Australian Coral Reefs: Pollution's Impact

Coral reefs are indispensable ecosystems that support about 25% of all marine species despite occupying less than 1% of the ocean floor. They are, however, in decline globally, with many scientists believing their existence may be in jeopardy unless efforts to protect them are intensified. Coral reefs in Australia, Thailand, Indonesia, and Myanmar are heavily contaminated with plastic, which clings to coral, causing sickness and death. Australia's Great Barrier Reef, the largest living structure on Earth, is also threatened by pollution from rivers, agricultural runoff, and sewage. This pollution includes silt and clay that smother corals, nutrients that reduce coral's thermal tolerance, and pathogens that cause disease.

Plastic pollution

The sources of plastic pollution that affect coral reefs include marine debris, such as plastic bags, bottles, and discarded fishing gear. This debris can snag on corals, blocking sunlight and hindering photosynthesis, which is essential for coral survival. It can also entangle and kill reef organisms, break or damage corals, and be ingested by coral and other marine life, blocking their digestive tracts and potentially introducing toxins.

Agricultural activities are a significant contributor to plastic pollution in coral reefs. Excessive fertiliser use on crops, such as sugarcane in Queensland, can wash into rivers and eventually reach the Great Barrier Reef. This nutrient pollution, particularly nitrogen, fuels harmful algal blooms, which block sunlight, reduce coral resilience, and promote coral bleaching. Additionally, sedimentation runoff from agriculture can smother corals, cloud the water, and hinder photosynthesis, further endangering the delicate coral ecosystem.

To address plastic pollution and protect Australia's coral reefs, comprehensive measures are necessary. These include improving wastewater treatment infrastructure, enforcing stricter regulations on wastewater discharges, educating communities about the impacts of plastic pollution, and promoting sustainable agricultural practices that reduce nutrient runoff and sedimentation. By tackling these issues, we can help ensure the longevity and recovery of Australia's precious coral reef ecosystems.

Agricultural runoff

The Queensland coast, with its extensive sugarcane and cattle farming, is a prime example of the impacts of agricultural runoff. Heavy rainfall can wash substantial quantities of fertilisers, herbicides, and soil into the waterways that feed into the Great Barrier Reef. This contributes to nutrient pollution, reduced water quality, and the proliferation of algae.

Excess nitrogen and phosphorus from fertilisers are key pollutants in agricultural runoff. They fuel the excessive growth of algae in coastal waters, a phenomenon known as eutrophication, leading to algal blooms that block sunlight from reaching the corals. This reduces the corals' ability to photosynthesise, making them weaker and more susceptible to disease and death. Additionally, the unchecked growth of algae can outcompete corals for space, hindering their colonisation and recovery.

Sedimentation runoff from agricultural activities also poses a significant threat to coral reefs. Increased sediment loads reduce the clarity of coastal waters, hindering the growth of light-dependent organisms such as corals. Silt and clay can smother corals, either by directly covering them or by clouding the water and reducing light availability. This interference with light availability can have detrimental effects on the coral's ability to photosynthesise and survive.

The impact of agricultural runoff on coral reefs extends beyond the immediate vicinity of agricultural areas. Even moderate levels of runoff can have disproportionately larger effects on reef systems situated in enclosed bays or lagoons, where water circulation is restricted, and pollutants accumulate more readily. Additionally, reefs that have already experienced stress from factors like climate change or overfishing may be more susceptible to the negative consequences of agricultural runoff.

To address the issue of agricultural runoff, the Queensland government passed a bill in 2019 to improve water quality flowing into the Great Barrier Reef from agricultural properties. However, stronger laws and effective pollution regulations are needed to ensure the protection of this fragile ecosystem.

River pollution

The Great Barrier Reef, which stretches along Australia's northeastern coast, is a spine of 3,800 reefs and atolls in the Coral Sea. It is the largest living structure on Earth. The delicate ecosystem of the Great Barrier Reef is threatened by river pollution.

