Pollution's Impact: Coral Bleaching Explained

Coral reefs are incredibly diverse ecosystems that support thousands of marine species. However, they are under constant threat from human activities, including pollution. Pollution from land-based sources such as coastal development, deforestation, and agricultural runoff can have detrimental effects on coral health and contribute to coral bleaching. Additionally, plastic pollution has been identified as a significant stressor, with plastic waste entangled in coral reefs causing abrasion and increasing the likelihood of disease. Climate change, which is influenced by human activities, also plays a crucial role in coral bleaching, as even small increases in water temperature can trigger corals to expel their symbiotic algae, resulting in a loss of colour and, eventually, their death.

Land-based pollution

One of the primary land-based pollution sources is coastal development and impervious surfaces, which contribute to sedimentation and the release of toxins. This includes activities such as deforestation, road construction, and urbanization, which can alter drainage basins and increase the volume of pollution released into adjacent coral reef ecosystems. Sedimentation can smother corals, blocking their access to sunlight and causing stress, which can lead to bleaching.

Agricultural runoff and the use of fertilizers and pesticides are also major contributors to land-based pollution affecting coral reefs. Excess nutrients from agriculture can cause an overgrowth of algae, which can smother corals and reduce water quality. Pesticides and herbicides can also be toxic to corals and disrupt the ecological balance of reef ecosystems.

Another critical land-based pollution source is plastic waste. Plastic items such as bags, bottles, diapers, and wrappers end up in the ocean, where they can entangle and smother coral reefs. Plastic pollution also provides a surface for bacteria and other harmful microorganisms to accumulate, increasing the risk of coral diseases. According to a study by Drew Harvell at Cornell University, the presence of plastic can increase the likelihood of disease in corals from 4% to 89%.

To protect coral reefs from land-based pollution, it is essential to address the root causes and implement proper waste management practices. This includes reducing the use of single-use plastics, improving recycling programs, and properly disposing of waste to prevent it from ending up in the ocean. Additionally, individuals can make small changes such as reducing stormwater runoff, using eco-friendly sunscreens, and avoiding the use of herbicides and pesticides to minimize their impact on coral reefs.

Plastic pollution

Firstly, plastic can cause physical damage to coral tissues. The more complex and branched the shape of the coral, the more likely it is to retain plastic debris, which can tear and abrade the coral's surface, making it susceptible to infection by bacteria and other harmful microorganisms present in the water.

Secondly, plastic can act as a vector for pathogens, carrying and spreading diseases between reefs. The presence of plastic debris has been linked to an increased prevalence of common coral diseases, including skeletal eroding band, white syndromes, black band, growth anomalies, brown band, and atramentous necrosis, which are associated with rapid coral mortality.

Additionally, plastic can block sunlight from reaching the coral, hindering its ability to photosynthesize and affecting its growth and health. Plastic pollution can also alter the nutrient balance in the water, leading to excess nutrients that promote the growth of algae and microorganisms that can be harmful to corals.

The impact of plastic pollution on coral reefs is a growing concern, and it is important for individuals and communities to reduce their plastic consumption, properly recycle and dispose of plastic waste, and prevent plastic from ending up in the environment to mitigate these effects.

Increased likelihood of disease

Coral reefs are already highly susceptible to disease due to warm water temperatures, and pollution only increases this likelihood. A study in the Pacific Ocean found that the likelihood of disease in corals increased from 4% to 89% when corals were in contact with plastic. This is because plastic abrades corals, creating openings that allow pathogens to infect the coral. In addition, plastic can block sunlight from reaching the coral, which may also contribute to higher rates of coral disease.

The presence of plastic pollution in coral reefs is a significant issue, with an estimated 11 billion plastic items entangled in coral reefs in the Asia-Pacific region alone. This region is home to over half of the world's coral reefs, and the problem is exacerbated by the proximity of dumps to the ocean in many of these countries. The issue is not limited to the Asia-Pacific region, however, as reefs in Australia, Thailand, Indonesia, and Myanmar were all found to be heavily contaminated with plastic.

