Catchment Pollution: Killing The Great Barrier Reef

The Great Barrier Reef is the largest living structure on Earth, stretching along Australia's northeastern coast. It is under threat from a combination of rising water temperatures, poor water quality from sediment runoff and pollution, and more frequent severe weather events. Catchment pollution, where contaminants are carried by water runoff into the ocean, is a significant contributor to the decline of the reef. This has been caused by a variety of factors, including agricultural practices, deforestation, and coastal development, which have led to increased sedimentation, nutrient runoff, and toxicants in the water. These pollutants have disrupted the delicate ecosystem of the reef, impacting the health and resilience of coral reefs and the species that depend on them.

Land-based runoff

The primary sources of land-based runoff are agricultural activities in the catchments, including grazing lands and sugarcane crops. Grazing lands are the main contributor of fine sediment and particulate nitrogen on the Reef, while sugarcane crops are the primary source of excess nutrients and pesticides. The excessive use of fertilisers on crops like sugarcane can wash into rivers and, ultimately, out to the Great Barrier Reef. Nitrogen from these fertilisers is linked to harmful algal blooms, which can block sunlight, reducing coral's resilience to bleaching and coral diversity. Increased nutrients in the water also stimulate the growth of algae, which compete with coral for space.

In addition to agriculture, other sources of land-based runoff include other agricultural industries (e.g. horticulture), urban areas, sewage treatment plants, aquaculture activities, mining, industrial areas, ports, and defence activities. These activities can contribute pollutants such as petroleum hydrocarbons, marine debris and microplastics, pharmaceuticals, and personal care products.

The impact of land-based runoff on the Great Barrier Reef is significant. Increased concentrations of sediment, nutrients, and pesticides can harm the Reef's plants, animals, and ecosystem functions. High concentrations of fine sediment can reduce coral diversity, affect reproduction, disrupt coral recruitment, and increase susceptibility to disease. It can also damage the gills and affect the metabolism of some fish species. Suspended sediment, along with nutrients and other organic particles, reduces the amount of available light for seagrass and corals to grow, a process known as sedimentation.

To address the issue of land-based runoff, the Queensland government passed a historic bill in 2019 to improve water quality flowing into the Reef from agricultural properties. The bill targets practices that pose the greatest risk to water quality, such as the overuse of fertiliser. Additionally, there have been efforts to improve land management practices by landholders and the community in partnership with governments, natural resource managers, industry, research, and conservation groups.

Climate change

The increase in ocean temperatures causes corals to undergo heat stress, expelling the microscopic algae that live inside their tissues and revealing their white skeletons. While bleached corals are not dead, they are at a higher risk of starvation and disease. The Great Barrier Reef has already experienced four mass coral bleaching events in seven years, reducing shallow water coral reefs by 50%. While coral reefs can recover from bleaching, this is only if temperatures decrease. As the ocean continues to absorb carbon dioxide, becoming more acidic, it becomes harder for corals to build their skeletons and form reefs. This ocean acidification also reduces the resilience of corals and other calcifying organisms to climate impacts.

The rise in ocean temperatures also forces marine species to migrate to cooler waters, creating increased competition for food and shelter and threatening the entire ecosystem. The loss of marine life has a devastating impact on local ecosystems, food sources, and industries such as tourism and fishing. Climate change is also increasing the frequency and intensity of severe weather events, with damaging cyclones, floods, and storms causing significant damage to the Great Barrier Reef.

To protect the Great Barrier Reef from climate change, urgent global action is required to drastically reduce greenhouse gas emissions. This includes strengthening key ecosystems like seagrass meadows, mangroves, and wetlands, which absorb carbon dioxide from the atmosphere and play a vital role in combating climate change. Additionally, interventions are needed to help coral reefs adapt to the warmer temperatures and rebuild resilient reefs.

Water quality

The Great Barrier Reef, the largest living structure on Earth, is under threat from declining water quality. Land-based runoff is the greatest contributor to poor water quality in the inshore areas of the reef and is a major factor in the current poor state of many inshore marine ecosystems. The main sources of land-based runoff are agricultural activities in the catchments, including grazing lands and sugarcane crops. These activities contribute to increasing levels of sediments, nutrients, and contaminants in the water, which smother and block sunlight from corals and seagrasses, reducing their growth and resilience.