Agricultural practices, such as excessive fertiliser use in sugarcane fields, contribute to nutrient-rich runoff that flows into rivers and ultimately out to the Great Barrier Reef. This nutrient pollution reduces coral's thermal tolerance, making them more susceptible to bleaching events. The increased nutrients in the water also promote the growth of algae, which can outcompete corals for space and provide a food source for the crown-of-thorns starfish (COTS), a native marine invertebrate that feeds on coral. While COTS typically do not cause significant harm, outbreaks fuelled by nutrient-rich conditions can lead to widespread damage and loss of coral cover.

Sediment runoff is another critical issue. Silt and clay from erosion and deforestation can smother corals, hindering their photosynthesis and growth. The runoff also reduces water clarity, further impacting light-dependent plants and animals, including corals. This reduction in water clarity can last for several months, affecting even middle and outer shelf reefs.

To address river pollution and protect the Great Barrier Reef, the Queensland government passed a bill in 2019 to improve water quality flowing from agricultural properties. The regulations target practices that pose the greatest risk to water quality, such as overusing fertiliser. Additionally, conservation efforts focus on prioritising interventions in areas that contribute the most pollution to target funding and resources effectively.

Crown-of-thorns starfish

COTS outbreaks can cause significant damage to the reef, including the loss of coral cover and a decline in biodiversity. During an outbreak, the number of starfish can explode, and their feeding habits can lead to extensive coral loss. COTS feed by extruding their stomachs through their mouths and secreting digestive enzymes to liquefy and absorb coral tissue. A single adult COTS can consume up to 10 square metres of coral per year, with a preference for fast-growing corals.

The survival of COTS larvae is closely linked to the abundance of their food source, algae. Nutrient pollution from land-based runoff can cause algal blooms, providing an ample food source for the larvae and contributing to COTS outbreaks. High nutrient levels in the water, particularly nitrogen and phosphorous, can also fuel phytoplankton blooms, which serve as food for the starfish larvae, ensuring their higher survival rates.

COTS breed through spawning during the summer months, with females releasing over 200 million eggs annually. The resulting larvae drift as plankton for 10 to 30 days before settling on the reef and developing into juvenile starfish. Juvenile COTS feed primarily on crustose coralline algae, a type of hard algae that forms a pink crust across reefs. Within two years, they mature and begin reproducing, continuing the cycle.

The impact of COTS on the Great Barrier Reef has been significant, with outbreaks leading to the loss of reef-building corals. Scientists are working to understand these outbreaks and develop methods to control them. While COTS are native to the region, their population must be kept in check to prevent widespread damage to the delicate coral ecosystem.

Climate change

Coral reefs are in decline globally, and climate change is a significant factor in this decline. Corals are sensitive to changes in ocean temperature, and as atmospheric temperatures rise due to climate change, seawater temperatures also increase. This warming places stress on corals, causing them to expel the microscopic algae that live within them and provide them with nutrition. Without the algae, the coral loses its coloration, a condition known as coral bleaching. Prolonged bleaching can eventually kill the coral or leave it more vulnerable to other threats such as infectious diseases.

The Great Barrier Reef in Australia has experienced multiple mass bleaching events due to rising ocean temperatures, with severe bleaching events occurring in 1998, 2002, 2016, 2017, 2020, and 2022. These bleaching events have led to widespread coral die-offs and the collapse of coral reef ecosystems. The increased frequency and severity of marine heatwaves are causing significant damage to the reefs, and the future of these ecosystems looks vastly changed.

In addition to ocean warming, climate change is also causing ocean acidification, which corrodes coral skeletons. The oceans absorb a significant amount of human-made carbon dioxide from the air, and this excess carbon dioxide dissolves in seawater, increasing its acidity. This acidic water weakens the coral polyps and makes them more susceptible to damage and disease.

The impacts of coral reef degradation affect not only the marine ecosystem but also human communities. Coral reefs provide food and livelihoods for millions of people worldwide, and their degradation can have significant economic and social consequences. The tourism industry, for example, is directly impacted by the decline of coral reefs, as visitors are drawn to the beauty and biodiversity of healthy reef ecosystems.

Scientists predict that if urgent action is not taken to reduce greenhouse gas emissions, we could lose over 90% of our coral reefs by 2050. This would be a devastating loss for the planet's biodiversity and for human communities dependent on these fragile ecosystems.

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