The impact of plastic pollution on coral health is not fully understood, but studies have shown a clear link between plastic contact and increased disease in corals. For example, a survey of 150 reefs found that corals in close proximity to each other showed a noticeable difference in health, with those in contact with plastic being much more likely to be diseased. This suggests that the presence of plastic itself is a significant factor in coral disease, independent of other pollutants that may be present in the water.

In addition to plastic pollution, other forms of pollution, such as toxicants, sediments, and nutrients, can also impact coral health and increase the likelihood of disease. These land-based sources of pollution, such as coastal development, deforestation, agricultural runoff, and oil and chemical spills, can impede coral growth and reproduction, disrupt ecological functions, and cause disease and mortality in sensitive species. Therefore, it is essential to address both plastic pollution and other forms of pollution to protect coral reefs from the increased likelihood of disease.

Blocked sunlight

Coral reefs are in decline, and scientists believe their existence is in jeopardy unless we intensify our efforts to protect them. One of the main threats to coral reefs is pollution, which can impede coral growth and reproduction, disrupt ecological functions, and cause disease and mortality in sensitive species.

One way that pollution affects coral reefs is by blocking sunlight. Excess nutrients from agricultural and residential fertilizer use, sewage discharges, and animal waste can lead to the growth of algae that blocks sunlight and consumes oxygen, which corals need for respiration. This often results in an ecosystem imbalance. Trash and microplastics from improper disposal and stormwater runoff can also block the sunlight needed by corals for photosynthesis. Plastic bags, bottles, and discarded fishing gear can snag on corals, entangle and kill reef organisms, and break or damage corals.

Sedimentation from coastal development, urban stormwater runoff, forestry, and agriculture can smother corals and interfere with their ability to feed, grow, and reproduce. This has been identified as a primary stressor for the existence and recovery of coral species and their habitats.

In addition, the chemicals in sunscreens can accumulate in areas with significant marine tourism and recreational use and have been shown to cause bleaching in corals. Oxybenzone, in particular, has been found to cause deformities in coral larvae and baby corals, damage to coral DNA, and abnormal skeletal growth.

The blocking of sunlight due to pollution can have detrimental effects on coral reefs, hindering their growth, reproduction, and overall health.

Climate change

Rising ocean temperatures

The primary cause of coral bleaching is rising ocean temperatures caused by climate change. A change in water temperature—as little as 2 degrees Fahrenheit—can cause coral to expel the symbiotic algae that give them colour and provide them with nutrients. If the temperature stays high, the coral will starve and die.

The average temperature of tropical oceans has increased by 0.1˚ C over the past century. This, combined with natural fluctuations of warmer ocean temperatures, has resulted in extensive coral bleaching around the globe, involving thousands of square miles of reefs.

During the 1997-1998 El Niño, widespread and severe coral reef bleaching occurred in the Indo-Pacific region and the Caribbean, killing 16% of the world’s coral reefs in 12 months. The 2010 El Niño also resulted in massive bleaching around the world. Another coral bleaching event in October 2015 extended into 2017, becoming the longest and most damaging ever recorded. More than 80% of the northern part of the Great Barrier Reef has now suffered severe bleaching.

Ocean acidification

The ocean absorbs carbon dioxide from the atmosphere, making it more acidic. Since the late 18th century, the ocean has absorbed about 30% of the carbon humans have generated, decreasing its pH level. A more acidic ocean means corals are less able to build skeletons and form coral reefs, which help protect coastlines from storms and provide habitats for thousands of species of marine life.

Severe weather events

Marine species migration

As water temperatures rise, many marine species are being forced to move south to cooler habitats. This shift creates increased competition for food and shelter in cooler waters, threatening the entire ecosystem. For Reef communities, the loss of marine life can have a devastating impact on local ecosystems, food sources and other industries such as tourism.

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