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Coral reef plastic pollution increases with depth and near protected areas

In the most comprehensive survey of plastic pollution on coral reefs to date, an international team of scientists has found that – rather counter-intuitively – the amount of plastic debris increases with depth.

The researchers determined that coral reef plastic pollution largely stems from fishing activities, and is associated with proximity to marine protected areas. 

Mapping the abundance, distribution, and drivers of plastic pollution at various depths could enable scientists to identify what conservation efforts should be prioritized – and where – to protect the Earth’s essential yet highly vulnerable coral reefs.

“Plastic pollution is one of the most pressing problems plaguing ocean ecosystems, and coral reefs are no exception,” said lead author Hudson Pinheiro, a marine biologist at the University of São Paulo, and a research fellow at the California Academy of Sciences

“From macroplastics that spread coral diseases to fishing lines that entangle and damage the structural complexity of the reef, decreasing both fish abundance and diversity, pollution negatively impacts the entire coral reef ecosystem.”

Visual surveys

The scientists conducted over 1,200 underwater visual surveys across 84 shallow and mesophotic coral reef ecosystems spanning more than two dozen locations across the Indian, Pacific, and Atlantic oceans. 

To survey hard-to-reach mesophotic reefs located between 100 and 500 feet (30 to 150 meters) deep, in the ocean’s so-called “twilight zone,” they relied on specialized diving gear which few scientists are currently trained to use safely.

The investigation revealed that coral reefs are more contaminated by plastics and other human-related debris than many other previously examined marine ecosystems, although they are less polluted than shoreline ecosystems such as beaches or wetlands. 

Plastic increased with depth

Of the total debris that the researchers identified, 88 percent was macroplastics (particles larger than five centimeters). Surprisingly though, the amount of plastic was found to increase with depth, peaking in the mesophotic zone, and was mostly derived from fishing activities.

“It was surprising to find that debris increased with depth since deeper reefs in general are farther from sources of plastic pollution,” said senior author Luiz Rocha, a curator of Ichthyology and co-director of the Academy’s Hope for Reefs initiative. 

“We are almost always the first humans to set eyes on these deeper reefs, and yet we see human-produced trash on every dive. It really puts the effect we have had on the planet into perspective.”

According to the scientists, the there are many potential causes of plastic pollution reaching such depths. These include increased wave action and turbulence near the surface dislodging and carrying away plastic particles, recreational divers removing debris from more accessible shallow reefs, and shallow corals with higher growth rates overgrowing and hiding the trash from surveys.

Widespread debris

Significant levels of plastic debris were found in nearly all locations, including some of the most remote and pristine reefs, such as those located near the central Pacific’s uninhabited islands.

While the lowest densities of pollution (about 580 items per square kilometers) were observed in the Marshall Islands, the highest were found near Comoros, a chain of islands off the southern coast of Africa (84,500 items per square kilometer). 

Unfortunately, since mesophotic reefs are more difficult to access, they are rarely included in conservation efforts, despite harboring unique biodiversity rarely found on shallower reefs. 

“Our findings provide more evidence that the mesophotic is not a refuge for shallow reef species in a changing climate as we once thought,” said co-author Bart Shepherd, the director of the Academy’s Steinhart Aquarium and co-director of Hope for Reefs.

“These reefs face many of the same pressures from human society as shallow reefs, and have a unique and poorly-studied fauna. We need to protect deeper reefs and make sure that they are included in the conservation conversation.”

Fishing gear

Although the scientists found much consumer debris (such as food wrappers or water bottles, which are the main sources of plastic pollution in many ecosystems), almost three-quarters of the plastic items polluting coral reefs were related to fishing (including ropes, nets, and fishing lines).

“Fishing gear, which even as debris continues to catch marine life through what we call ghost fishing, appears to contribute a large proportion of the plastic seen on mesophotic reefs,” explained co-author Lucy Woodall, a principal scientist of Nekton and associate professor of Marine Conservation Biology and Policy at University of Exeter.

“Unfortunately, fishing gear debris is often not reduced by general waste management interventions; therefore specific solutions related to the needs of fishers should be considered, such as no-charge disposing of damaged gear in ports or individually labelling gear to ensure fishers take responsibility for misplaced equipment.”

