Bronte Wells
Coral reefs are incredibly important ecosystems that are currently under threat from a variety of sources, one of the most significant being pollution. Pollution can damage reefs in a multitude of ways, from plastic waste and marine debris to nutrient and chemical runoff. Human activities such as coastal development, dredging, and agricultural practices all contribute to the pollution that is killing coral reefs. As human populations expand in coastal areas, the landscape is altered, increasing land-based sources of pollution and threatening the health of coral reefs.
| Characteristics | Values |
|---|---|
| Plastic pollution | Bags, bottles, diapers, cotton swabs, wrappers, toys, beads in soap, microplastics, etc. |
| Marine debris | Abandoned fishing nets, household products, outdoor items, etc. |
| Land-based sources of pollution | Coastal development, dredging, deforestation, agricultural runoff, sewage treatment plant operations, etc. |
| Nutrient pollution | Nitrogen, phosphorus |
| Eutrophication | Algal blooms, phytoplankton blooms |
| Excess nutrients | Intensify diseases like Aspergillosis, a fungal infection, and yellow band disease, a bacterial infection |
| Air pollution | Coal-burning, volcanic eruptions |
| Oil spills | Petroleum products |
| Chemical spills | Pesticides, anti-fouling paints and coatings |
| Poor water quality | High levels of nutrients, wastewater, industrial effluent, etc. |
| Destructive fishing practices | Dynamite, cyanide, trawling, blast fishing |
| Overfishing | Reduces grazing fish populations, altering food-web structure |
| Coral harvesting | For the aquarium trade, jewelry, curios |
| Increased ocean temperatures | Rise in seawater temperatures |
| Climate change | Coral bleaching |
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What You'll Learn
Plastic pollution
Plastic provides an ideal surface for disease-causing microbes to attach to and colonize. It can carry bacteria and pathogens, promoting the development of diseases in corals. Three major diseases of corals—skeletal eroding band disease, white syndromes, and black band disease—increased with plastic contact. Plastic debris also stresses coral through light deprivation, toxin release, and anoxia, further increasing the risk of disease.
The impact of plastic on coral reefs is exacerbated by the fact that coral reefs are already susceptible to bleaching and disease due to unusually warm water temperatures, whether from seasonal shifts or human-caused global warming. Plastic pollution, therefore, adds to the existing stressors on coral reefs, making them even more vulnerable.
The plastic pollution affecting coral reefs originates from land, with an estimated 80% of marine plastic debris coming from coastal regions with poor waste management. Ten major rivers, passing through heavily populated areas, contribute to the majority of plastic trash in the ocean. The Asia-Pacific region, in particular, struggles with mismanaged plastic waste, with nine of the ten countries with the highest global levels of plastic waste entering the ocean located in this region.
To protect coral reefs from plastic pollution, it is essential to reduce plastic consumption, promote reusable alternatives, and improve waste management and recycling practices, especially in coastal regions.
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Nutrient pollution
Nutrients such as nitrogen and phosphorous enter coral reefs through agricultural and residential fertilizer use, sewage discharges, and animal waste. These nutrients can also enter reefs through stormwater runoff, which carries large quantities of nutrients from agricultural areas and sewage outflows. This runoff also contains toxins, sediments, and pathogens that can be harmful to coral reefs.
Excess nutrients can support the growth of microorganisms, like bacteria and fungi, that can be pathogenic to corals. Bacteria and parasites from fecal contamination can cause disease in corals, especially if they are already stressed by other environmental conditions. Coral disease occurs naturally, but the input of pathogen-containing pollution can increase the frequency and intensity of disease outbreaks.
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Land-based runoff
Sedimentation has been identified as a primary stressor for the existence and recovery of coral species and their habitats. Sediment deposited onto reefs can smother corals and interfere with their ability to feed, grow, and reproduce. Nutrients (nitrogen and phosphorus) from agricultural and residential fertilizer use, sewage discharges, and animal waste can lead to the growth of algae that blocks sunlight and consumes the oxygen corals need for respiration. This often results in an imbalance affecting the entire ecosystem.
