Researchers peer into the twilight zone and discover…even more human-produced trash!

It’s something we’ve heard time and time again: no place on Earth is free from plastic waste. Plastic pollution poses a critical and escalating threat to our planet, with the extensive use and improper disposal of plastic products leading to the accumulation of vast amounts of plastic waste even in our oceans.

Plastic items such as bags, bottles, and microplastics disintegrate into smaller particles, infiltrating every level of the marine food chain. The persistence of plastics in marine environments, lasting for decades or even centuries, exacerbates the problem.

As a result, marine habitats and the diverse array of species that inhabit them are facing severe consequences. And this not only impacts marine animals directly, but also poses potential risks to our health through the consumption of contaminated seafood!

But some of the reefs are 150 metres deep, so surely plastic pollution isn’t a big problem there?

Over the past few years, coral reef scientists have found that these crucial ecosystem are losing the capacity to sustain their biological functions, and plastic pollution is an emerging threat to coral reefs, spreading throughout reef food webs, and increasing disease transmission and structural damage to reef organisms.

Although plastic pollution has been recognized as a global concern, the distribution and quantity of plastics trapped within the crevices of our reefs is largely unknown.

To figure this out, a team of scientists set out to study 84 coral ecosystems in the Pacific, Atlantic, and Indian Oceans to look for man-made debris. The team reported this month in Nature.  

“Plastic pollution is one of the most pressing problems plaguing ocean ecosystems, and coral reefs are no exception,” says Dr Hudson Pinheiro, the study’s lead author, a biologist at the Center for Marine Biology of the University of São Paulo, and a postdoctoral 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.”

The researchers performed 1,231 meticulous transects over each habitat – including unoccupied atolls and reefs at depths of 150 meters. To examine mesophotic coral reefs located at depths ranging from 30 to 150 meters (commonly known as the ‘twilight zone’), they used specialized diving equipment that only few scientific dive teams know how to operate. 

The results were sobering — coral reefs exhibit higher levels of plastic and human-derived debris contamination compared to other surveyed marine ecosystems. Even the most remote and pristine reefs had plastic pollution, like uninhabited central Pacific atolls!

Locations like the Marshall Islands exhibited the lowest pollution densities, with approximately 580 items per square kilometre. Conversely, Comoros, an island chain off the southeast coast of Africa, displayed the highest density of pollution, reaching nearly 84,500 items per square kilometre – equivalent to approximately 520 debris pieces on a single football field.

But in contrast to research done on near-shore environments, the scientists found that the deeper the reef, the more plastic junk it’s likely to have. A notable increase in plastic concentration with depth was observed, reaching its peak in the mesophotic zone.

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

“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.”

The researchers propose several potential reasons for pollution reaching such depths, such as heightened wave action and turbulence near the surface which dislodges and transports trash downward.

Disturbingly, although these plastic-laden deeper reefs harbor unique biodiversity not typically found on shallow reefs, they often face limited inclusion in conservation efforts, management targets, and discussions due to their challenging accessibility for study.

“Our findings provide more evidence that the mesophotic is not a refuge for shallow reef species in a changing climate as we once thought,” says co-author Bart Shepherd, 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.”

Among the entirety of debris, approximately 88% constituted macroplastics larger than two inches (five centimetres). Despite discovering a significant amount of varying consumer debris, such as the usual culprits such as water bottles and food wrappers, the researchers observed again the damage by commercial fishing – nearly three-quarters of all plastic documented on the surveyed reefs were associated with 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,” says co-author Dr Lucy Woodall, principal scientist of nekton (living organisms that are able to swim and move independently of currents) and associate professor in marine conservation biology and policy at University of Exeter. Ghost nets are often made of durable materials like plastic, allowing them to persist in the marine environment for years or even decades, often smothering or destroying delicate ecosystems like corals.

During this time, they also continue to fish indiscriminately – even after they are abandoned or lost. This ongoing fishing pressure on marine populations can deplete fish stocks and disrupt the natural balance of marine ecosystems.

“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,” says Woodall.

Ultimately, the team hopes their investigation will lead to a redirection of conservation efforts.

“The results of our global study shine a light on one of the many threats that deep reefs face today,” says study author and University of Oxford marine biologist Dr Paris Stefanoudis.

“Because these ecosystems are ecologically and biologically unique, much like their shallow-water cousins, they need to be conserved and explicitly considered in management plans.”

Specifically, the researchers emphasize expanding the depth of marine protected areas to encompass mesophotic reefs as being crucial. They also believe updating international agreements on combating plastic pollution should encompass fishing gear (as discussed at the recent Intergovernmental Negotiating Committee on Plastic Pollution) and developing low-cost biodegradable alternatives to fishing gear are essential.

The researchers believe that those places where they found relatively little debris show there are effective strategies for preventing plastic pollution.

Hope for Reef’s Shepherd says: “If we act fast and employ science-based solutions, there absolutely is hope for coral reefs.”