Coral Reefs: Pollution's Devastating Impact Revealed

Coral reefs are susceptible to damage from pollution, which can have a devastating impact on their health. Sources of pollution include land-based activities such as coastal development, agriculture, and fishing practices. Sedimentation, for example, can smother corals and interfere with their ability to feed, grow, and reproduce. Plastic pollution is also a major threat, with a four-year study finding that plastic waste is sickening and killing reefs from Thailand to Australia. In addition, coral reefs are vulnerable to the effects of global warming, with unusually warm water contributing to disease and bleaching.

Sedimentation from coastal development

Sedimentation is a form of pollution that originates on land but finds its way into coastal waters. It is caused by coastal development, urban stormwater runoff, forestry and agriculture. Sedimentation has been identified as a primary stressor for the existence and recovery of coral species and their habitats.

Coastal development can also cause physical damage or destruction to coral reefs. This includes dredging, quarrying, destructive fishing practices and gear, boat anchors and groundings, and recreational misuse (such as touching or removing corals).

In addition to sedimentation, coral reefs are also threatened by other forms of pollution, including plastic pollution, organic and non-organic pollution, and excess nutrients in the water (eutrophication) which can lead to enhanced algal growth on reefs, crowding out corals and degrading the ecosystem.

Eutrophication

The increased nutrient levels in the water promote the growth of algae, leading to algal blooms. These blooms can smother coral reefs, blocking their access to sunlight and nutrients. Algal blooms can also produce toxic compounds that are harmful to corals and other marine organisms. As a result, corals may experience reduced growth rates, decreased reproductive success, and even death.

In addition to the direct impacts on coral reefs, eutrophication can also have indirect effects. For example, the increased nutrient levels can lead to an overgrowth of algae and other aquatic plants, which can alter the marine food web. This can result in a decrease in the availability of food sources for coral reefs, as well as an increase in competition for resources.

The effects of eutrophication on coral reefs can be long-lasting and difficult to reverse. Once algal blooms have taken over, it can be challenging to restore the balance of the ecosystem. Preventative measures, such as reducing nutrient runoff from land and improving sewage treatment processes, are crucial to mitigating the impacts of eutrophication on coral reefs. By addressing the root causes of eutrophication, we can help protect and preserve the delicate coral reef ecosystems.

Pesticides

A study by Warne et al. (2018) assessed the toxicity of eight insecticides and one fungicide on coral larvae. However, few of these toxicity data meet the experimental criteria required for inclusion in national water quality guidelines. To address this gap, a subsequent study identified the toxicity thresholds of three insecticides (diazinon, fipronil, imidacloprid) and two fungicides (chlorothalonil, propiconazole) on the larval metamorphosis of the common reef-building coral *Acropora tenuis*. The larvae were exposed to these pesticides for 48 hours, and the results showed that they were affected by all five pesticides.

Agricultural runoff is a significant source of pesticides in coral reef ecosystems. A study by Porter et al. (2018) reported on the widespread accumulation of organochlorine pesticides (OCPs) in reef organisms from the iSimangaliso Wetland Park in South Africa. Acetochlor, alachlor, and hexazinone were the predominant herbicides detected at all sites, with atrazine and simazine detected less frequently. Multiple herbicide residues were detected in 95% of the samples collected from ten sites along the Maputaland coast, with average concentrations ranging between 25.2 ng g−1 to 51.3 ng g−1 dw.

The presence of pesticides in coral reef ecosystems can have devastating effects on coral health and resilience. It is important to improve water quality guidelines and subsequent risk assessments to protect these valuable ecosystems.

Plastic pollution

Coral reefs are susceptible to bleaching due to unusually warm water, either from seasonal shifts in water temperature or from human-caused global warming. A four-year study found that plastic pollution is also killing coral reefs. Bags, bottles, diapers, cotton swabs, and wrappers are littering reefs and causing widespread damage.

Millions of tons of plastic waste end up in the ocean every year. This waste originates from land-based sources, such as sedimentation from coastal development, urban stormwater runoff, forestry, and agriculture. These sources of pollution threaten coral reef health. Excess nutrients result in poor water quality, leading to decreased oxygen and increased nutrients in the water (eutrophication). This can lead to enhanced algal growth on reefs, crowding out corals and significantly degrading the ecosystem.

Sediment deposited onto reefs smothers corals and interferes with their ability to feed, grow, and reproduce. Pesticides also interfere with coral reproduction and growth. Sewage discharge and runoff may 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.

Human activities have a substantial impact on coral reefs, contributing to their worldwide decline. Damaging activities include coral mining, pollution (both organic and non-organic), overfishing, blast fishing, and the excavation of canals and access points to islands and bays. Additional threats include disease, destructive fishing practices, and the warming of oceans.

Global warming

Coral reefs are susceptible to bleaching due to unusually warm water, which can be caused by seasonal shifts in water temperature or human-caused global warming. Global warming is a significant threat to coral reefs, as it can lead to coral bleaching and disease. The warming of the oceans can cause corals to expel their symbiotic algae, leaving them white and vulnerable to infection and disease. This process is known as coral bleaching and can lead to the death of the coral.

The effects of global warming are felt disproportionately in the oceans, with seawater absorbing over 90% of the excess heat trapped by greenhouse gases. This has led to a rise in sea surface temperatures, which can have devastating effects on coral reefs. Corals are sensitive to changes in water temperature, and even small increases can cause stress and damage.

In addition to coral bleaching, global warming can also contribute to ocean acidification. As the ocean absorbs more carbon dioxide, the water becomes more acidic. This can make it difficult for corals to build and maintain their calcium carbonate skeletons, leading to weakened structures and increased vulnerability to disease and other threats.

The impacts of global warming on coral reefs are already being felt, with coral bleaching events becoming more frequent and severe. In 2016, for example, a massive coral bleaching event affected over 70% of the Great Barrier Reef, leading to widespread coral death and ecosystem degradation. It is crucial to address global warming and reduce greenhouse gas emissions to protect coral reefs and the diverse array of marine life that depends on them.

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