By examining how the abundance of human-derived plastic debris correlated with geographic and socioeconomic factors, the scientists found that reef pollution generally increases with proximity not only to densely populated cities and markets (as expected), but also – counterintuitively – to marine protected areas.

This phenomenon could be explained by the fact that, since most marine protected areas allow some fishing within or near their borders and are usually more productive due to their protected status, they are heavily frequented by fishers. 

Complex challenges 

“Our findings reveal some of the complex collective challenges we face when dealing with plastic pollution. As marine resources around the world dwindle, humans that rely on those resources are turning to deeper habitats and those closer to marine protected areas where fish remain abundant,” Pinheiro explained.

By clarifying the major drivers of plastic pollution and showing that it increases with depth, this study could help redirecting conservation efforts to better protect our planet’s reefs.

The findings highlight the need to expand the depth of marine protected areas to include mesophotic reefs, and update international agreements on combating plastic pollution to include fishing gear, while developing low-cost biodegradable alternatives to plastic fishing gears that would not impact the well-being of coastal communities relying on sustainable fishing. 

“Despite the disturbing overall trend, there were some places where we found relatively little debris, which shows us that there are effective strategies for preventing plastic pollution. If we act fast and employ science-based solutions, there absolutely is hope for coral reefs,” Shepherd concluded.

The study is published in the journal Nature.

More about ocean plastic pollution

Every year, about eight million metric tons of plastic make their way into our oceans. This human-made disaster poses an escalating threat to marine life, human health, and our economies. Our oceans suffocate under this burden, their once-clear blue depths blurring into a murky haze of discarded plastic.

Plastic, by design, is incredibly durable and resistant to degradation. These attributes that once seemed beneficial now haunt our ecosystem.

A single plastic bottle may persist in the ocean for up to 450 years, fragmenting into minute particles, but never fully disappearing. These microscopic fragments, known as microplastics, permeate the oceans, sediments, and even the bodies of marine creatures.

Plastic’s impact on marine life

Plastic pollution’s environmental impact is both devastating and wide-ranging. About 700 species, including endangered ones like sea turtles and the Hawaiian monk seal, are known to ingest or get entangled in plastic debris. This exposure often leads to injury, malnutrition, and death.

Moreover, as plastic disintegrates into microplastics, it enters the food chain. Plankton, the base of this chain, inadvertently consume these particles, resulting in bioaccumulation and biomagnification as we move up the trophic levels.

Plastic’s impact on humans

Humans are not exempt from the impact of plastic pollution. We rely on the oceans for food, jobs, and recreation. Alarmingly, studies indicate that people may consume up to 5 grams of microplastics each week – equivalent to a credit card’s weight.

The long-term health consequences of this ingestion remain largely unknown. However, some research points to potential links between microplastics and a host of health issues, ranging from hormonal disruption to cancer.

Moreover, plastic pollution has significant economic implications. It directly affects industries reliant on clean oceans, including fishing, tourism, and shipping. The Asia-Pacific Economic Cooperation (APEC) estimates that the economic loss due to ocean plastic pollution ranges from $1.26 billion to $2.5 billion per year.

Immediate action is needed

We cannot ignore the plastic pollution problem any longer. The implementation of effective waste management systems is crucial, especially in developing nations where much of the plastic waste originates.

Reducing our plastic usage, promoting recycling, and developing biodegradable alternatives are critical steps we can take immediately. Governments, industries, and consumers must join forces to battle this crisis.

Research plays a pivotal role in this fight. Scientists are investigating the feasibility of plastic-eating bacteria and exploring the extent of plastic pollution, even in remote areas. Such studies could lead to breakthroughs in our battle against this pervasive pollutant.

In conclusion, the issue of plastic pollution is an urgent global concern that threatens marine life, human health, and our economies. However, with concerted global effort, responsible practices, and scientific innovation, we can navigate our way toward cleaner, plastic-free oceans.

The future health of our planet depends on our actions today. We must confront the plastic pollution problem head-on, for the sake of our oceans, our health, and generations yet to come.


By Andrei Ionescu, Staff Writer

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