Excessive fertiliser use on crops, such as sugarcane, can wash into rivers and, ultimately, out to the Great Barrier Reef. Nitrogen from these fertilisers is linked to harmful algal blooms, which further block sunlight, reducing coral diversity and making corals more susceptible to bleaching. Increased nutrients in the water also stimulate the growth of algae, which competes with coral for space, and promotes the breeding success of the destructive crown-of-thorns starfish, a significant contributor to the loss of coral cover.

Pesticides and herbicides used on crops also wash into the reef, inhibiting the growth of non-target plants such as seagrasses, which are depended on by dugongs, turtles, and fish. These chemicals have been detected in high concentrations in inshore areas of the World Heritage Area and pose a significant risk to marine life.

In addition to agricultural activities, other sources of water pollution in the Great Barrier Reef catchment area include urban areas, sewage treatment plants, aquaculture, mining, industrial areas, ports, and defence activities. These activities contribute pollutants such as petroleum hydrocarbons, marine debris and microplastics, pharmaceuticals, and personal care products.

The declining water quality in the Great Barrier Reef is a critical issue that needs to be addressed urgently. It not only threatens the health and resilience of the reef ecosystem but also compromises reef-dependent industries such as tourism, fishing, and recreation. Improving water quality is crucial for the long-term survival and recovery of the Great Barrier Reef.

Erosion

The clearing of trees and overgrazing of grasslands in the catchment areas of the Great Barrier Reef have left the soil vulnerable to erosion. Without the roots of trees to hold the soil together or sufficient vegetation cover to protect the ground, heavy rainfall can easily wash large amounts of soil into nearby creeks and rivers, which eventually flow into the reef. This erosion results in increased levels of sediment in the water, reducing its transparency and causing it to appear murky or muddy. This condition, known as turbidity, leads to reduced light availability for coral reefs and seagrass, hindering their growth and survival.

Agricultural practices, such as grazing, gully erosion, and streambank erosion, are major contributors to the increased sedimentation in the Great Barrier Reef. In particular, grazing pressure within and around gullies has been identified as a primary cause of hillslope gully erosion. By excluding cattle from gullies and maintaining ground cover, land managers can help minimise sediment runoff and promote healthy soil.

The sediment entering the Great Barrier Reef is often very fine and can remain suspended in the water for extended periods, travelling great distances. This fine sediment interferes with filter feeding by organisms such as clams and can smother corals, reducing their growth and impacting their reproductive cycles. Additionally, the nutrients and pollutants attached to the sediments can be released into the marine environment, creating further problems for coral and seagrass ecosystems.

The Australian and Queensland governments have recognised the significance of this issue and are investing in gully remediation projects to address erosion. These projects aim to improve land management practices and reduce the amount of sediment entering the Great Barrier Reef, thereby improving water quality and enhancing the resilience of this valuable ecosystem.

Crown-of-thorns starfish

COTS breed through spawning in the summer months, with females and males releasing their eggs and sperm into the water simultaneously to fertilise. A large female COTS can release over 200 million eggs in a year. The resulting larvae drift as plankton for 10-30 days, feeding on microscopic plants called phytoplankton before metamorphosing into a small starfish with five arms. Within six months to a year, a juvenile starfish will change into its adult form and begin eating coral. Within two years, it will be mature and ready to reproduce.

The starfish get their name from the numerous sharp spines that cover their upper body, which can be up to 4cm long. These spines are effective in deterring predators and are covered in plancitoxins that can cause liver damage. When threatened, COTS also release saponins, compounds that destroy red blood cells. Together, these toxins can cause great pain to any animal or human that comes into contact with them. However, some predators are brave enough to feed on this cocktail of spines and slime, including the giant triton snail, humphead Maori wrasse, starry pufferfish, and titan triggerfish.

COTS outbreaks have been a major source of coral loss on the Great Barrier Reef since 1962, and a fourth outbreak is currently underway in the World Heritage Area. These outbreaks are responsible for extensive loss of reef-building corals and can kill up to 90% of the corals on affected reefs. Scientists and managers are working together to understand and control these outbreaks, which generally start offshore from Cairns or further north and take about a decade to spread south along the Reef.

Nutrient pollution from land-based runoff, which increases algal blooms, is one of the factors contributing to COTS outbreaks. Increased algal growth provides a boost in the food source for COTS larvae, increasing their survival rates. Therefore, reducing nutrient pollution from rivers flowing into the Reef is critical in helping to sustain its health and reduce the impact of COTS outbreaks.

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