Excess nutrients can also support the growth of microorganisms, like bacteria and fungi, that can be pathogenic to corals. Pathogens from inadequately treated sewage, stormwater, and runoff from livestock pens can cause disease in corals, especially if they are stressed by other environmental conditions. Sewage discharge and runoff may also introduce pathogens into coral reef ecosystems. For example, Aspergillus sydowii has been associated with a disease in sea fans, and Serratia marcescens has been linked to white pox, another coral disease.
Trash and microplastics from improper disposal and stormwater runoff can snag on corals and block the sunlight needed for photosynthesis. Derelict (abandoned) fishing nets and other gear—often called "ghost nets"—can entangle and kill reef organisms and break or damage reefs. Even remote reef systems suffer the effects of marine debris. Natural disasters such as hurricanes, tropical storms, tsunamis, and landslides have the potential to be the source of a tremendous amount of marine debris.
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Destructive fishing practices
Coral reefs face a variety of threats from destructive fishing practices and gear. One such practice is overfishing, which can alter the food-web structure and cause a reduction in the number of grazing fish that keep corals clean of algal overgrowth. This can lead to an imbalance in the biological checks and balances of a reef ecosystem, making it more susceptible to disease and invasive species.
Blast fishing, which involves using explosives to kill fish, can also cause physical damage to corals. In addition, certain types of fishing gear, such as nets and traps, can inflict serious physical damage to coral reefs and other important marine habitats. Abandoned fishing nets, often referred to as "ghost nets," can entangle and kill marine life, including reef organisms, and damage reefs.
Coral harvesting for the aquarium trade, jewelry, and curios is another destructive fishing practice that can lead to the over-harvesting of specific species, destruction of reef habitats, and reduced biodiversity. This can decrease the overall resilience of the reef and increase its susceptibility to disease and invasive species.
Fishing plays a central social and cultural role in many island and coastal communities, providing food and income for over a billion people worldwide. However, unsustainable fishing practices, such as the use of efficient fishery technologies and inadequate management, can lead to the depletion of key reef species and habitat damage. Nearshore habitats serve as nurseries for many fish, and catching young fish in nets can remove them before they can help replenish the population.
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Climate change
Coral reefs are extremely sensitive to changes in their environment. As ocean temperatures increase, corals become stressed and expel the microscopic algae that live within their tissues and produce food for them. This loss of algae, known as coral bleaching, causes the corals to turn white, revealing the colour of their underlying skeleton. Bleached corals are more vulnerable to other stressors, and if the stressor is not removed, they can die. With climate change, the frequency of marine heatwaves is increasing, leaving corals with insufficient time to recover between events.
The warming of the oceans is caused by the absorption of carbon dioxide from the atmosphere. Human activities, such as burning fossil fuels, have produced a significant amount of carbon dioxide, leading to increased levels in the ocean. This process is called ocean acidification, which reduces the pH of the seawater and decreases calcification rates in reef-building organisms, negatively impacting their growth and structural integrity.
In addition to ocean warming and acidification, climate change will also affect coral reefs through sea-level rise, changes in the frequency and intensity of tropical storms, and altered ocean circulation patterns. These changes will further stress coral reefs, making them more susceptible to disease and other threats.
To protect coral reefs, it is crucial to address the root cause of climate change by reducing greenhouse gas emissions and limiting global temperature rise. The Paris Agreement, which aims to keep global temperature rise well below 2°C above pre-industrial levels, provides a framework for mitigating the impacts of climate change on coral reefs. By reducing emissions and transitioning to sustainable economic practices, we can slow down ocean warming and give coral reefs a chance to recover and adapt.
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Frequently asked questions
Plastic pollution can kill coral reefs by clinging to coral, blocking sunlight, and providing a surface for pathogens to infect the coral. Plastic debris can also entangle and kill reef organisms, and corals can ingest microplastics, which blocks their digestive tracts and introduces toxins.
Nutrient pollution, such as nitrogen and phosphorus, can cause eutrophication, enhancing the growth of algae and other organisms that smother corals. This process can also create hypoxic conditions by depleting oxygen levels.
Oil spills can damage the eggs and sperm of corals during spawning, as they float near the surface. Oil pollution can also compromise water quality, making corals more vulnerable to other disturbances.
