Water Pollution's Impact On The Great Barrier Reef

The Great Barrier Reef, the largest living structure on Earth, is under threat from water pollution. Stretching along Australia's northeastern coast, the reef is formed of 3,800 reefs and atolls that together create a unique ecosystem. Water pollution, largely from land-based activities, is one of the highest risks to the health of the reef. With climate change posing an overarching threat, reducing water pollution is crucial to alleviating further pressure on the reef and aiding its recovery.

The impact of toxicants, sediments, and nutrients

Water pollution is one of the highest risks to the Great Barrier Reef's health. As human populations and development expand in coastal areas, the landscape is altered, increasing land-based sources of pollution and threatening the health of coral reefs.

Toxicants, sediments, and nutrients are all major contributors to water pollution and have a detrimental impact on the Great Barrier Reef. These pollutants originate from land-based sources such as coastal development, deforestation, agricultural runoff, and oil and chemical spills. The increased volume of land-based pollution released into the ocean has impeded coral growth and reproduction, disrupted ecological functions, and caused disease and mortality in sensitive species.

Toxicants

Toxicants, or toxins, are harmful substances that can be released into the water through activities such as coastal development and oil and chemical spills. These toxins can impede coral growth and reproduction and disrupt the overall ecological function of the reef. They can also cause disease and mortality in sensitive species.

Sediments

Sedimentation, the process of solid matter being carried and deposited by water, has a significant impact on the health of the Great Barrier Reef. Fine sediment can stay suspended in the water for long periods, causing turbidity and reducing light for seagrasses and corals. This reduced light can interfere with filter feeding, reduce coral recruitment, and alter the quantity and quality of light available for photosynthesis, essential for the growth of coral and seagrass. Sediment can also smother corals, affecting their reproductive cycle and early development and damaging gills. The main source of sediment is erosion from the mainland, carried by rivers and streams as runoff during the wet season. Agricultural land use, especially overgrazing, is a significant contributor to sedimentation.

Nutrients

While nutrients are essential for the growth of plants and animals, excessive levels can upset the natural balance of reef ecosystems. Annual nutrient discharge into the Great Barrier Reef has more than doubled since European settlement, with sugarcane farming being the greatest contributor. Excess nutrients can lead to increased outbreaks of coral-eating crown-of-thorns starfish, lower coral diversity, algal blooms, and increased susceptibility to coral bleaching and diseases. Algal blooms, caused by high levels of nitrogen from fertilisers, block sunlight, reducing coral resilience and diversity. Additionally, while additional nutrients can enhance seagrass growth, they also favour plankton and algae growth, reducing light availability for seagrass leaves, which can lead to algae domination.

The role of climate change

Climate change is the greatest threat to the Great Barrier Reef, with water pollution from land-based activities like agriculture being a close second. The Great Barrier Reef is the largest living structure on Earth and is one of the most complex natural ecosystems in the world. It is also one of the world's most powerful blue carbon sites, absorbing carbon dioxide from the atmosphere and storing carbon. However, it is in danger due to climate change.

The planet is currently experiencing a steady rise in temperatures on land and in oceans, caused primarily by an increase in carbon dioxide and other greenhouse gases. This temperature increase is due to human activities, mainly the emission of greenhouse gases. Since the late 18th century, the ocean has absorbed about 30% of the carbon generated by humans, decreasing its pH level. This process is known as ocean acidification, which makes it harder for corals to build skeletons and form coral reefs.

As the ocean absorbs most of the heat trapped in the Earth's atmosphere, marine ecosystems are at risk. Rising ocean temperatures put coral reefs under stress, causing them to expel the microscopic algae that live inside their tissues. This is known as coral bleaching, and while bleached corals are not dead, they are more susceptible to starvation and disease. Marine heatwaves have already triggered mass coral bleaching events on the Great Barrier Reef, reducing shallow-water coral reefs by up to 50%. If global temperatures rise by 2 degrees, coral reefs may be lost forever, devastating thousands of marine species and the livelihoods of around a billion people worldwide.

Climate change also increases the frequency and intensity of severe weather events such as storms, flooding, and cyclones, to which coastal regions like the Great Barrier Reef are particularly vulnerable. Between 2004 and 2018, 10 cyclones of category three or higher crossed the Great Barrier Reef, causing significant damage to coral reefs. As water temperatures rise, marine species are forced to move south to cooler habitats, creating increased competition for food and shelter and threatening the entire ecosystem.

To protect the Great Barrier Reef from climate change, urgent global action is needed to drastically reduce greenhouse gas emissions. Additionally, it is crucial to strengthen key ecosystems like seagrass meadows, mangroves, and wetlands, which play a vital role in absorbing carbon dioxide and combating climate change. While reducing emissions is crucial, it is also important to help coral reefs adapt to the warmer temperatures already caused by climate change. This involves developing and scaling up interventions that buy time for coral reefs, protect them from severe bleaching, and help them adapt to higher temperatures.

