical Reefs Alliance
Qld Museum - Corals of Moreton Bay
NICA - Marine Bio. Assessment
Coffs Harbour SRD

 

What is the area and extent of coral reefs in SEQ?

Coral reef ecosystems in SEQ cover approximately 20 km2. The map to the right shows where coral reefs are located in SEQ. Major coral reef ecosystems include:

- limestone reefs at Mud, Green, King and Bird islands;
- inshore rocky reefs at Sandgate, Woody Point, Scotts Point, Reef Point (Scarborough), Toorbul Point, Goat Island and a few rocky  outcrops on the southern bay islands; and
- artificial reefs such as Tangalooma Wrecks and Curtin artificial reef are not included in this map but should be assessed under this category.  

 

What is the vulnerability of coral reefs and threats to this ecosystem in SEQ? 

Some threats to coral reefs come from activities within the marine area and some come from land-based sources. There are undeniable links between catchments and corals. Excessive nutrient runoff can allow algae to outcompete corals for light and space. Excess sedimentation can smother corals or block the light required by the symbiotic algae living in their tissues to photosynthesise and provide corals with much of their nutrition. Habitat loss for corals or intimately connected habitats (e.g. mangroves that filter terrestrial runoff) can also increase stress on coral habitats.

Direct human actions such as damage from boating and anchoring can physically harm corals and reef inhabitants. Over-harvesting of marine species both for food and the aquarium trade collection can interfere with critical ecosystem processes (e.g. removing key functional groups like herbivorous fish, that help keep reefs healthy). On the global front, climate change threatens SEQ reefs in two ways. Ocean acidification reduces growth in all calcifying species, including corals, making it harder for them to secrete their limestone structure. Ocean temperature and current changes may stress corals and result in noticeable species range shifts on the border between tropical and temperate regions.

Coral is extremely sensitive to their environment and yet reefs themselves are resilient ecosystems that have been around for millions of years. Most coral species found in SEQ are hearty species, capable of withstanding environmental extremes including changes in temperature, severe weather and siltation. However, multiple compounding specific stressors (like floods) and chronic stressors (like overfishing or climate change) without sufficient recovery time between events can reduce reef resilience and limit their ability to recover and maintain essential ecosystem processes.

 

How do we manage coral reefs in SEQ? 

State management agencies have jurisdiction in Queensland Coastal Waters, which extend 3 nautical miles out to sea. In 2009, the heritage-listed Moreton Bay Marine Park managed by the Queensland Department of Environment and Heritage Protection increased the coverage of green zones, where no fishing is allowed, from 0.5% to 16% of the Park. Local councils also contribute to managing issues through land-use planning which in turn affects catchment water quality and runoff flowing into estaurine and marine ecosystems. When coral reefs become stressed they expel the tiny algae (zooxanthellae) from their tissues resulting in what is commonly termed 'coral bleaching' (the whitening of diverse invertebrate taxa). If a coral remains stressed then it cannot reclaim zooxanthellae back into their tissues and they will not survive. 

It is essential that management frameworks for subtropical reef ecosystems are based on a sufficient scientific understanding relating to both terrestrial and marine ecological processes. Currently, there is further research needed to understand the complex dynamics that occur in this transitional marine habitat. This includes improved understanding about the impacts of global issues such as climate change as well as localised issues like water quality. It is also essential that communities understand their relationships with reefs and how they can take action to improve reef management and protection. Reef Check Australia is a not-for-profit environmental organisation that empowers the community to participate in reef conservation, including regular monitoring and reporting on reef health using a standardised global scientific protocol.  

Impacts of complex threats such as climate change are not well-understood for subtropical reefs. In November 2010, scientists and marine managers reviewed the current status of and called for greater effort to understand and protect unique subtropical reef habitats in a document titled The Coffs Harbour Subtropical Reefs Declaration. The declaration identifies 7 priorities for improving the management of subtropical reefs, including: improved knowledge and monitoring for management decisions, development of more holistic management frameworks that stretch across political boundaries and increased understanding of threats and identification of refuges.