Agricultural runoff

The Great Barrier Reef is a beautiful yet fragile ecosystem that is under threat from water pollution caused by agricultural runoff. This runoff is a result of excess water flowing across the land surface or into groundwater after rainfall or irrigation, carrying with it fertilisers, pesticides, and sediment. With 35 major catchments draining into the Great Barrier Reef Region, a vast array of reefs and ecosystems are at risk.

Agricultural activities in these catchments are a significant source of water pollution, with grazing lands and sugarcane crops being the primary contributors. The excessive use of fertilisers on crops, such as sugarcane, leads to a wash of nitrogen into the rivers and, ultimately, the Great Barrier Reef. This excess nitrogen is linked to harmful algal blooms, which block sunlight and reduce the resilience of corals, leading to bleaching and a decline in coral diversity. The growth of seagrasses, upon which dugongs, turtles, and fish depend, is also hindered by these algal blooms.

Pesticides and herbicides used in agriculture have been detected in high concentrations in the inshore areas of the Great Barrier Reef. These chemicals inhibit the growth of non-target plants like seagrasses and harm marine plants and animals, further disrupting the delicate balance of the ecosystem. Tree clearing and overgrazing have also led to soil erosion, with an estimated 17 million tonnes of sediment washing into the rivers and eventually smothering the reefs.

The impact of agricultural runoff on the Great Barrier Reef is far-reaching. It reduces the resilience of the reefs and affects biodiversity. The inshore areas of the Marine Park are particularly vulnerable, with high concentrations of sediment, nutrients, and pesticides impacting the plants, animals, and ecosystem functions. The reefs' ability to recover from disturbances is also compromised, with coral recovery rates declining over the years.

To protect and restore the health of the Great Barrier Reef, addressing agricultural runoff is crucial. This includes implementing best practices in agricultural land management, reducing the use of fertilisers and pesticides, and restoring and protecting the land to prevent soil erosion. By tackling these issues, we can give the Great Barrier Reef the clean water it needs to restore its health and ensure its long-term survival.

Oil and chemical spills

The impacts of oil spills on corals depend on the species, maturity, and level of exposure to oil. Even small amounts of oil over extended periods can be harmful. Once oil comes into contact with corals, it can impede their reproduction, growth, behaviour, and development. The entire reef ecosystem can be affected, including fish, crabs, and other marine invertebrates. Oil spills can also lead to the use of chemical dispersants, which can further harm corals and the marine life they support.

In the case of the Shen Neng 1 incident, the ship's fuel tank was damaged, creating a narrow oil slick near the vessel. While the initial estimate suggested a loss of up to 150 tonnes of oil, it was later determined that only 3 to 4 tonnes were released. However, the oil spill still had significant consequences, with oil tarballs washing up on the beaches of North West Island, a critical bird and turtle habitat. The ship also created the longest grounding scar on the Great Barrier Reef, measuring approximately 3 kilometres long and 250 metres wide, and some areas were left devoid of marine life.

The response to the oil spill involved the use of tugboats to stabilise the vessel and prevent it from breaking apart in rough seas. The ship's master and officer-on-watch were charged, and the maximum fines for shipping companies causing damage to the reef were increased. Beach clean-up operations and oil removal from North West Island commenced, and the ship was eventually towed to Singapore. The Australian government settled with the ship's owners for $29 million in 2016, and remediation efforts began in 2019.

To protect the Great Barrier Reef from oil and chemical spills, regulations and interventions are necessary. This includes improving land-use practices, such as reducing tree clearing and implementing more targeted approaches to funding and incentives for farmers to reduce nutrient runoff. Additionally, stronger clearing regulations and restoration efforts are crucial to preventing erosion and reducing sediment runoff into the reef.

Crown-of-thorns starfish outbreaks

Crown-of-thorns starfish (COTS) are native to the Great Barrier Reef and occur naturally throughout the Indo-Pacific region. They are large marine invertebrates, growing up to 80cm in diameter, and are the world's second-largest starfish species. They have up to 21 arms, hundreds of venomous, toxin-tipped spines, and feed on coral.

COTS prey on almost all types of coral and can consume up to 10 square metres of coral per year. They feed by pushing their stomachs out through their mouths, covering the coral with digestive enzymes, and then converting the coral tissue into a soup. COTS breed by spawning during the summer months, with females releasing over 200 million eggs per year.

COTS outbreaks have been a major source of coral loss on the Great Barrier Reef since 1962, causing extensive damage to reef-building corals. During an outbreak, COTS can strip a reef of up to 90% of its living coral tissue. These outbreaks can take about a decade to spread south along the reef and are one of the major causes of coral decline in the region over the past 40 years.

The outbreaks are likely caused by a combination of factors, including increased larval survival due to higher nutrient levels in the water, which boost phytoplankton—the food source for COTS larvae. Nutrient runoff from land-based activities, such as agriculture, has been linked to these outbreaks. Additionally, the removal of natural COTS predators, such as the giant triton snail, may also contribute to the increased outbreak frequencies.

To control COTS outbreaks, targeted programs have been implemented, with trained divers injecting the starfish with bile salts or vinegar to kill them without harming the surrounding ecosystem.

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